MPEG-2 basic training, part 2
Nov 6, 2011 3:13 PM, By Ned Soseman
Devices such as this can perform receive site analysis to the PID level with offline verification and inspection. Image courtesy IneoQuest.
Is MPEG compression your friend? Of course, the answer to this question is that MPEG compression is your friend, unless it’s not working properly. When that happens, it’s our job to make it friendly again. This “Transition to Digital” tutorial continues the discussion from the preceding mid-October “Transition to Digital” tutorial about monitoring and evaluating MPEG-2 streams.
Streams are made of packets with headers and are filled with metadata, compressed video or compressed audio. To reconstruct a program from a stream, all of its video, audio and table components, and the corresponding PID assignments, must be correct. Also, there must be consistency between PSI table contents and the associated video and audio streams. This is a good place to look for trouble in a suspicious MPEG-2 stream.
Program Specific Information
Program Specific Information (PSI) is part of the Transport Stream (TS). PSI is a set of tables needed to demultiplex and sort out PIDs that are tagged to programs. A Program Map Table (PMT) must be decoded to find the audio and video PIDs that identify the content of a particular program. Each program requires its own PMT with a unique PID value.
The master PSI table is the Program Association Table (PAT). If the PAT can’t be found and decoded in the transport stream, no programs can be found, decompressed or viewed.
PSI tables must be sent periodically and with a fast repetition rate so channel-surfers don’t feel that program selection takes too long. A critical aspect of MPEG testing is to check and verify the PSI tables for correct syntax and repetition rate.
Another PSI testing scenario is to determine the accuracy and consistency of PSI contents. As programs change or multiplexer provisioning is modified, errors may appear. One is an “Unreferenced PID,” where packets with a PID value are present in the TS that are not referred to in any table. Another would be a “Missing PID,” where no packets exist with the PID value referenced in the transport stream PSI table.
Good broadcast engineers never forget common sense. Just because there aren’t any unreferenced or missing PIDs doesn’t guarantee the viewer is necessarily receiving the correct program. There could be a mismatch of the audio content from one program being delivered with the video content from another.
Because MPEG-2 allows for multiple audio and video channels, a real-world “air check” is the most common-sense test to ensure that viewers are receiving the correct language and video. It’s possible to use a set-top box with a TV set to do the air check, but it’s preferable to use dedicated MPEG test gear that allows PSI table checks. It’s also handy if the test set includes a built-in decoder with picture and audio displays.
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