Satellites around the world have been receiving and transmitting video signals for decades, but most of these signals have not been used directly by consumers. Recently, many new satellites have been launched to provide DTH broadcast services. These satellites use new digital coding and compression schemes to provide crystal-clear television pictures with CD-like audio.
Many of these new satellites transmit using the Ku band, and consumers require only an 18-inch dish to receive the signal. A large number of these satellites already transmit compressed digital video signals, most often using the MPEG standard (ISO/IEC 13818).
The systems enabling consumers to receive a multitude of channels via satellite are quite convoluted. The original broadcast signal must pass through at least four stages before it reaches the viewer:
Encoding for compressing video and audio signals.
Multiplexing or remultiplexing for interleaving the compressed digital signals along with MPEG overhead tables and stuffing packets.
In-home process that demodulates the RF signal, demultiplexes the transport stream, decodes the MPEG signal and converts the digital signal to analog.
DTH providers usually multiplex an array of tape players and live signals to complete their service offering. See Figure 1 for an example of MPEG equipment used in the DTH process.
Definition of the System and MPEG-2
Before delving into the complexities of test processes, some definition of the equipment used throughout the system and the MPEG-2 standard is needed. The only thing lacking from the popular MPEG standard is a specification of a physical interface and transmission medium.
As a result, the DVB committee created several recommendations to fulfill these needs, including some for satellite transmission (ETS 300 421). Specifically, DVB-S describes the satellite coding procedures for transforming a 188-byte MPEG-2 packet into a 204-byte packet ready for QPSK modulation. QPSK modulation is a common scheme used by satellites to receive and send digital signals.
DVB-S coding includes Reed-Solomon and Viterbi FEC to provide a very robust digital signal. This Reed-Solomon coding is the same redundancy coding scheme used by CD-ROM makers to allow CD players to recover the data even in the presence of errors from flawed media. Similarly, television broadcasting will use FEC, redundancy coding, and digital compression to provide consumers with advanced technology, enabling them to receive more channels and clearer pictures at less cost.
DTH consumers now use an 18-inch satellite dish along with an IRD to receive up to 200 digital TV programs from one service provider. IRDs, also called TV Set Top Boxes because they usually are found on top of the TV set, come in different levels of capability and price, such as consumer, business, and professional.
Each IRD has an RF input directly from the dish. The IRD also has video and audio outputs. The consumer IRD allows TV program control through an infrared remote. The professional IRD usually allows complete access to the incoming Elementary Streams and the demodulated MPEG-2 output, and provides Serial Digital Video and Serial Digital Audio outputs. The digital video and audio outputs are useful in cable head-end applications where the head end mainly redistributes the video and audio.
The MPEG-2 (ISO/IEC 13818) standard, finalized in 1994, includes several sections on the system layer (Program and Transport Stream) as well as a definition of the Elementary Streams (video, audio, and data). The standard was written for MPEG-2 decoders. There is no specification for MPEG-2 encoders.
This approach allows encoder designers to implement their own design with the knowledge that the output must be properly decoded by any MPEG-2 decoder following the ISO/IEC 13818 standard. Consequently, IRDs deployed prior to 1994 might not be completely MPEG-2 compliant because of last-minute changes to the standard.
Although encoders are more complicated and much more expensive in comparison to decoders, testing an encoder for compliance to the MPEG-2 standard is somewhat simpler than verifying a decoder. MPEG-2 analyzers available today make testing as simple as acquiring the input and testing if the MPEG-2 encoder complies with ISO/IEC 13818.
Conversely, running a similar test on an MPEG-2 decoder does not mean the decoder complies with the standard. This only means that the decoder can decipher a specific MPEG-2 sequence.
Checking a decoder for compliance requires an enormous number of combinations and variations of MPEG-2 sequences. The compressed video rates can range widely up to 15 Mb/s, and the decoder must handle each rate.
The MPEG-2 standard offers scores of different parameters for providing highly compressed video. An encoder should not be expected to generate every combination of MPEG-2 parameters, although the standard does define the capability for anyone choosing to take advantage of it. Consequently, testing an MPEG-2 decoder is quite different from testing an MPEG-2 encoder.
Testing the design of a decoder is also quite different from production testing of decoders. Recently, INTELSAT completed its third round of open-invitation tests of MPEG-2 encoders and decoders. These tests were for interpretability and not for compliance verification, although the testing and results were very informative. These interoperability results can be seen on INTELSAT’s web page at: http://www.intelsat.int/.
Design testing is extremely important for several reasons, mainly because you never want to find a problem after thousands or millions of units have been manufactured. As a result, it is critical to thoroughly examine every aspect of the decoder.
A good method of testing isolates individual modules of an IRD. Four of the most important IRD modules are the RF demodulator, the demultiplexer, the decoder, and the D/A converter. Both the RF demodulator and D/A converter are important, but the demultiplexer and decoder contain the newest technology and are potentially the most difficult to integrate.
Figure 2 shows a typical setup where an MPEG generator stimulates a decoder module. These are four important areas that require in-depth exercising:
1) Test of the MPEG-1 and MPEG-2 video at different rates up to 15 Mb/s (specifically Sarnoff stress patterns).
2) Addition of MPEG-2 stuffing packets to vary the transport rate up to 30 to 60 Mb/s.
3) Addition of PCR jitter to stress clock recovery.
