DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Some video switchers incorporate
digital video effects (DVE)
capability; a DVE unit necessarily
includes a framestore.
When a studio needs to accommodate an asynchronous
video input – one whose frame rate is within
tolerance, but whose phase cannot be referenced to
house sync, such as a satellite feed – then a framestore
synchronizer is used. This device contains a frame of
memory that functions as a FIFO buffer for video. An
input signal with arbitrary timing is written into the
memory with timing based upon its own sync elements.
The synchronizer accepts a reference video signal; the
memory is read out at rates locked to the sync elements
of the reference video. (Provisions are made to adjust
system phase – that is, the timing of the output signal
with respect to the reference video.) An asynchronous
signal is thereby delayed up to one frame time, perhaps
even a little more, so as to match the local reference.
The signal can then be used as if it were a local source.
Some studio video devices incorporate framestores, and
exhibit latency of a field, a frame, or more. Low-level
timing of such equipment is accomplished by introducing
time advance so that 0 V appears at the correct
instant. However, even if video content is timed
correctly with respect to 0 V , it may be late by a frame,
or in a very large facility, by several frames. Attention
must be paid to delaying audio by a similar time
interval, to avoid lip-sync problems.
138 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES