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Reference
SDA 601 Serial Digital Analyzer User Manual
39
coded signals are not available. This check requires a fixed (un-
changing) video signal.
The SDA 601 calculates the Full Field CRC (FFCRC) that is used for
EDH error checking. Then, for this check, the SDA 601 compares
the calculated CRC in each field to the CRC calculated for the same
field in the previous frame. If the incoming video signal is fixed,
then the calculated CRCs should be identical from frame to frame. If
the CRCs are not identical, either the transmitted signal has been
deliberately changed, or a transmission error has altered the signal.
This check has some limitations. As noted, the signal must be pre-
sumed to be fixed, and the equipment being checked must therefore
be temporarily removed from service. Also, embedded audio ancil-
lary data is by its nature changing from field to field; therefore, this
check will be useless if audio is embedded in the signal. In the case
of embedded audio, APCRC Changed is a more appropriate check
method.
APCRC Changed — Is also an out-of-service check, very similar to
the FFCRC Changed checking method, except the checking is lim-
ited to the active picture data only. This allows the check to be used
with signals that have constantly changing ancillary data, as do sig-
nals that contain embedded audio.
With this checking method, data in the vertical and horizontal re-
gions is completely unchecked.