North American MPEG-2 Information
Updated November 24, 2003
TSReader - Analyze, Decode and Record MPEG-2 Transport Streams - click here!
|Introduction Section. Terms defined|
|Symbol Rates, FEC and that kinda stuff|
|SCPC, MCPC, PIDs and Formats|
|The MPEG-2 Transport Stream. How receivers know what's where|
|DVB/MPEG-2 video available in North America|
|Time Division versus Statistical Multiplexing|
|Pros and Cons of the listed receivers|
|Nokia Mediamaster specific information|
|Before you buy an MPEG-2 receiver|
|European information - explains some of the terms used in Europe as they apply here in North America when you buy an MPEG-2 receiver from overseas|
|What can be received with Echostar and AlphaStar receivers|
|Conditional Access - the key to private and pay TV systems|
|HDTV - way cool looking TV (opens on a seperate page)|
|DCII - info about the "other" North Amercian MPEG-2 satellite standard|
|DSS - the "other other" MPEG-2 satellite standard used in North Amercia|
|Links - more MPEG-2 info on the WWW|
This document is © 1997-2003 by Rod Hewitt. Permission is granted for personal use of this document only. This document may not be copied, mirrored or otherwise used without express written authorization from the author.
Why is this here?
I made this information available as I see a large number of questions posted to the rec.video.satellite.* news groups about MPEG-2 video services and more importantly because quite often, the answers that people post are incorrect. Although MPEG-2 is a very wide ranging standard that covers more than satellite distribution, this article concentrates on just satellite distribution.
What is not here
This page does not cover any topics relating to compromising encrypted programming. All the odd signals that people have received in North America using MPEG-2 receivers have been received because the broadcaster has not elected to encrypt the programming. I believe in paying for programming, even if that means a grey market subscription. That is quite different than stealing programming.
What is MPEG?
MPEG stands for Moving Picture Experts Group. It is a standard method of transmitting digital video and sound in a compressed format using less bandwidth than the traditional analog method.
The first MPEG standard introduced was MPEG-1 which is used to compress film onto regular compact discs (VideoCDs). MPEG-1 uses a low bit rate resulting in a picture similar to VHS video tape. The MPEG-1 data stream supports only one video signal and is therefore not used for satellite transmissions. MPEG-1 uses either 25 or 30 frames per second and is therefore not very well suited to storage of interlaced video.
Broadcasters wanted the enconomy of digital transmission, but because MPEG-1 was not suitable for satellite and MPEG-2 was still being developed, a "bastardized" flavor of MPEG which I call MPEG-1.5 was created. This format is not a official standard, but is still used for satellite (CNN Airport network uses MPEG-1.5). MPEG-1.5 uses a wide bandwidth MPEG-1 flavor of video encoding along with multiplexing of data streams which allows multiple programs to be transmitted across one satellite channel at a time.
MPEG-2 is becoming the de-facto standard in the digital TV world. MPEG-2 fixes many of the problem inherent in MPEG-1, such as resolution, scalability and handling of interlaced video. It allows for a much better picture (studio quality and up to HDTV levels) and allows multiple channels at various bitrates to be multiplexed into a single data stream. It was officially adopted by ISO and has the catalog number ISO 13818-1.
Program producers (like NBC, HBO et al) prefer to use MPEG-2 to distribute their programming because they can transmit multiple programs in the same space as a single analog transmission. Satellite and cable companies also like the idea of digital compression and it allows them to offer much more programming versus analog with the same amount of bandwidth. All licensed US DBS providers (DirecTV, USSB, Echostar etc.) are required by their licenses to transmit in digital format.
What is DVB?
DVB stands for Digital Video Broadcast and is a standard based upon MPEG-2 video and audio. DVB covers how MPEG-2 signals are transmitted via satellite, cable and terrestrial broadcast channels along with how such items as system information and the program guide are transmitted along with the scrambling system used to protect the signal.
With the exception of the United States of America, Mexico, Canada, South Korea and Taiwan, DVB has been adopted by just about every country in the world for digital TV & radio. This document concentrates on DVB-S, the satellite format of DVB - DVB-C is the specification for DVB/MPEG-2 over cable and DVB-T is DVB/MPEG-2 over terrestrial transmitters.
What is Digicipher?
Please see our new section on DCII
What is ATSC?
ATSC is Advanced Television Systems Committee which is destined to replace NTSC as the method of terrestrial television transmissions in the United States, Canada, Mexico, South Korea and Taiwan. Like DCII, ACTS uses the MPEG-2 video specification, but bastardizes everything else, making North American (and South Korea/Taiwan) an island in a world of standards.
ATSC is almost exactly the same as Digcipher 2 and of course it's no surprise that General Instrument was on the comitee that recommended ATSC to the FCC. In theory, ATSC and Digicipher 2 have a couple of advantages over MPEG-2/DVB, especially in the area of signal aquisition time, however, this is not enough to justifying a totally different standard than the rest of the world.
An interesting tidbit about why ATSC uses AC3 for audio and not Musicam recently surfaced. In the field trials during the development of the ATSC specification, both AC3 and Musicam were tested. Technically both have the same merits, including the ability to do 5.1 audio in the same bandwidth. However, AC3 was chosen because in one area, it was tested to have better performance than Musicam. It was later discovered that the testing procedure was flawed and that subsequent re-testing after the standard was published showed that AC3 and Musicam performed equally as well.
Like DVB/MPEG-2, ATSC supports HDTV.
Will there ever be a receiver than can do ATSC, DCII and MPEG-2/DVB?
Yes - Motorola's DSR-4800 receiver is able to process both DVB and DCII formats, however, it's worth pointing out that this is a commercial receiver with a $4,000 price tag.
Obviously, because of the differences in audio encoding, the receiver handles both AC3 and MPEG-1, as the second generation of MPEG-2/DVB silicon is now coming onto the market has the capability to do both audio standards.
Additionally, because DCII and ATSC are so similar, the DCII specification is now 95% public information, whereas in the past it was considered proprietary to General Instrument. In a complete turn about, GI now licenses the DCII specification and has recently signed a cross license agreement with Scientific Atlanta, one of the early adoptors of the MPEG-2/DVB standard.
Symbol Rates, FEC and that kinda stuff
What's a symbol?
Like just about any form of digital transmission, the receiver has to know the rate at which the transmitter is sending information. In the computer world, we call this the bit rate. For example, PCs can transmit from their serial ports at up to 115,200 bits per second. Bit rate and baud rate are not the same, despite the fact that some people will turn blue trying to tell you that they are. The bit rate specifies how many bits per second are carried across the channel (phone line, serial cable or satellite transponder), however, the baud rate describes the rate that data is sent within the channel.
For example, suppose you invented a simple modem that transmitted at 50 bps by using two tones. One tone could signal a 1 needed to be sent and the other would signal 0. Now imagine that you wanted to double the transfer rate across the channel. By using four tones instead of two, you could signal two sets of bits at the same time by switching various combinations of the four tones. The baud rate is still 50 baud (i.e. the tone pairs change 50 times per second), however, the bit rate is now 100 bps. The combination of the sets of tones is called a "symbol" because too many people are confused by the term baud.
What's QPSK modulation?
When satellite transponders are used to transmit MPEG-2 signals, Quadrature Phase Shift Keying is used to modulate the digital information onto an RF carrier.
Rather than using the amplitude or frequency of the carrier to convey the information, QPSK modulates the phase of the carrier signal. Depending on the data being modulated, the carrier is forced into one of four different phase states, known as a symbol. The great advantage of this method is that each symbol contains two data bits, thus doubling the potential amount of data that is transmitted over conventional amplitude or frequency modulation (AM or FM) techniques.
The diagrams below illustrate a typical implementation of QPSK:
Figure 1 shows each possible pair of data bits is represented by a different phase angle and figure 2 shows and example of a QPSK waveform.
Because of QPSK, data rates are quoted in Symbol Rate rather than bit rate. In the case of QPSK modulation, the bit rate is twice high as the symbol rate. For example an SR of 20MS/s (20 mega-symbols) means 40Mb/s (40 mega-bits bits per second).
Satellite transponders are rather noisy communications channels are are therefore subject to a large number of errors when a signal is sent through them. Because satellite transmissions are broadcast, the receiver cannot send a message to the transmitter to say "I didn't get that last piece of information, please re-transmit it". As a result, Forward Error Correction is used, where the transmitter sends error correction information along with the actual signal so that should errors occur, the receiver can re-generate the bit stream.
FEC when used with QPSK modulation uses two forms of error correction. The first, called convolutional coding with the Viterbi algorithm code is quoted as a fraction, for example, 2/3. The fraction defines the amount of the symbol rate that's used for real data, with the remainder used error correction purposes.
After the convolutional error correction code has been removed and used as needed, a second error form of error correction is used called the Reed-Solomon code. This correction results in 188 bytes out for every 204 bytes coming in with the remainder used as parity bits to help correct any remaining errors. Additionally, the FEC scheme also uses interleaving of the data stream to prevent noise bursts from interrupting the flow of data in much the same way that CDs use it to prevent scratches from causing drop-outs.
