MPEG-2 basic training
Oct 16, 2011 4:25 PM, By Ned Soseman
One of the advantages of digital video is that it can be compressed and transported in an MPEG stream across an IP network. MPEG compressed digital video requires new sets of test tools and troubleshooting skills using bit stream monitoring and testing to accurately identify problems, or preferably, recognize and identify potential problems before they occur.
The MPEG-2 standard is defined by ISO/IEC 13818 as "the generic coding of moving pictures and associated audio information." It combines lossy video compression and lossy audio data compression to fulfill bandwidth requirements. The foundation of all MPEG compression systems is asymmetric because the encoder is more sophisticated than the decoder.
MPEG encoders are always algorithmic. Some are also adaptive, using a feedback path. MPEG decoders are not adaptive and perform a fixed function. This works well for applications like broadcasting, where the number of expensive complex encoders is few and the number of simple inexpensive decoders is huge.
The MPEG standards provide little information about encoder process and operation. Rather, it specifically defines how a decoder interprets metadata in a bit stream. MPEG metadata tells the decoder what rate video was encoded at, and it defines the audio coding, channels and other vital stream information.
A decoder that successfully deciphers MPEG streams is called compliant. The genius of MPEG is that it allows different encoder designs to evolve simultaneously. Generic low-cost and proprietary high-performance encoders and encoding schemes all work because they are all designed to talk to compliant decoders.
Before SDI
Asychronous Serial Interface (ASI) is a serial interface signal where a start bit is sent before each byte, and a stop signal is sent after each byte. This type of start-stop communication without the use of synchronized fixed time intervals was patented in 1916 and the key technology making teletype machines possible. Today, an ASI signal is often the final product of MPEG video compression, ready for transmission to a transmitter, microwave or fiber. Unlike uncompressed SDI, an ASI signal can carry one or multiple compressed SD, HD or audio streams. ASI transmission speeds are variable and depend on the user's requirements.
There are two transmission formats used by the ASI interface, a 188-byte format and a 204-byte format. The 188-byte format is the more common. If Reed-Solomon error correction data is included, the packet can grow an extra 16 bytes to 204 bytes total.
What’s the purpose of a general-purpose oscilloscope (GPO) in troubleshooting MPEG? Not much. Specialized technology demands specialized test gear. MPEG steams are complicated, and MPEG-2 streams are more so. Examining MPEG-2 streams is reminiscent of measuring the front porch or counting the number of sync pulse serrations to manually validate an analog video sync pulse. Well, kind of, anyway.
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