Triple-Play Service Deployment

Chapter 6: Troubleshooting Video in the Headend
tables is a critical task performed in an MPEG-2 multiplexer. If the
PIDs are incorrectly identified in the tables, the program
components are lost to the set top box. Because the PIDs for each
program component in the stream must be unique, the PIDs must
be remapped anytime one or more programs are combined into a
new transport stream.
MPEG-2 Synchronization
The second half of the MPEG-2 transport process is the synchronization
of program elements. At the PES layer, the headers
contain the PTSs for the audio components and both PTS and
DTSs for the video components. These time stamps are created
against the encoders system time clock (STC), and have meaning
only if that STC exists at the decoder. This is accomplished by the
periodic stamping of a program clock reference (PCR) in the
MPEG-2 transport packets. The decoder will use the PCR to lock its
own 27 MHz clock to the encoder’s STC. Additionally, it will use a
phase lock loop to track the PCRs and keep its own STC in sync.
That 27 MHz clock is very sensitive to small inaccuracies, so the
jitter of PCR arrivals and the accuracy of the PCR are of paramount
importance to the set top box.
Testing MPEG-2 Transport
Digital video test is based heavily on MPEG-2 transport test as this
is the end-to-end medium. The audio and video content has been
created by an encoder in the compression stage, and whether that
was done in-house or by a content-provider, it is a stand-alone
stage and distinctly separate from transport. Good or bad, the
compression stage hands the content to transport, and from the
encoder throughout the network and right to the set top box, all
digital video programming is running in an MPEG-2 transport
stream. This is particularly true within the headend, where the
video services have not yet been exposed to the risks inherent to
Ethernet transport.
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