Consider the following message:
If interleaved, it might look like:
Should an error occur and say wipe out the 'mgi' part of the message, the de-interleaved message will now read
As a result, only single characters are missing from the message (shown here as asterix), rather than an entire word missing in the case of non-interleaved data.
As a final step, the QPSK symbols are scrambled to ensure that long runs of the same symbol value don't cause a lack of change in phase of the carrier. Since the QPSK demodulator obtains its signal clock from directly from the signal, there must be a large number of phase changes in order to re-generate the clock and of course scrambling results in this. Note: this form of scrambling is not the same as scrambling of the decoded signal.
Why use different SR/FEC values?
When people purchase time on a satellite, in effect they are primarily paying for the bandwidth. Therefore if a programmer wanted to transmit three video channels via a transponder, he would use less bandwidth than a service that transmitted six. However, the bandwidth of a transponder is finite and therefore an upper limit is placed on the SR (typically between 28 and 29 MS/s). By reducing the amount of FEC information sent along with the actual data, the number of channels can be increased. However, this then means that errors are harder to correct and that the down link stations must be able to receive a certain signal strength (i.e. use a certain size dish) in order to receive quality programming via the transponder.
What's QAM and Vestigal Sideband?
Quadrature Amplitude Modulation is the cable version of QPSK. Using many different symbol phases (the initial standard for the US is 64 different phases), a given 6MHz of cable bandwidth will be able to carry the same amount of data as a single 30MHz transponder. Given a 125 channel cable system, this means that they will be able to carry 625 video and audio programs assuming compression levels where five video services are sent on a single RF channel.
Vestigal Sideband modulation (otherwise known as VSB-8) is the technique that will be used in the US for terrestrial ATSC transmission. VSB-8 uses AM transmission with phase information within the sideband. The other sideband is almost totally surpressed and a pilot carrier is inserted to help receivers initially acquire the signal. VSB-8 uses eight phases with 3 bits encoded per phase which are then reduced to two bits in the receiver. I could try to explain how it works, but Harris Semiconductor has written a much better explanation which is linked at the bottom of this page.
How do I make sense of the SR/FEC/PID listings on the Lyngsat Chart?
If you've seen something like:
12,177 V SR 23000 FEC 2/3
V 0FF0 A 0100 ATN
V 0FF1 A 0101 RTN
V 0FF3 A 0103 HealthSouth
V 0FF4 A 0104 RE/MAX TV
This means that the transmission is centered at 12.177 GHz, uses Vertical polarity for the down link, uses a symbol rate of 23.000 MS/s and FEC of 2/3. This is a multi-channel package that contains four video services with the Video and Audio PIDs for the individual packages listed. The PIDs are shown in hexadecimal format.
SCPC, MCPC, PIDs and Formats
MCPC stands for Multiple Channel Per Carrier. Given an average satellite transponder with a bandwidth of 27MHz, typically, the highest symbol rate that can be used is SR 26MS/s. Obviously, with this large bandwidth, multiple video or audio channels can be transmitted via the transponder at the same time.
MCPC uses a technique called Time Division Multiplex to transmit the multiple programs at the same time. As one can expect from the name, TDM sends data for one channel at a certain time and then data for another channel at another time.
Many encoder manufacturers are currently experimenting with statistical multiplexing of MPEG-2 data. Using this technique, channels that need high data rate bursts in order to prevent pixelization of the picture (such as live sports events), will obtain the bandwidth as they need it by reducing the data rate for other services that don't.
Statistical multiplexing should improve perceived picture quality, epecially on video that changes rapidly and has the advtange of requiring no changes in the receiver equipment.
SCPC stands for Single Channel Per Carrier. In the case of this type of transmission, only a part of the available transponder is used for the signal. The satellite operator can sell the remaining space on the transponder to other up linkers. SCPC is typically used for feeds rather than for direct programming. SCPC has the advantage over MCPC that the signals up linked to the same transponder can be transmitted up to the satellite from different locations (SNG trucks for example), but has the disadvantage of not being quite as efficient as MCPC because of "guard bands" which keep the SCPC signals on the same transponder separated from each other.
NBC uses SCPC MPEG-2 for its back haul feeds and is able to use up to four SCPC transmissions on a single satellite transponder (GE-1 Ku-Band). Microspace uses the same type of transponder on the same satellite, but in MCPC format and is able to transmit six video channels and a few audio channels in the same space.
What are PIDs?
MPEG-2 transmits its data in packets of 188 bytes each. At the start of each packet is a package identifier (or PID) that tells the receiver what to do with the packet. Because the MPEG-2 data stream might be in MCPC mode, the receiver has to decide which packets are part of the current channel being watched and therefore pass them onto the video decoder for further processing. Those packets that aren't part of the current channel are simply discarded.
There are typically four types of PIDs used by satellite receivers. The VPID is the PID for the video stream and the APID is the audio stream. Occasionally, a PCR PID (program clock reference) is used to synchronize the video and audio packets, however, most of the time, this data is embedded into the video stream. The forth data PID is used for data such as the program guide, information about other frequencies that make up the total package etc. This data is called the System Information and uses a PID value of between 0000 and 0014 (hex).
The System Information stream
SI packets tell the receiver about the format of the transmission along with information such as multiple language selections, program guide information and other transponders that are related to the current transponder.
The primary reason that MPEG-2/DVB receivers cannot handle Digicipher 2 and ATSC signals is because the SI packets are totally different between the two standards. In theory, it should be possible to make an MPEG-2 receiver receive DCII/ATSC, however, this would either require access to the source code of the MPEG-2 receiver's firmware (and probably a license from General Instrument) or the DCII/ATSC signal being transmitted using both DCII/ATSC and MPEG-2/DVB SI packets. This is possible (see the ATSC technical documentation page for information on how this is done), however, the audio will either have to be sent twice or the receiver will need to handle both Musicam and Dolby AC3 as this is another big difference between the systems.
What's 4:2:2 and HHR MPEG-2?
When MPEG-2 encodes color and picture information, it samples the analog picture at certain resolution both as horizontal and vertical pixels, but seperately as color (chrominance/hue) and brighness (luminance). The DVB specification calls for 4:2:0 encoding which put simply means that the resolution of the color information is one quarter of the resolution of the video information.
Since studios need better quality than DVB offers, an extension to MPEG-2 has come about that isn't part of the DVB spec but has its own specialized defintion within the MPEG-2 standard. This is called 4:2:2 format or MP@4:2:2SP meaning "Main Profile 4:2:2 Studio Profile". In this system, double the amount of vertical color information is transmitted.
Another format exists that is in very common use today. Called HHR for half horizontal resolution, this part of the MPEG-2/DVB standard transmits only half of the normal 720 pixel horizontal resolution while maintaining normal vertical resolution of 480 pixels (although, since it's 4:2:0 format, the color information is only encoded at 240 pixels vertically and 176 pixels horizontally. A lot of the smaller DBS (like the ethnic packages on T5 etc) use HHR format since it dramatically reduces the bandwidth needed for channels - of course at the expense of picture quality. Special logic in the video decoder chip in the set top box, re-expands the picture to its normal horizontal size by interpolation prior to display.
4:2:2 video at Standard Definition looks just as good as the NBC analog feeds on GE-1 Ku. High bandwidth 4:2:0 video like the NBC digital feeds on GE-1 Ku come very close to studio quality and the low bandwidth stuff encoded in HHR format, looks a lot like VHS quality.
The following diagram shows the ratios of 4:2:0, 4:2:2 and HHR resolutions. I could explain why the ratio used for 4:2:0 is written as 4:2:0 but that gets mega-complex and is beyond the scope of this document. If you want to know more, I highly recommend getting a copy of Digital Video: An Introduction to MPEG-2 by Barry Haskell, Atul Puri and Arun N. Netravali - ISBN 0-412-08411-2.
MPEG-2 Sample Shots
When using DVB2000 software on a Nokia Mediamaster receiver and a PC equipped with a SCSI bus and DVBEdit, it's possible to capture the recontstructed video directly out of the MPEG-2 decoder's buffer memory. In the following screen shots from Dish Network, you can see how each of the individual components of the picture are transmitted and how pan/scan is used to interpolate both the base video and chroma information.
The MPEG-2 Transport Stream
As mentioned above, MPEG-2 transmissions are either transmitted as SCPC or MCPC feeds. However, at an individual channel level, both techniques use the same method for building a data stream containing the video, audio and timing information. In this section, I'll concentrate on MCPC because once this is understood, SCPC becomes obvious. This combination of compressed video and audio is called the PES or Packet Elementary Stream and is built as follows:
The time field isn't the actual time that the encoding was done, but timing information to allow the audio and video to stay synchronized together. This part of the PES is called the PCR (Program Clock Reference) and may be sent either as part of the video stream or as a seperate stream (hence the reason that some MPEG-2 receivers like the d-box have a seperate field for the PCR).
Multiple PES streams get multiplexed together into a faster stream and the System Information or SI stream gets added, resulting in the final MPEG-2/DVB multiplex that gets uplinked to a transponder on the satellite:
The SI is responsible for telling the receiver all kinds of useful information about the data stream, so that the receiver can write the appropriate data into its program guide. The first part of the SI is called the Program Association Table or PAT. The PAT is always transmitted on PID 0000 and contains a list of Program Map Tables or PMTs that are part of the data stream. For example:
PAT (PID 0000) = 0100, 0200, 0300, 0400
PMT 1 (PID 0100) = Video PID 0101, Audio PID 0102, Audio PID 0103, PCR 01FF
PMT 2 (PID 0200) = Video PID 0201, Audio PID 0202, PCR 01FF
PMT 3 (PID 0300) = Video PID 0301, Audio PID 0302, PCR 02FF
PMT 4 (PID 0400) = Video PID 0401, Audio PID 0402, PCR 0401
Given this information, the receiver knows that the DVB transport on the current frequency contains four programs. The first channel contains two audio services (perhaps for multiple languages) and all of them except for the fourth program contain seperate timing information - the fourth has the PCR timing embedded into its video stream.
The reason that the PCR might be transmitted seperately from
the video stream is in the case of multiplexed channels which
were encoded with a common clock reference. In this case, it would
be redundant to send the PCR again, since the reciver would always
use the same clock refernece for all the signals within the multiplex.
In the above example, one might assume that the first three video
channels came from a common encoder and the fourth stream was
multiplexed in, perhaps after being received from an SCPC feed
or line-line and not re-encoded prior to multiplexation.
Thanks to Scott Bidstrup at Vyvx for his contributions relating to the above.
Obviously, the PMT contains other information, such as pointers to the name of the channel in the SDT table and things like information about data services that might be mutliplexed in as part of the PES. But in addition to the PAT and PMT, there are a few more interesting ones. The Network Information Table (NIT) on PID 0010 contains a list of associated transponders that make up the package along with their SR and FEC values, which can be different.
When doing a search on a single channel on the Echostar DBS service, most smart MPEG-2 receivers (like the d-box) will automatically go off and search the other frequencies used by Echostar since the NIT on each transponder points to all the other transponders. The NIT can also point to transponders on other satellites, so that in the case of Echostar, the receiver would know to switch to another dish to receive programming from the Echostar 3 DBS satellite at 61.5 degrees when you tune the receiver to a channel carried on this satellite. In its own strange and totally non-standard way, Digicipher 2, uses a similar technique to allow the 4DTV receiver to know where other satellites are and turn to them when a particular channel is chosen.
Now the receiver knows all the frequencies associated with a package, there are few other PIDs that make a DVB receiver work the way it does. The optional BAT or Boquet Association Table tells the receiver about programs of the same type (such as sporting events, movies, news etc.) that are part of the package. Echostar uses this part of DVB for their "Themes" menu. The EIT or Event Information Table on PID 0012 contains a list of the programs (or events) that when interpreted by the receiver's firmware, make the program guide. The EIT allows for up to two weeks worth of programming to be sent ahead of time.
And finally, if you wondered how MPEG-2/DVB receivers know what the time is, the TDT (Time and Date Table) tells the receiver what the date and time is in Universal Time - the smartcard or non-volatile memory in the receiver contains the UTC offset, so that you see local time on the screen.
SCPC signals are transmitted pretty much the same way as MCPC, but obviously only contain one PES since they occupy less bandwidth. Because SCPC channels are normally feeds, they typically do not carry many of the DVB SI streams such as the NIT, BAT and EIT.
What about moving MPEG-2 transport streams around facilities?
At sites where MPEG-2 transport streams are processed, there are a number of different interfaces used to move transport streams between devices. It's worth mentioning some of them since they come up from time in discussions about professional-level equipment.
DVB-ASI: This is a local area (300 metres) serial interface is based on coaxial cable. The data is simplex (i.e. from one device to another - not the other way around) and runs at 270 Mbps, although due to overhead the actual data rate is around 240 Mbps.
DVB-SPI: Again, a local network typically used to interconnect professional MPEG-2 equipment. Data is sent in parallel using LVDS (Low-Voltage Differential Signaling) balanced transmission. Data rates up to 40 MBps (B = bytes b = bits) can be used with this interface over short distances (a few metres).
ATM: ATM networks are WAN, MAN or LAN (Wide, Metropolitain
and Local Area) networks used by communications companies as the
protocol on fibre (and other) high speed networks. ATM has 53
byte packets, 48 of which are available for payload. When using
AAL1 (one of the possible standards for transmitting MPEG-2 over
ATM networks), the payload for data is 47 bytes, so one 188-byte
MPEG-2 packet can fit exactly in four ATM cells. This is the main
reason for the 188 byte packet length used in MPEG-2. As a side-note,
frequently another standard called AAL5 is used to encapsulate
DVB packets over ATM networks - in this mode, two MPEG-2 packets
plus 8 bytes of overhead adds up to 384 bytes which fits nicely
into the payload of eight ATM cells.
Thanks to Michael Clawson and Paolo Bevilacqua for info about the AAL5 encapsulation
Time Division versus Statistical Multiplexing
When MPEG-2/DVB carriers broadcast Multiple Channels Per Carrier (MCPC), packets for each of the channels within the transponder are mixed into the higher rate stream that's carried on the transponder. This process is called multiplexing and can be done two different ways. Before we get into how the two work, it's good to have an understanding of how the uplink works.
At the uplink, signals that are to be carried on the service are first received. This can be via a number of different methods such as from another satellite, off air antenna or via a leased line circuit. As an example, many of the DISH Network local channels are sent back to Cheyene in MPEG-2/DVB format using 45Mbs T-3 leased lines.
The signals are then converted back to composite video by decoding incoming any digital inputs which may be either in MPEG-1, MPEG-2, Digicipher 1 or Digicipher 2 - obviously the analog video is already in composite format. As an example of where DISH Network for instance get's some of its programming:
|TV5||Digicipher II from Galaxy 7 C-Band|
|Food Channel||Digicipher I from Galaxy 1R C-Band|
|Denver Locals||MPEG-2/DVB over leased-line|
|USA||Analog from Galaxy 5 C-Band|
|TV Polonia||MPEG-2/DVB from Orion F1 Ku-Band (SCPC)|
Next, the analog video and audio is re-encoded using very expensive encoders which generate an MPEG-2/DVB video and audio stream. This stream along with the other streams that will make the channels on the transponder and then multiplexed together and the System Information and Conditional Access streams are inserted before the resulting stream is modulated onto QPSK DVB-complaint carrier and transmitted up to the satellite.
At first, you may think that it's rather wasteful to decoded and then re-encode the signals that are already in digital format. However, this is done to allow the uplinker control of how much bandwidth his system has allocated fo the particular channel.
Also keep in mind that the DVB-SI and CA streams are transmitted in parallel across all transponders within the system (even across multiple satellites at different orbital locations). Obviously, this is wasteful from a bandwidth perspective, but it's a necessary evil to keep the EPG and authorization of the receiver working.
Graphically, the resulting stream looks something like the diagram below.
Time Division Multiplexing
In the TDM system the bandwidth is divided up at a fixed rate for each of the streams on a particular transponder. The uplinker has to make some decisions about how much bandwidth to allocate to each channel taking into consideration the type of programming that will be carried on each channel. For example, the DISH Network system uses SR 20.000 MS/s FEC 3/4 on its transponders. This results an available bitrate on each transponder of about 28Mb/s:
20.000MS/s = 40.000Mb/s
Minus 3/4 convolutional coding = 30.000Mb/s
Minus 188:204 Reed-Solomon coding = 27.647Mb/s
Given that the uplinker wants to make a certain number of channels on this transponder, he might choose a scheme like:
Movie Preview Channel 3Mb Cable Channel 3.5Mb Sport Channel 4Mb Sport Channel 4Mb Movie Channel 4.5Mb PPV Channel 4.5Mb News Channel 3Mb Audio Channel 128Kb Audio Channel 128Kb Audio Channel 128Kb Audio Channel 128Kb Audio Channel 64Kb Audio Channel 64Kb Audio Channel 64Kb Low-speed Data Service 19.2Kb High-speed Data Service 512Kb DVB-SI (EPG, authorization etc) 630Kb Firmware Update 128Kb Total 27.993Mb Unused (null PID) 7Kb
The problem with TDM is that channels that are given low bandwidth tend to contain lots of overcompression artifacts when there is too much motion, many frame changes or huge differences in luminosity. Additionally, when high bandwidth channels contain very compressable video, their bandwith is lost.
Statmux in MPEG-2/DVB systems is very new - the second generation encoders only hit the streets a few months ago. The only service known in North America to be using statmux at this time is DISH Network, where it has made a huge difference to the quality of the video and at the same time allowed all the transponders FEC to be backed down to 3/4, therefore improving rain fade performance.
