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38
SDA 601 Serial Digital Analyzer User Manual
Using the CRC Change Watch Group
The three items in the CRC change Watch group—ØAPCRC,
FFCRC Changed, and APCRC Changed—are designed to facilitate
the testing of equipment or signals that don’t have EDH encoding.
ØAPCRC — If you are testing equipment that won’t pass the verti-
cal interval, but will pass the entire active picture area, then
ØAPCRC is an excellent test method. Vision mixers (production
switchers) and some VTRs or still stores are typical examples of
equipment that can benefit from this test method.
ØAPCRC is an encoding process in which the last five video words
in the last full line of active video are altered so that the CRC of the
active picture words for the entire field will be exactly zero (Ø).
The SDA 601 calculates the same Active Picture CRC (APCRC) that
is used for EDH error checking. The SDA 601 can recognize that this
CRC is consistently Ø for each field, and will then assume that any
occurrence of a non-zero CRC is the result of a signal propagation
error. This check is as reliable as the APCRC check in the EDH
checking method.
Because ØAPCRC encoding does modify the active video signal, the
method may not be acceptable in all operational cases. As a practical
consideration. however, the encoding results in a small bright spot in
the very bottom right corner of the active picture, where it is not nor-
mally visible on overscanning monitors or receivers. Furthermore,
the encoding is designed to not introduce any illegal data values that
might provoke video signal processing errors in conventional opera-
tional video equipment.
Another advantage of this method is that it doesn’t require a fixed
test signal and may be applied to any legal digital component or
composite format signal.
This method of signal checking will not work if the equipment being
tested does not pass the entire active video line, or if a 10-bit signal
encoded with ØAPCRC is passed through an 8-bit transmission link.
Eight-bit signals which are encoded may be used to check either
8-bit equipment or the eight higher-order bits of 10-bit equipment,
and 10-bit signals which are encoded may be used to check 10-bit
equipment.
FFCRC Changed — Is an out-of-service check for either 8-bit or
10-bit equipment which will not pass EDH, or for which EDH en-