Triple-Play Service Deployment

Chapter 6: Troubleshooting Video in the Headend
any addition or deletion of some element of the MPEG-2 transport
stream. Even dropped packets would create some errors in the
PCR spacing. Once the decoder has locked to a PCR, it expects to
see each subsequent PCR at regular intervals. The delta between
the expected and actual arrival can be significantly impacted by
the network, and this jitter can cause many problems for a
decoder. Likewise, inaccuracies in the PCR value itself will cause
similar effects as jitter. The PCR is the most critical element for a
high-quality presentation at the set top box. Many picture quality
issues are related to some aspect of PCR performance, including
basic errors such as pixelization as well as more complicated ones
such as chroma/color problems.
In addition to these errors, several composition and timing/spacing
errors can occur, as can application-specific errors. Because packet
spacing and order, as well as rate and bandwidth, are so critical in
MPEG, any unrecognized PID in the stream can cause trouble, as
can PIDs that are present but with negligible bit rate. Likewise, the
PSI table data must be accurate and it must also be systematically
present in the transport so that any receiver can access it almost
immediately after tuning to the MPEG transport stream.
MPEG Testing Guidelines
All of these factors combine to yield a cumulative effect—MPEG
transport testing is an inescapably complex and broad task.
Technicians must interpret and confirm protocols, identify packet
flows and track the rates for each of them, decode header
information, parse tables, read descriptors, check references and
cross-references, replicate a system time clock and make countless
iterative calculations. And, this covers testing the basics only.There
are literally hundreds of errors or events that could transpire in just
a few minutes, and each has a temporal relevance. When they
happen and how long they persist is important to know when
trying to troubleshoot a problem.
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