Path: utzoo!attcan!utgpu!watmath!iuvax!mailrus!tut.cis.ohio-state.edu!ucsd!brian
From: brian@ucsd.Edu (Brian Kantor)
Newsgroups: sci.electronics
Subject: Re: Time Base Correctors
Message-ID: <10021@ucsd.Edu>
Date: 21 Sep 89 14:22:57 GMT
References: <11991@cit-vax.Caltech.Edu>
Reply-To: brian@ucsd.edu (Brian Kantor)
Organization: The Avant-Garde of the Now, Ltd.
Lines: 33

Simple time-base correctors work by using a double-buffered two-line
digital memory.  The take the incoming video signal and digitize it,
then store one horizontal line at a time into one of the line buffers.
That buffer is then swapped with the other one, and the data just
received is then sent back out using the station horizontal sync and
chroma reference while the other buffer is being filled.  Clearly this
introduces a one-line delay in the video signal path.  Often all the
other inputs to the switcher console will be run through a 1H delay line
to correct this.

This simple scheme does not correct for vertical sync errors.  When
using a pair of TBCs to edit tapes into an effects console, for example,
the playback decks have their vertical timing synchronized mechanically
to station sync.

A full-frame TBC, often refered to as a "framestore", has enough
memory to perform the above on a full frame (sometimes just a field),
and can thus correct for both horizontal and vertical sync errors.
They can also can be made to do scan rate conversion, stillframing,
sampled-slo-mo, etc.  You'll often see these in use for
outside-broadcast work, such as "minicam remotes".  Since they clock
the input signal in from its sync, and output at station sync, they
have the interesting effect of providing stillframes when a seagull
lands on the microwave and the input signal disappears.  You've
probably seen remotes where the picture kept freezing; this is often
the reason why.

The resolution of the stuff we have here (for student-production educational
use) is 512 pixels luminance, 128 pixels I-carrier, and 64-pixels Q-carrier
per line.  (I and Q are the Inphase and Quadrature subcarriers used to
transmit chroma [color] information in NTSC.)  Each is stored in 8 bits.
It actually looks pretty good.
	- Brian