From: nwickham@nyx.cs.du.edu (Neal Wickham)
Subject: Re: TBC use
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Reply-To: nwickham@nyx.UUCP (Neal Wickham)
Organization: Nyx, Public Access Unix at U. of Denver Math/CS dept.
Date: Wed, 30 Dec 92 22:39:54 GMT

In article <1hs96kINN227@crcnis1.unl.edu> bberg@unlinfo.unl.edu (bruce berg) writes:

>Funds are very limited so I'm forced to 
>be very carefull in what I get.


Get a loan.  Although there is probably no hurry on the A4000 since not
much 3rd party stuff is available yet.


>1. Will a GVP G-Lock give me a broadcast quality output?
>2. Will a DPS TBC I, II, or III also give me a Broadcast output?


I think broadcast quality has a couple of different meanings (someone
please correct me if I am wrong).  One is that FCC regulations require
that signals broadcasted on public airwaves be of a certain quality as
far as the sync pulses go and what not.  You can make crummy old video
from an old VHS recording a broadcast quality signal no matter how fuzzy
the picture might be.  But sometimes people talk of broadcast quality
as far as the picture is concerned in the same context as professional
quality.  It means that the picture is sharp and clear.
 
The only thing a TBC is for is to sync or steady signals off tape.  Video
runs in frames;  so for any switching between two signals, the frames
need to start and end at the same time.  This is not easily done where
your signals are coming off two separate electro-mechanical tape drive
systems.  The TBC stores a frame in a buffer then pushes it out when a
commom sync pulse tells it to.  It also corrects the unstable speeds
inevitible in a tape drive.  Live video doesn't need TBC since you can
use a common sync pulse for all you sources.  But tape is the way 
most of us will do editing.

This is why JPEG compression has such good prospects.  You don't need
TBCs and the quality can match that of the best of tape systems. 

>3. If anyone uses the DPS TBC, can you tell me if it has an SVHS out?
>4. Do you run the output from the genlock back into the TBC and then 
>   out to the recording VCR?

What the TBC is for is to steady and sync signals off tape.  You never
need it to record really; only to sync or steady the output off tape.  If 
you run a tape *source* through a TBC, you don't need to run it through
again to the destination tape.  A TBC is to fix tape output. ...not input.

I am not sure about genlocking.  You can genlock without TBC but I think
the unsteady speeds of a tape source will degrade the quality of any
output to the destination deck.  The Amiga dosn't really buffer the signal.
It only overlays stuff on the video signal.  I am not sure what you'd
gain by a TBCed signal through a genlock.  The genlock just makes sure
that the Amiga frames and source frames start and end at the same time.
The Aimga gets synced to the source.  A TBCed source is going to be of
higher quality where the tape speed problems are corrected.  I don't know.
Someone else please comment.  It probably depends a lot on the quality
or your source deck.

Uh... to do any switching off tape, you'd need a TBC.  And start-stop
type swiching I believe will make for non-broadcast quality video.  There
the sync pulse just stops and then starts up again.  To be of broadcast
quality, I think, the sync pulse has to be constant throughout the clip. 
Again, the JPEG compression boards can do switching without TBC, but
you are limmited by the amount of video you can get on your hard drive.
But I guess you could take all your short clips and peice them together
on an inexpensive editing deck.  ...not sure.  Comments???


                                    NCW
 

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