Triple-Play Service Deployment

170
Chapter 6: Troubleshooting Video in the Headend
Conflicts of Interpretation
Video is a very dynamic service, and it requires flexibility in the
application of any rules. This begins at the encoder, where
compression that works for one type of programming may be
completely unacceptable for another. It continues at the transport
level—the previously illustrated case of CBR vs. VBR is an example.
This creates an environment where two vendors may have a
conflict of interpretation, or implementation, of the MPEG
transport. To continue the CBR vs. VBR example, it is perfectly valid
for MPEG transport streams to use a Null PID – common in fact. It
is equally valid for the vendors to not have a Null PID, perhaps less
common but not “illegal.” If equipment vendor A outputs CBR
streams with a Null PID, and equipment vendor B expects to
receive streams without the Null PID, the operator encounters a
conflict of interpretation. Neither vendor is technically incorrect,
but the service cannot be delivered while the debate over
adjustments needed to resolve the issue takes place.
Middleware is an area susceptible to this risk, as it is tempting here
to tweak the MPEG transport or compression to enable valueadded
features. The tweaks may not be against the MPEG
standards, but they may create conflicts with other video
equipment, or even with the decode process in general.
Comparative analysis is the most useful tool for identifying the
issues at the heart of these conflicts of interpretation because
nothing is technically wrong with the video. When the technician
cannot test to determine what is wrong, comparing on either side
of the conflict to identify differences is the best alternative
approach.