Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cmcl2!uupsi!fozzie!stanley
From: stanley@phoenix.com (John Stanley)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: VGA presentation on a TV
Message-ID: <2N08R7w163w@phoenix.com>
Date: 6 Nov 90 18:39:12 GMT
References: <90309.220833MHS108@psuvm.psu.edu>
Organization: One Man Brand
Lines: 37


MHS108@psuvm.psu.edu (Mark Solsman) writes:

> allowing you to see what you are doing when recording. The card also has
> "GEMLOCK". This allows you to combine a NTSC input with your VGA and produce
   ^^^^^^^
> NTSC output of the combination. (This is what your weather forcaster uses for
> displaying maps on the nightly news)

   It is called genlock, and is shorthand for saying "sync GENerator phase
LOCK". All this means is that the color sync generator on the card can be
timed from an external source.

   I have not seen the card in question, but would bet that it does not
perform the video mixing. You will still need a video mixer to combine
the another NTSC signal with the NTSC graphics. All the genlock says is
that both NTSC signals will be mixable. 

   The same function as a genlock can be performed with a full frame time
base corrector (TBC). The TBC reads the incoming signal into memory, and
sends it out in phase with the master timing signals it receives from the
master sync generator. The TBC has the advantage of being designed for TV
work, and so produces good NTSC output. Many low cost NTSC graphics
systems generate NTSC signals good enough to run a TV, but not to be used
for broadcast use.

   (FYI: if you ever watch a baseball game, and hear the audio get
staticy, and the picture freezes, that is a TBC at work. Somewhere in the
chain the video signal got lost. A TBC detected loss of input and kept
displaying the last good frame it got.)



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