Statmux encoders in-effect "talk" to each other about the amount of bandwidth required for the video they are currently compressing and they share this information with a central processor that talks to the other encoders and knows some basic rules, like the amount of space to allocate for fixed rate services, like the DVB-SI etc.
The end result is that a particular channel's bandwidth utilization might be 6Mb one second and 2Mb the next, depending on how much bandwidth that particular channel needs at that time. Obviously, there is still an upper limit to the number of channels that can be transmitted on each transponder, but the number is generally increased by going to statmux encoders since the bandwidth is now shared.
MPEG-2 Video Available in North America
Although there is a lot of Digicipher 2 video in North America, MPEG-2 is very popular here for the simple reason that it's cheaper to produce MPEG-2 encoders and receivers than Digicipher encoders and receivers because a) everyone else around the world uses MPEG-2/DVB and b) no-one has to pay General Instrument royalties or license fees for their oddball system. At the start of 2000, more than 1,000 DVB compatible services can be received from the East Coast of North America.
Echostar, ExpressVu and Microspace
All three of these program providers use the same encoder and receiving equipment to transmit digital video. All share the same SR 20.000 and transmit using MPEG-2/DVB.
Note that ExpressVu changed satellites in November of 1999,
when they switched from the Anik E2 satellite to Nimiq 1. At the
same time ExpressVu changed LNBFs, switching from linear polarity
FSS Ku-band to the circular DBS part of the Ku-Band. Although
Echostar hardware is now identical to that of ExpressVu, right
down to the LNB used, Echostar will not activate subscriptions
to receivers with ExpressVu serial numbers, nor will ExpressVu
activate subscriptions to receivers with Echostar serial numbers.
Thanks to John Koperski for this update
Both ExpressVu and DISH Network have some free to air video on their services and most of their audio services are also unscrambled.
Because DTV went on the air before the MPEG-2/DVB standard was ratified, they don't use standard MPEG-2. Much like GI, they use MPEG-2 video encoding, but use strange audio encoding and system information packets. Additionally, the DSS service uses variable FEC values depending on how they have the the transponders on the satellite configured. The DSS data stream is transmitted the same way as all other MPEG-2 satellite services, however, due to its differences, is incompatible with MPEG-2/DVB receivers.
AlphaStar used to transmit on Loral's Telstar 5 satellite until they went out of business due to bankruptcy. Recently, a number of MPEG-2/DVB signals have popped up on T5 using the same SR 23.000 and FEC 2/3 as AlphaStar used. These services are up linked by various companies and are typically ethnic or niche programs that were previously available to AlphaStar clients.
Associated Press's Global Video Wire is transmitted around the globe on a host of different satellites. This SCPC signal can be received on the east coast of North America from no less than four different satellites (Intelsat K, TDRS, PAS-5 and GE-3) using the same SR 5.565 and FEC 3/4. By using the same SR and FEC for all their feeds, in order for them to have their signal hop between satellites, only the RF signal has to be received and re-transmitted. Most probably, APTV uses one MPEG-2 encoder in London and uses RF retransmission to get the signal around the globe.
Latin American Cable TV
Panamsat 5 at 58 west carries a host of MPEG-2 video services targeted at cable companies in Central and South America. This includes CCTV (Chinese TV), CBC Tele Noticias, Locovision (Cartoons), ESPN, Playboy TV, BBC World, NHK and Deutche Welle. Check Lyngsat for an up to date list.
Hispasat Digital Service
The long running Canal Hispavision on Hispasat (can be seen on the East Coast) recently went digital and picked DVB/MPEG-2 for its transmission format. This package contains five video channels (TVE Internacional, Canal 24 Horas, Hispavision, EuroNews and TVE Internacional America) and five radio channels. Sadly, this service which has been free since the launch of the Hispasat system has now encrypted with Nagra and can only be received by a General Instruments DVB receiver (probably one of the worst DVB receivers available).
Obviously, the cheapest way to obtain a DVB/MPEG-2 receiver is to go and buy an Echostar receiver. However, this receiver is targeted at a particular service (i.e. Dish Network) and therefore will only receive signals that have the same SR and FEC as Echostar.
A receiver that can handle variable SR / FEC and receive non scrambled programming is called a Free To Air (or FTA) receiver. Currently, in North America only one of these beasts exists, the Scientific Atlanta PowerVu receiver.
Scientific Atlanta PowerVu 9223 - about $1600
This receiver is designed for cable companies to allow them to receive MPEG-2 signals up linked for their benefit. As a result, its user interface is very complex and is not designed for channel surfing. It locks virtually all known MPEG-2 signals, but has some video problems with a few signals, especially those encoded by Wegener equipment.
In order to lock a signal with this receiver, the exact frequency, SR and FEC must be known ahead of time. Therefore, this receiver cannot be used for hunting new signals. Since the 9223 was designed before the official publication of the MPEG-2/DVB specification, it does have difficulty with some MPEG-2/DVB signals and PIDs must be entered manually.
Scientific Atlanta PowerVu 9234 - about $750
The 9234 "Business Satellite Receiver" does all that the 9223 receiver does, but adds a remote control, is fully MPEG-2/DVB compatible, searches for the FEC and does frequency tracking (i.e. you don't have to be exactly "on the spot" in order to get a lock), has RF, composite video and S-Video outputs and has memories for up to 24 transponders.
This receiver, like the 9223, can be authorized for some Canadian
programming which is a major enhancement over any of the other
receivers. Channels which can be received include TV5, RDI (Reseau
De l'Information), Canal Vie, MusiMax, Musique Plus, LCN (Le Canal
Nouvelles TVA) and SRC (Societe Radio-Canada).
Thanks to Roland Babin for this info.
Standard-Firmware Nokia MediaMaster 9500s (the d-box) - about $800
This European receiver is the MPEG-2 hunter's dream receiver as it only needs the frequency of a target signal. Once the frequency has been entered, the receiver will automatically find the SR and FEC, which is a unique feature of this receiver. However, this receiver is designed for use in Europe and is therefore somewhat of a pain to use in North America. Specifically, it requires a PAL TV and 220v to operate and has a nasty bug that causes the screen to flicker once every 20 seconds when viewing 525 line 60Hz video, the normal standard for North America.
However, the d-box contains many hidden menus that allow control over virtually all aspects of the receiver. Additionally, because of the popularity of this receiver in Europe, there are many commercial and free d-box control programs available for the PC that drive the receiver through the serial port on the back of the receiver.
One very nice thing about the Nokia receiver is its ability to be upgraded via the serial port from a regular Windows 95 PC. I recently upgraded my receiver to a version called Dream 5.0 which fixes many problems with this receiver. You can read my review on the C&J Electronics web site.
DVB-98 Equipped Nokia MediaMaster (9200, 9500 or 9600) - about $800
Since the Nokia Mediamaster is very popular receiver in Europe, an enterprising German chap by the name of Uli Herrmann decided to write his own firmware for the Nokia receiver. Since its introduction in early 1998, it has become probably the most comprehensive piece of software on any digital MPEG-2/DVB receiver. Not only does it find the Symbol Rate and FEC of transmissions but also locates the services running in the stream by either reading the DVB-SI or by using a brute force technique of scanning each possible PID for a video stream.
It's complimented on the PC by the DVBEdit Windows application that the author of this document developed specifically to add PC to Mediamaster connectivity. This utility allows editing of the channel information stored in the Mediamaster and also does firmware upgrades using the serial port. It contains a band scanner that allows the PC and Mediamaster to scan an entire satellite and report the frequency, SR and FEC of any transmissions found and includes a spectrum analyzer capability. DVBEdit also runs on Windows CE based handheld and palm-sized PC devices.
Since many MediaMaster receivers include a SCSI bus, DVB98 can play CD discs containing MPEG-1 Level 2 audio files and VideoCD movies. SCSI to Mediamaster interfaces are also under development. This capability will allow recording and playback of video and audio using a PC and SCSI controller card.
DVB-98 and DVBEdit are both free and can be downloaded from many sources on the Internet:
I recently posted a list of good things about DVB98 on the Nokia to one of the newsgroups. Here's a copy of that info since it really does show the wonderful things that DVB98 can do that virtually all other receivers can't.
With the addition of a SCSI CD-ROM:
With the addition of a serial cable and a PC:
With the addition of a SCSI controller in the PC:
Hyundai HSS-100C - about $600
This Korean receiver is becoming the most popular in North America for a number of reasons. First, it operates on both 110v and 220v, second it outputs NTSC when it encounters a 525 line 60Hz video source and third is quite easy to operate. It cannot search for the symbol rate like the d-box.
To run this receiver, you'll need a PAL TV to initially set it up, but it will then drive a normal NTSC TV once it's tuned into video. It handles most MPEG-2 feeds however, locks up on some signals, such as half of the NBC SCPC feeds on GE-1 Ku-Band.
There are two versions of the HSS-100C available. Those running firmware version 2.x contain the same Nokia tuner that the d-box uses and the 5.x version which contains a Sanyo QPSK tuner. It appears that the Sanyo tuner isn't as sensative as the Nokia tuner, however, the 5.0 version of firmware does add an automatic FEC mode.
