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From article <11991@cit-vax.Caltech.Edu>, by stevel@tybalt.caltech.edu (Steve Ludtke):
> Hi. Does anyone out there know how time base correctors (like the ones used
> to sync 2 video signals) work ? If so, could you post a general description
> of how they work ? Just curious ...
> 
> ---------------------------------------------------------------------------
> stevel@tybalt.caltech.edu          CS-> | <-Ph
> stevel@citiago  (Bitnet)               \|/               I DO ...
> 72335,1537      (Compuserve)            ?

There are two basic types of TBC's on the market, analog and digital.
They vary in the number of lines that can be stored (corrected)
(known as 'window') some units are full field and frame window units.
Analog ones usually max out at 8 or 16 lines of correction.

Basically VCRs that can be connected to TBC's (not all can be connected
to analog models, but full frame units known as frame synchronizers
will handle any video input) have a video input that accepts
advanced compensation sync signals (ADV COMP).

The idea is that a tape player will speed up  and slow down minutely
while playing a tape, and these 'broadcast' VCR's can be told to
speed up and slow down by sending an advancing sync signal into
the VCR. Analog and small window units (16 lines is considered small
these days) advance the signal coming from the VCR until the 
vertical retrace signal is aligned with 'house sync' (the signal
that you're trying to lock on to). Then, regular wow and flutter in
the VCR causes the signal to advance or delay by a certain amount,
at which point the analog and digital line delay units jump in
to compensate for the speed change.  Thats the basic synchronization
part. (of course the ADV COMP signal is going all the time and the
line delay is always working, its quite fast).
Some TBC's also offer drop out compensation (usually handled by the
VCR) in which a lost video line is substituted with the previous
line by the TBC. Most VCR's do this (up to 4 or 5 times in a row)
before actually showing a snowy line. Drop outs happen all the time
and the DOC in the VCR's work quite well.

Most TBC's also get into the actual time-base-
correction process, in which horz and vert sync, and color burst
are stripped off the incoming signal and digitally regenerated, and
added on the final output to give a 'clean' output signal. Some
TBC's have a proc amp (process amplifier) section which allows
video level, color level, hue and pedestal adjustments. You can
also buy stand alone proc amps that don't have the TBC functions.

Full frame TBC's, also known as frame synchronizers, have enough
digital memory to store an entire frame of video, therefore the
advanced comp sync signal is not required, but may be used anyway. 

Digital units (the common ones) have 8 bits for luminance, 4 bits
for saturation and 4 bits for hue (8-4-4) (don't quote me on this one).

Some TBC's are dual units, containing two frame buffers that handle
two input signals, extra cpu power can be added to mix and match
the input buffers to produce effects on the output side, such
as compression, push off/on, tumble (needs lots of cpu).

I speak from experience w/ the following TBC's
	FOR-A  FA-200
	Microtime S320D, T120D E120 (yuck).
	Quantel
	Adda

hope that answers your question.

-- 

| Brad Clements          bkc@omnigate.clarkson.edu        bkc@clutx.bitnet 
| Network Engineer       Clarkson University              (315)268-2292