4) Providing fixed audio/video delay and measuring lip-sync offset.
MPEG-2 decoders are backward compatible and must decode MPEG-1 video too. Since the video rates used by DTH systems range up to 15 Mb/s, it is important to exercise as many as possible.
Since all video elementary streams are created by software or hardware encoders, it is difficult to limit to parameters within the stream. Video elementary streams, created by David Sarnoff Research Labs, are handcrafted to exercise one parameter at a time. Each stream (or file) stresses a single video parameter. Overall transport stream rates may vary up to 60 Mb/s. Therefore, many programs and stuffing packets should be added to a transport stream to verify the different rates.
Each MPEG-2 transport stream contains at least one program with PCRs. The legal limit for the PCR values is up to 500 ns of tolerance. PCR variations should be tested to find the amount of induced jitter required to cause the phase lock loop to produce unacceptable video.
An MPEG-2 decoder extracts the digital video and audio and then uses the PTS to accurately generate the continuous video and audio. Failure to keep the audio and video synchronized causes measurable lip-sync errors.
All of these transport stream patterns are critical for design verification but could not be tolerated in manufacturing because of the long test time. Quicker methods must be used to provide high-volume testing.
The goal of production testing is to check as much as possible in a very short time. Production volume of MPEG decoders can be so high that test time is limited to just a few seconds.
The most important decoder module parameters to check are:
Locking to L-Band RF signal.
Demodulating QPSK signal.
Demultiplexing one program from an MPEG transport stream.
Verifying analog performance of video and audio.
The tests can be performed quickly only if simple audio and video test patterns are used. It also may be important to have a subjective viewer to give final approval on motion or moving sequences.
Figure 3 shows a complete IRD under test. The MPEG-2 generators used in manufacturing are quite different from those used in design engineering. Where the MPEG-2 generator used in design test (Figure 2) requires the capability to create custom transport streams for exercising various rates, PCR modulation, and special video elements, the MPEG-2 generator used in manufacturing test (Figure 3) has a seamless PCR along with multiple programs in a single transport stream.
Testing in the manufacturing environment (Figure 3) includes two modulators. The first modulator has FEC and transforms the transport stream into a QPSK signal. The second modulator moves the QPSK signal into the L-band (950 to 2,150 MHz). The 11- to 12-GHz signal is not required since the satellite dish automatically performs demodulation down to the L-band.
The infrared remote control is available for changing from one program to another. Most transport streams contain five to seven programs.
More programs are obtained when many different satellite transponders are used. This technique provides up to 200 separate TV programs from one satellite. The analog video/audio analyzer performs measurements on the decoded streams. It is critical that the patterns in the generator be chosen to match the measurement capabilities of the analyzer. A color picture monitor is not required but could be very useful.
There are two different types of testing requirements for MPEG-2 IRDs. Testing of an IRD design requires an in-depth, methodical search of all signal paths used inside the IRD. The design test usually is a very long process covering many different data rates. The test equipment must create a near infinite number of these variables.
In contrast, the production test requirements call for a very quick check of a limited number of parameters. The test equipment for production testing must produce multiple programs within a transport stream using a seamless PCR. A seamless PCR means that its values are continuously increasing without discontinuities.
Different tests require different equipment. Each piece of test equipment is optimized to perform one or more special tasks. Always review your requirements before making test-equipment selections.
Publications from the International Organization for Standardization:
ISO/IEC 13818, Part 1, 2, and 3, Generic Coding of Moving Pictures and Associated Audio: Systems, Video, and Audio.
Publications from the European Telecommunications Standards Institute:
ETS 300 158, Satellite Earth Stations (SES) Television Receive-Only (TVRO-FSS), Satellite Earth Station operating in the 11/12-GHz FSS bands.
ETS 300 249, Satellite Earth Stations (SES) Television Receive-Only (TVRO) equipment used in Broadcasting Satellite Services (BSS).
ETS 300 421, Digital broadcasting systems for television, sound, and data services; Framing structure, channel coding and modulation for 11/12-GHz satellite services (DVB-S).
ETS 300 473, Digital broadcasting systems for television, sound, and data services; Specification for Service Information (SI) in Digital Video Broadcasting (DVB) systems
ETS 300 468, Digital broadcasting systems for television, sound, and data services; Specification for Service Information (SI) in Digital Video Broadcasting (DVB) systems.
Publications from Digital Video Broadcasting—Technical Module: Physical Interfaces:
DVB-PI-232, TM1449, Interfaces for CATV/SMATV Head Ends and Similar Professional Equipment.
About the Author
Dennis Kucera has been an applications engineer at Tektronix for 13 years. He is a graduate of the University of Oregon with degrees in computer science and mathematics. Mr. Kucera also is credited with the U.S. patent for automatic eye-diagram testing. Tektronix, Measurement Business Division, P.O. Box 500, Beaverton, OR 97077, (503) 627-4979.
Glossary of Terms
DTH—Direct To Home
DVB—Digital Video Broadcast
ETS—European Telecommunications Standard
FEC—Forward Error Correction
IEC—International Electrotechnical Committee
INTELSAT—International Telecommunications Satellite Organization
ISO—International Organization for Standardization
MPEG—Moving Pictures Experts Group
PCR—Program Clock Reference
PTS—Presentation Time Stamps
QPSK—Quaternary Phase Shift Keying
Copyright 1997 Nelson Publishing Inc.