On the negative side, this receiver is very slow. This is because it uses EEPROM for its channel memory and EEPROM is very small, so it can't save things like PIDs in its memory. As a result, every time you switch between transponders, the receiver has to lock the signal and process the DVB-SI to figure out which channels are there. Needless to say, this is very time consuming.
Pansat 100A - about $500
This receiver is basically the same as the Hyundai HSS-100C but starts in NTSC mode initially. It has all the other drawbacks or advantages of the HSS-100C (depending on your point of view).
RSD Communications ODM 300
This Scotish FTA receiver hasn't really been distributed at all in North America. I managed to get hold of one to play with and write some software for, but it certainly does have a lot of potential. It uses a 33MHz Coldfire processor from Motorola, so compared to all the other receivers, it flies, even with complex multi-color graphics. This receiver is like a graphically slick version of DVB98, i.e. it's designed for satellite enthusiasts. It doesn't have all the wonderful technical capabilities of DVB98 but in return is much simpler to operate.
Note: The ODM-300 supports DiSEqC 1.2 rotors, which is a very simple way to motorize an offset Ku-Band dish. See the links section for links to DiSEqC 1.2 rotor manufacturers.
This technically isn't a FTA receiver but with a lot of coaxing can receive some FTA programming. This is basically a Dish Network 3000 receiver but with European things like SCART connectors and a Common Access CAM slot. It contains a Nagra Conditional Access circuit built into the board since it's designed for use with the Spanish Via Digital DBS service. The reason for the built CAM slot is so that it can receive programming from other providers that provide CAMs since this is a legal requirement for all digital satellite receivers in Spain to prevent people from being locked into a particular service (I wish the same were true in North America!).
This recevier has two LNB configurations (one set for Universal and the other programmable) and has memories for 8 base transponders (it searches the DVB-SI for other transponders that are part of the boquet). Only one base transponder memory can be active at a time, but then if you consider the fact that it's designed for DBS services, this isn't too much of a restriction. The front-end is only capable of 18MS/s and above, so SCPC or small MCPC packages are not possible - it also doesn't process the PIDs in many packages correctly. Of course pointing it at one of the Echostar DBS slots, ExpressVu or the Microspace package on GE-1 Ku results in a perfect channel map since all three of these services (along with Via Digital in Spain) use the same TV/COM / Divicom encoder equipment that was put together by Echostar's engineering team. Of course, most of these services are encrypted.
This is probably the least expensive and also one of the fastest MPEG/DVB receivers available. Although it doesn't allow direct PID entry, it does a very good job with virtually all North American signals it has been pointed at. You can read my review here.
Pros and Cons of the Receivers
Scientific Atlanta PowerVu D-9223
Standard-Firmware Nokia Mediamaster 9500s (d-box)
DVB-98 Equiped Nokia Mediamaster (9200, 9500 or 9600)
Nokia Mediamaster Specific Information
The Nokia Mediamaster is the generic name for a series of digital MPEG-2 receivers made by the Finish company, Nokia. Many people refer to this type of receiver as a d-box, but that's really a confusing because there are so many variations of this type of receiver.
The d-box and dreambox
The d-box is a receiver sold in Germany for the DF-1 package. It contains a Conditional Access Module (CAM), which is a descrambler along with a card slot which uses a card that gets married to the CAM. Echostar receivers also have a CAM, however, theirs is built directly onto the main receiver board. If you have a look at an AlphaStar receiver, you can see the CAM - it's a seperate board that connects between the QPSK receiver and main board. The d-box also contains a modem for PPV reporting (it also does faxing, but I'd probably never want to send a fax from my d-box!).
When the d-box was introduced, DF-1 subsidized its price in Germany in exactly the same way that DirecTV and Echostar subsidize the prices of MPEG-2 receivers here in North America. People in Holland found out that if you took a standard d-box and added a Multichoice card (Multichoice is a Dutch pay TV service), it would work fine. The result was that suddenly a lot of German d-boxes were showing up in Holland which was costing DF-1 plenty of money as they never received a subscription to the DF-1 pay TV service.
When the situation got too much for DF-1 they upgraded the firmware of all d-boxes to stop them from receiving Multichoice, which obviously left a lot of people in Holland very unhappy. Because the Nokia box is based on the Motorola 68340 processor and it has a debugging tool built directly into the chip (and a connector on the Nokia motherboard for the debug tool), some enterprising engineers in Holland figured out how to extract the old firmware from d-boxes that worked with Multichoice and re-programmed it (for a fee of course) into the d-boxes that had been in-effect switched off by DF-1. They also patched the code to prevent any future updates from the satellite, while preserving its ability to be upgraded via the serial port.
This receiver is called a "dreambox" from C&J Electronics. It receives all IRDETO scrambled signals (with the correct smart card of course) and also all FTA SCPC and MCPC signals without or without a smart card. Another modified Nokia receiver receiver is available from Bentley Walker and is based on firmware from Bakker Electronics, also in Holland. Since I don't have one of these types of receivers, I can't comment on the firmware, however, I've heard favorable reports about its operation. Their technical support, however, is somewhat lacking in comparison with C&J Electronics though. Links for all of these companies are at the bottom of the page.
Nokia Official FTA receivers
Nokia also realized the need for a pan-European (meaning usable all over Europe) FTA MPEG-2 receiver and introduced their 9200S model. This is exactly the same as the 9500S platform that's used by DF-1 (and Telepiu in Italy) except that it is missing the CAM connector and the modem. After all, neither of these would be needed for a FTA receiver.
Nokia later revised the 9200S model and introduced the 9600S which in theory works with both FTA and encrypted MPEG-2 signals. It uses the new Common Access type of CAMs that include the card-reader built directly into the CAM (CAMs are the same size as a type-3 PCMCIA card). The 9600S also included much improved software that does a much better job finding and storing signals than the 9200S.
DVB98 is replacement code for the Nokia Mediamaster series, developed independantly from Nokia by a satellite enthusiast. It's probably the most versitle operating system for an MPEG-2 receiver for both sky scanners and people that just want to watch the TV.
DVB98 can do some wonderful things when used in conjunction with DVBEdit software for Microsoft Windows, such as entire satellite scanning, channel editing and high-speed data transfer via the SCSI bus. See the receivers section for more details and links related to this wonderful piece of code.
Before you buy an MPEG-2 receiver
If you decide to invest in an MPEG-2 receiver, keep in mind that this is very new here in North America from a hobbyist point of view. Many MPEG-2 receivers have problems with some signals, yet work fine on others or require extra equipment like a PAL TV or a supply of 220v.
That said, there are quite a lot of signals in MPEG-2 here in North America. Lyngsat has a quite extensive list. Unless you have a spectrum analyzer, searching for new signals, is much harder than analog however. The only receiver that can easily find new signals is the Nokia receiver. However, this involves knowing the transponder frequencies for the satellite in question (especially important on Ku-band where most of the MPEG-2 video is) and going through each transponder and telling the d-box to search each potential signal for MPEG-2, which typically takes about three minutes per transponder. This process can be automated with DVB98 firmware and DVBEdit's scanner function, however, this process is still very slow taking roughly 3 hours to scan for both SCPC and MCPC signals in a 500MHz range.
In short, if you want to sit back and watch a bunch of channels easily, don't get an MPEG-2 receiver at the moment. MPEG-2 receivers are expensive and not generally easy to use, but the situation is improving.
When buying an MPEG-2 receiver from Europe, there are a number of terms used there that need to be understood in order to use a receiver designed for that market.
Ku-Band is King
In Europe, the most common way of transmitting feeds and video programming is via Ku-Band and not C-Band as it is here in North America. The reason for this simple - a) Ku-Band dishes are smaller and therefore easier to install b) the geographical distances there are much smaller than in North America c) if a signal is targeted towards say the Balkan states (Bulgaria, Romania, Albania etc.), a beam can be used for these services because generally no-one outside of these areas will want to receive the signals. The beam results in a stronger signal on the ground, which improves signal quality and therefore can also reduce the size of the receiving dish.
C-Band is used in Europe, however, it's typically used for Arabic feeds (again a large geographical area) and for hemispheric feeds (for example, the Deutche Welle feed on Intelsat at 1 west that covers all of Europe and Africa).
Because Ku-band is so popular in Europe, most people use an offset-style dish with a combined LNB and feedhorn (an LNBF). The LNBF uses variation of the supply voltage to switch between horizontal and vertical polarity (14v = vertical, 18v = horizontal). In the US, Echostar and DSS use the same technique to switch between left-hand and right-hand circular polarization.
In North America, Ku-Band is split into two bands. The FSS band covers 11.7-12.2 GHz and is used for some DBS services (ex-AlphaStar and Primestar), but also for video feeds between TV stations and data services. The DBS band is designed only for direct to home applications and uses 12.2-12.7GHz.
When DBS started in the Europe, the initial band was 11.2-11.7 GHz, however, this has now been expanded to cover from 10.7 to 12.7 GHz, all for direct to home services.
Intermediate Frequencies and LOs
Initial satellite receivers available in Europe received in the range 950-1450MHz. This meant that the LNB contained a local oscillator frequency of 10.25 GHz (10.25 GHz + 950 MHz = 11.2GHz). When the band was extended down to 10.7GHz, this meant that the receivers had to change in order to receive all the programming. This meant that the IF was extended with the range 950-2100MHz with an LO frequency of 9.75 GHz.
Next, along comes digital TV, which occupies the 11.7 to 12.7 GHz band. Trying build a tuner and LNB that handles the entire range of 10.7 to 12.7 is impossible, so therefore the LNB contains two LOs - one at 9.75GHz and the other at 10.6GHz. This type of LNB is called a Universal and it is switched between the two LO frequencies by the receiver modulating a 22KHz tone onto the power supply for the LNB.
Diseqc and Hot Bird
In Europe, the major orbital location for DBS services (both analog and digital) has always been the Astra slot at 19.2 east. The Astra fleet currently comprises six co-located satellites with more planned. All the satellites are owned by Astra, based in Luxembourg, who then lease the transponder time to programmers.
The other major European satellite consortium is Eutelsat (based in Paris) and they recently decided to also get into the direct to home market, much like Astra has. They currently have three high power satellites (called Hotbird) co-located at 13 east with more on the way.
Because of only a difference of 6.2 degrees between the two satellites, many Europeans that want to receive from both satellites use two LNBs pointed at the same dish with an adaptor to point each at the correct satellite. Obviously, switching between the two LNBs requires either a manual switch or something a bit more high-tech.
Diseqc (DIgital Satellite EQuipment Control) fills this gap by modulating digital commands onto the 22KHz signal that is used to switch between bands. With a Diseqc compatible receiver (like the d-box with its latest version of software), it is possible to have the receiver send a command to a switching device mounted at the dish to switch between the Astra and Hotbird LNBs without the effort of running an extra cable. With the correct external 12v switchbox and a 4 to 1 Diseqc switch box, it's possible to connect up to eight LNB inputs to many modern receivers. In the future, Diseqc will offer bi-directional communications between the receiver and equipment at the dish for features such as dish motorization and switching into circular modes.
The Astra 1D Frequency Extender (ADX)
When Astra 1D launched, it had sixteen transponders that were below the regular direct to home band, i.e. in the 10.2-10.7 GHz range. Because most receivers that were already in use couldn't tune the band, the ADX was invented. It shifts the IF frequency up or down by 500MHz.
In North America, most people use Ku-Band LNBs with a local oscillator frequency of 10.750GHz, which results in the tuning of 11.7-12.2 GHz with an IF frequency of 950-1450MHz. A few satellites (Intelsat K for example) have Ku-Band transponders below the normal North American range, so by using an ADX, the transponders below 11.2 GHz can be received by shifting the 11.2-11.7 GHz band up by 500MHz.
Typically, the maximum extra range that can be reached with a regular LNB is about 150MHz. You can tune down to about 11.55GHz, but that's enough for the transponders on Intelsat K, which include a few SCPC MPEG-2 signals.
Another North American use for ADXes is if you use a wide band LNB for Ku-Band that has a standard LO of 10.75GHz, but an output range of 950-2100 MHz. Here, you use an ADX to shift down the 11.7 to 12.2 GHz band by 500MHz to make it match the 950-1450MHz IF of most North American receivers.
The ADX does cause a couple of band edge spurious signals at the bottom of the band, but generally works very well. I've heard of people using it with a wide band Ku-band LNB on a big dish and getting a signal lock on Echostar 1/2 by shifting down the DBS band at 119 degrees. It's rather weak because of the mismatch of circular versus linear polarization though.
What can be received with Echostar and AlphaStar receivers
Not too much really. Both receivers are package receivers and therefore have fixed SR and FEC values. However, if you peruse Lyngesat, you'll most probably find something that matches.
Info about the Echostar Receiver
The Echostar receiver uses SR 20.000 with automatic FEC. Because it was designed to operate in the DBS band (12.2 to 12.7 GHz), it uses a local oscillator frequency of 11.25 GHz in the LNBF. Remember that the Echostar DBS satellites use circular polarity and uses the 14v and 18v technique to switch between right and left hand.
There are a few signals (other than Echostar's own) that will work with Echostar receivers.
One is the Microspace package on GE-1. Hook the receiver up to an LNB pointing at GE-1 Ku and tune to transponder 16. If you're using an LNB with a feedhorn, set for horizontal polarity. If you're using an LNBF, physically rotate the feedhorn by 90 degrees. You'll get a lock and the program guide will show lots of channels. Sad to say, there are all scrambled.
Echostar receivers will also lock onto ExpressVu on Nimiq Ku-Band. This isn't surprising as ExpressVu buys their receivers and LNB's from Echostar - the hardware is identical. Unfortunately, ExpressVu is scrambled, with the exception of the 30 "Galaxie" music channels. Unfortunately, you won't be able to see the music channels on the channel map unless you subscribe (or use an FTA MPEG2 receiver).
The third option is the SkyVista programming package on Telstar 5. The SkyVista programming package is a joint venture by Loral Skynet and Echostar, using EchoStar hardware. SkyVista requires the use of a KU Band LNB rather than the circular polarity FSS LNB's used by ExpressVu and EchoStar.. Like Microspace and ExpressVu, SkyVista is scrambled, with the exception of a few arabic channels.
Info about the AlphaStar Receiver
This receiver was initially made by Tee-Comm for the AlphaStar DBS service which used a symbol rate of 23.000 and FEC 2/3. Software upgrades since the demise of AlphaStar have made it work with a package uplinked by Spacecom Systems on T5 using the same SR/FEC, but this receiver is also being used for a Chineese package on T5 with SR 20.000 FEC 3/4, so obviously someone knows how to change the SR/FEC on this box by changing the firmware.
It was also used very briefly in Europe where a Dutch distributor re-wrote the firmware to use variable SR/FEC, along with making the menus in Dutch. Since the Tee-Comm 1000 uses the Nokia tuner, it can actually handle SR from 1-45MS/s. It was never sold though, since Multichoice (now Canal+) wouldn't license the CA to the distributor, which does seem rather odd, since AlphaStar used the same IRDETO CA as Multichoice.
Conditional Access - The Key to Private and Pay-TV Systems
Conditional Access (CA) is used to prevent unauthorized access to either private or pay-TV systems.
Note: As you will have read at the start of this document, this site does not provide any information relating to compromising scrambled signals. We do explain how CA systems work in general, but don't expect to find hacking information here.
How is Conditional Access on DVB-systems performed?
In the DVB specification, there are only a few of the stream types that must be transmitted without scrambling. Obviously, these only include some of the Systems Information streams such as the Program Association Tables (points to more info about each channel) and the Network Information Table (points to the other transponders used by the service). These streams must be transmitted without scrambling so that any DVB compliant receiver can at least tell "what's there". However, everything else (including the program guide streams in the EIT) can be scrambled.
Scrambling of the appropriate streams is performed at the uplink site. The MPEG-2 packets are encrypted by the usual techniques, based on a common key known to both the scrambling and decryption devices. The actual scrambling technique, i.e. how the bits are rearranged to make them nonsensical, is obviously kept a secret, as are the keys contained in both the scrambling computer and the decryption device (typically, a smartcard in DBS applications).
When a scrambled packet arrives, before it passed through to the demultiplexor, it's first sent through the CAM or Conditional Access Module. The CAM is the descrambling engine and can be either built directly into the receiver or inserted into the receiver via a PC Card (aka PCMCIA) connector. At the start of each MPEG-2 packet is a 2-bit field called the TSF or Transport Scrambling Flags - if zero or one, the packet is passed through the CAM onto the demux for display since this value indicates an un-scrambled stream. If the TSF is set to either two or three, then the packet is passed through to the CAM, which takes the key obtained from the smartcard and uses it to turn the packet packet back into an MPEG-2/DVB transport packet which can then be processed by the rest of the system.
Many people think the smartcard and CAM are the same thing. These are two different entities - the DVB transport cannot be sent through the serial card - the serial interface it way too slow! Instead, based on the card receiving authorization from the service provider (using the DVB EMM and ECM tables), the card will emit the keys required by the CAM which are in turn used to descrambled the program stream. Most smartcard's serial interfaces operate in the 9,600 to 38,400 bps range.
Obviously, the key used to scramble the channel changes over time. If you look at the serial communications between the CAM and the smart card with an oscilliscope, you will see a burst of data every few seconds. This is the CAM asking the smart card for the next set of decryption keys for the next few second's worth of video. This also explains why, on most systems, if you pull out the smart card, you'll often see a second or two worth of programming before the picture blanks out.
What are the different types of CA being used today?
The CA market is big business stuff and so the market is very competitive. There are a number of different companies proving DVB compatible CA systems, so when a DBS provider starts a service, they have many options to choose from. There is always the cost versus security issue, license fees and so on. The following table shows which scrambling systems are being used around the world. If you have information to add to this chart, please let me know via email to email@example.com.
|Scientific Atlanta (US)||PowerVu CA||Many feeds and a few private TV services|
|Kudelski (CH)||Nagravision||Dish Network (US), ExpressVu (CN), Microspace (US), Via Digital (ES)|
|Irdeto Access (SA)||IRDETO||Many European Systems, ABS Filipino (US), CCTV (CN), Multichoice (SA)|
|Canal+ (FR)||Seca, Mediaguard||D+ (IT), CanelSatellite (FR), On Digital (UK Terrestrial)|
|France Telecom (FR)||Via Access||Television Par Satellite (FR), U-Best (US)|
|Philips (NL)||Cryptoworks||Viacom (US), MEASAT|
DIRECTV* (US), DIRECTV Latin America *(Various), Sky Latin America
(Various), Sky Digital (UK), Star-TV (East Asia)
Note: systems marked with * use the DSS system and are not DVB complaint, but are listed here for completness.
|Motorola (US)||Mediacypher||Embedded system for set top boxes.|
|Telenor (NO)||Conax||Canal Digital (1 West) for Scandinavian countries.|
Why is some signal scrambled and yet another isn't?
Mostly to protect distribution rights. For example, RTPi and Deutche Welle on PAS5 (58 west) are unscrambled since both of these are public information channels transmitted for worldwide distribution and for use by pretty much anyone without fee, much like in the way that NASA TV is a free channel that anyone can redistribute without cost nor license.
Dish Network transmits their "Dish Information" barker channel without scrambling - that way, even if a card swap has occurred due to a hack on their security system, when an out of the box receiver powers up for the first time, it gets the barker channel, therefore verifying the correct operation of the receiver.
However, Dish Network also transmits many other interesting streams without scrambling, such as their audio channels. Despite the lack of scrambling, they do actually charge for these channels since the Nagravision security system has the ability to "hide" channels from the user if they are not subscribed. With an "official" receiver, the channels can't be received without a subscription and it obviously saves a lot of hardware for scrambling these streams that really are "low value". In effect, this is "Poor Man's" scrambling.
How can I subscribe to a certain signal?
Generally, unless it's a signal from an established Pay-TV provider, you can't. Many signals are encrypted for private or exclusive cable distribution and as a result cannot be subscribed to even if you have the right hardware. Many feeds in Canada use the Scientific Atlanta PowerVu scrambling system and even with the right receiver and an address in Canada, you still can't subscribe to many of the channels which are available on cable.
DCII - The "other" MPEG-2 satellite standard
Well, OK, there's DSS in North and South America along with ISDB in Japan, but one of the major video distribution methods in North America is Motorola's Digicipher II standard.
Digicipher is Motorola's proprietary video distribution system. The first version was a totally non-standard system called Digicipher I and was one of the first digital video compression systems available in the market. The largest Digicipher I user was the Primestar direct-to-home system which closed down in 2000 after the company was purchased by DirecTV. There are two or three other DCI multiplexes available in North America - mostly feeds for South America that are receivable on global beams.
Digicipher II is Morotola's MPEG-2 based distribution system. It's used by about 70% of "cable" channels in North America to distribute their video to cable headends, other satellite companies like DirecTV and Dish Network and also available to backyard dish owners via Motorola's 4DTV satellite receiver products. DCII is also used by Canada's StarChoice direct-to-home service.
DCII includes an uncompromized encryption and authorization system.
What's different between DVB and DCII?
Both systems are based on the MPEG-2 standard. Both use the MPEG-2 transport stream format, so both have a Program Association Table (PAT) and Program Map Tables (PMTs) along with elementary data multiplexed onto various PIDs.
The video format used by DCII can be just as varied as DVB - 720x480, 480x480, 576x480 and so on. DCII also supports 4:2:2 video and HDTV like DVB.
Moving onto audio, DVB's primary audio format is MPEG-1 Level 2, also called Musicam. DVB optionally supports the AC3 standard from Dolby (otherwise known as Dolby Digital) but DCII requires AC3 for all audio streams. This doesn't mean that all DCII channels are using 5.1 surround - most channels transmit "2.0" format encoded with analog Dolby Suround.
Where DVB and DCII are really different is in how the channel definitions get into the receiver. DCII was designed before the DVB standard was ratified and General Instrument (now a part of Motorola) designed their own scheme without any interfacing to the work being done by the DVB team was doing and as a result, we have two totally different standards to deal with. This part of a digital TV system is called the SI or System Information.
What SI differences are there between DCII and DVB?
During this discussion, keep in mind the two major places where DCII and DVB are used: DCII in the consumer realm where a receiver is hooked up to a motorized dish and DVB in the Dish Network model where an electronic switch is used to switch between stationary dishes pointed at differental orbital locations.
So, the fresh receiver gets turned on and due to factory programming and correct installation, is able to receive a "homing" channel. The homing channel contains all the info needed by the receiver to tune any one of the channels that the receiver can receive.
In the case of DVB, this means a Network Information Table (NIT - used to tell the receiver about other transport streams - contains frequency, symbol rate, orbital location etc), the Service Description Table (SDT - used to tell the receiver the names and types of programming available on each channel) along with the Time and Date Table (TDT - tells the receiver about programming events on the channels - used to build the electronic program guide).
DCII is a little more complicated. First, there's a thing called the Satellite Definition Table (SDT - tells the receiver about the name and orbital locations of satellites), next there's the Modulation Mode Table (MMT - lists the different symbol rate, FEC and modulation types used by DCII) and then the Carrier Definition Table (CDT - this just lists the L-Band frequencies required to tune a channel) followed by the Transponder Defintion Table (TDT - lists the polarity and CDT table index). Finally, the Virtual Channel Table (VCT) is the thing that ties everything together. This lists each channel on the entire DCII system across multiple satellites and contains an index the appropriate TDT, SDT, MMT and CDT tables so that the receiver can select the correct channel analog or digital. If the channel is digital, the VCT also contains the MPEG-2 Program Number (although it's called Service Number in Motorola parlence) so the receiver can process the MPEG-2 PAT and PMT to acutally display the program. Whew!
A simpler way to look at the differences is that DVB uses a flat list (much like a comma seperated file with lots of repetition) whereas DCII uses a very relational table structure (much like a SQL database over satellite).
What are these DCII modulation types?
DVB is pretty straight forward - it's pretty much always QPSK modulation at variable symbol rates and FEC coding. There have been enhancements to the DVB standard to support BPSK, 8PSK and 16QAM modulation modes. BPSK is like QPSK but only transports one bit per time period and is therefore more robust on degraded links. 8PSK and 16QAM increase the data rate by using more than two bits per time period but require a much more robust link than QPSK.
DCII has a number of different modes. First, there's regular-QPSK and Offset-QPSK. Offset-QPSK is quite similar to normal QPSK but one of the bits in the symbol is delayed by one bit period and phase changes are limited to 90 degrees, making performance in a non-linear environment much better than QPSK. All DCII signals with a symbol rate less than 19.51 MSps use OQPSK - anything above uses QPSK. In the DCII system, the encoders and receivers have pre-programmed symbol rates. For example, the 3.25 MSps rate supports one 576x480 video stream and one audio stream; the 4.88 MSps rate supports two 352x480 video streams and associated audio streams and so on.
There's one other variable for a DCII signal - it's mux mode. As you probably know from reading this document, the higher the symbol rate, the higher the actual bitrate that can be carried across the channel but this also depends on the FEC coding rate - the less fraction of the datarate used for error correction, the higher still the bitrate, so a 19.51 MSps 3/4 stream carries less data per second than a 19.51 MSps 7/8 stream.
In regular QPSK, the data is recovered from both the I and
Q phases that make up a QPSK symbol, i.e. each bit on the I and
Q phases ends up as a serial bit in the transport stream. Because
of limitations in either the QPSK demodulator or the transport
stream demultiplexor, most DCII streams with a rate above 19.51
MSps 4/5 FEC coding operate in "split" mode. In split
mode, the I phase contains a transport stream and the Q phase
contains another different transport stream. It's not really true
QPSK but more like dual-BPSK. The Mode Modulation Table sent as
part of the DCII SI contains an indication as to whether the transmission
is using QPSK or OQPSK and whether it's the regular "combo"
mode or split mode.
Thanks to Mark Hemstad for info about OQPSK.
There is one exception to this rule. Some DCII signals use a symbol rate of 29.27 MSps and yet operate in combo mode. One assumes that Motorola was able to correct the limitations that caused them to invent split mode, however, the bad news is that the only receiver that can do this high-speed mode (Motorola calls it MegaStream) is the DSR-4800 which is an expensive commercial receiver. It's worth pointing out that CBS uses MegaStream for it's HDTV network feeds which explains why they can't be received by consumer DCII receivers and the 4DTV HD decoder.
What's the "channel map" issue with DCII?
Remember when you first turn on a new DCII consumer receiver, it needs to be tuned to a homing transponder so it can download all the appropriate tables that allow it to tune channels. Because 4DTV receivers don't allow manual control of the tuning and video parameters, the only way for a receiver to get told what channels are out there is for Motorola to include them in the channel maps they send on the homing transponders.
Motorola used to be quite friendly towards the 4DTV receiver owner and included many channels such as PBS's PBS You and PBS Kids as part of the 4DTV channel maps. However, despite their unscrambled transmission on satellite, PBS insisted that Motorola remove them from the maps being sent to 4DTV receivers and as a result, 4DTV receivers can only tune one of the many PBS feeds that are available and free-to-air.
The only known solution to this problem is to use a program called 4Play developed by the author. It works very well with the DSR-905 digital-only DCII receiver, but also supports the DSR-920 and DSR-922 receivers with some limitations. It can add about 50 or so unscrambled channels to the receiver's maps. You can read more about 4Play and download it for free from this link.
Why exactly can't DCII receivers receive DVB and vice-versa?
Although both DCII and DVB have exactly the same performance, their differences, especially in the area of transmission standard, prevent DCII receivers from receiving DVB and DVB receivers from receiving DCII. This table shows the differences between the two systems:
|Transmission Format||QPSK/dual BPSK||QPSK|
|Randomization||After interleaver using a truncated 2^16 bit polynominal sequence||Before Reed-Solomon coding using a truncated 2^13 polynominal sequence|
|Reed-Solomon Coding||DCII polynominal||DVB polynominal|
|Interleaving||Convolutional I=12, J=19||Convolutional I=12, J=17|
|Convolutional Encoding||Length 7 (64-state Viterbi trellis)||Length 7 (64-state Viterbi trellis)|
|Puncturing rate||1/3 underlying||1/2 underlying|
|QPSK shape||Butterworth pulse shaping||Square-root-raised-cosine pulse shaping|
|Multiplex||MPEG-2 standard||MPEG-2 standard|
|System Information||DCII (similar to ATSC with extensions)||DVB|
|Audio||Dolby Digital (AC3)||Musicam (MPEG-1 layer 2) or AC3|
|Video||MPEG-2 standard 4:2:0 (MP@ML)||MPEG-2 standard 4:2:0 (MP@ML)|
What's the deal with the way some pictures are initially displayed on DCII receivers?
MPEG video is comprised of compressed video frames, much like a JPEG file. However, these complete pictures (called I-pictures) are sent infrequently. MPEG's ability to compress video so well relies on the similarity between pictures over time, so most of the time, MPEG video is sending the difference between prior and upcoming pictures using B and P-pictures. Obviously, these pictures take a lot less space than I-pictures.
The I, B and P-pictures are sent in a specifc order called a Group Of Pictures or GOP. Typically with 60Hz video, there are two GOPs per second with 15 pictures in each GOP using IBBPBBPBBPBBPBB ordering. When a DVB receiver tunes an MPEG video stream for the first time, it will typically wait until an I-picture comes along before displaying video, so when you tune to the channel after a short pause, you get a complete picture.
Some DCII channels use long GOPs - perhaps one I-picture every two seconds. As a result, when you first tune some DCII channels, you'll see the picture being built as a mosaic from the B and P-pictures and eventually, the entire picture is built when the next I-picture arrives.
DSS - the "other other" MPEG-2 satellite standard used in North Amercia
Before the MPEG-2 and DVB standards were ratified, the first all-digital DBS service was launched in the USA - DIRECTV. DIRECTV uses a unique system called DSS, invented in part by Thomson of France. Systems like DCII and DVB both model themselves on the MPEG-2 transport stream model, but DSS really is very different.
Today, the DSS system is used for DIRECTV's DBS service in the USA (broadcast from 101, 110 and 119 degrees west), the DIRECTV Latin America service (broadcast on Galaxy 8i at 95 degrees west) and for some DirecPC Internet over satellite channels. DSS supports AC3 audio as an option (most channels use MPEG-1 Level 2) and also supports HDTV.
What details are known about DSS?
A slight warning - the DSS "standard" is proprietary belonging to DIRECTV. Unlike DVB, documentation on the standard is not available - this information has been put together by talking to people in the industry. If you find errors in this section or have information to add, please contact us.
DSS uses QPSK modulation for the satellite downlink, just like DVB, however, it's packets are 127 bytes in length and not 188 bytes. The MPEG-2 specification chose 188 byte packets for compatibility with ATM networks. We don't know the reasoning for DSS using 127 byte packets.
DSS uses the same type of error correction and detection algorithms used by DVB (Viterbi and Reed-Solomon), but again there are differences - DSS uses a 6/7 FEC coding for many of it's high-power transponders and has a different puncture rate and randomizer. These differences along with the transport stream packet length make DSS and DVB receivers totally incompatible at this time.
How does DSS handle things like the program guide, network tables etc?
The DSS system uses a thing called the Master Program Guide (MPG). This stream has all of the information found in the EIT, SDT, NIT, etc. in the DVB system. It tells the IRD what satellite transponders are available and what code rates they have (like DVB's NIT), it tells the IRD which virtual channels exist ("viewer channels") and which PIDs and PID types comprise each channel (rather like the DVB SDT).
The single MPG gives all of the PID and transponder assignments for all channels and all transponders, and so the MPG is a "one-stop shop" vs. the DVB approach of many smaller tables. The MPG also has two hours worth of programs and program titles, and then "points" to extension guides for data out to two days in the future. New US DIRECTV IRDs also have a feature called "Advanced Program Guide", which works a little more like the DVB EIT and allows program info out to weeks in the future.
What are the technical differences between DSS and DVB transmissions?
|Viterbi Code Rates||1/2, 2/3, 6/7||1/2, 2/3, 3/4, 5/6, 7/8|
|Multiplex||Proprietary - MPEG-2 like (has PIDs)||MPEG-2 standard|
|Audio||Musicam (MPEG-1 layer 2) or AC3||Musicam (MPEG-1 layer 2) or AC3|
|Video||MPEG-2 standard 4:2:0 (MP@ML)||MPEG-2 standard 4:2:0 (MP@ML)|
MPEG-2 Related Links
Real World Info
The DVB Project
homepage - the people responsible for the world standard for digital
ATSC's homepage - the North American terrestrial MPEG-2 standard
Hobbiest Information / Sites
Satellite Chart is one of the best sources on the Internet
for MPEG-2 channel listings
Chris Muriel's MPEG-2 FAQ is very good source of information about MPEG-2 and DVB
SatForums has a forum dedicated to MPEG-2/DVB in North America. Registation is required but the forums are free. Highly recommended.
John Rainer has written an excellent article about upgrading Nokia 9600 receivers to run DVB98.
Mike Frisch's DVBWave.com is a great source of info and DVB FAQs.
DVBResource.com has some useful info.
DVB Receiver Vendors
supplies many different digital receivers and accesories, especially
for Telstar 5 and Ku-band reception
Bentley Walker is another supplier of modified d-boxes in the UK
Global Communications can provide MPEG-2 receivers in the United States
Telsat Communications in New Zealand can supply Nokia, Scientific Atlanta, Hyundai receivers and more
Gavilan Communications is probably the only place where you can get a Nokia receiver in North America.
Hisat in the UK will ship Nokia receivers to North America and has pretty good prices on the 9600
BNW distributes the Sat Cruiser DSR-101 receiver out of New York state
Satworld in Victoria, Australia says they always have Nokia receivers available
Sadoun Satellite Sales in Ohio has a wide range of DVB receivers and accesories
DVB Receiver Manufacturers
in Scotland makes the ODM300 FTA receiver
Samsonic has an interesting looking DVB receiver and a wide selection of LNBs
Fortec Star from Canada makes a wide range of DVB satellite receivers
Echostar DVB receivers are very popular in Europe. Not much use for them in North America though - especially since they can't be subscribed to Dish Network
Dream multimedia in Germany make the Dreambox series of Linux based satellite receivers. These receivers have an open API so you can add your own softare!
DVB Technical Equipment Manufacturers
makes some pretty fancy looking DVB stream analyzers.
Agilent (previously a division of HP) makes DVB and ATSC verification and test equipment
Computer Modules makes a set of PCI-bus DVB-ASI cards (DVB Master) along with Windows drivers that have some very interesting applications
publish the highly recommended Digital Video: An Introduction
to MPEG-2 book that you'll want to read if you really want
to know how MPEG-2 works.
Halhed Enterprises has a very interesting article about the fomat used on the DSS system.
Techtronix has a really informative guide to MPEG fundementals.
NTV in St. John's, Newfoundland can get you a PowerVu receiver with a free subscription to NTV!
DiSEqC 1.2 Rotor Manufacturers
Stab in Italy makes the Rotor Sat series of
Thanks to Alan Terry for the link.
If you spot any incorrect information here or would like to contribute to the page, please drop me a line at firstname.lastname@example.org.