olivier@butler.UUCP (Charles Olivier) (09/06/85)
(For the line eater [if its still around])
Hello,
Does anybody know of any cheap method of doing video overlay with
an IBM PC. I have an IBM PC and a VCR, and I would like to over
text (& graphics data from my pc). Can this be done cheaply?
Does anyone know of some hardware that can plug into or be attached
to the pc/or VCR to allow overlaying?? I would appreciate any
HELP that I can get.
THANKS in ADVANCE !!
P.S: I am capable of building any hardware and writing
the required software.
Charles Olivier
P.O. Box 2249
Kirkland Wa 98083
uucp: ...uw-beaver!{tikal,teltone}!dataio!butler!olivier
& Email
dsi@unccvax.UUCP (Dataspan Inc) (09/09/85)
You probably didn't want to hear this, but there just isn't any
way to do this cheaply, if at all. Sounds like you need a frame store
time base corrector (I take it that you want to take SCH phased colour
video from a camera, colour video from another VCR, and use your PC as
the poor man's Chyron or Vidifont character generator.
Real character generators used for television are "genlocked" to
the station's master sync generator. The timing requirements of this
are nontrivial (i.e. you must synchronize the colour subcarrier within
a few nanoseconds). Thus, with the other video locked, and the character
generator locked, you can then algebraically add the video from the
character generator to whatever video you already have (also locked).
The reason you need a frame store TBC is that there is probably
no way to send "advanced" vertical sync back to your IBM-PC. (There are
"line store TBC's" which maintain a correction window of + or - so
many lines). You would also have to convert the RGB outputs of your
PC to NTSC (if this isn't done already) and your TBC would have to
have the "pre heterodyning" option to force the colour subcarrier to
be in a precise relationship with the horizontal sync.
You also need a TBC for your video tape recorder, unless the
video coming from your PC is of reference stability. All kinds of
factors enter into time base instability of helical scan VTR's, such
as varying "stiction" around the headwheel, runout in the headwheel
capstan and pinchroller, varying tension in the takeup spool due to
the ever-changing tape pack size, relative humidity, etc. Although
the video from your VCR ** looks ** stable, it in reality is very
disgustingly changing frequency and so on. (To see how bad, get
two TV station video sources, display one, and superimpose the other
with a resistive matrix. You will see the other station's image but
most likely, the syncrhonizing pulses will "cross" over the other
station's image. Then, try this with a VCR and a TV station, and
notice how much the sync pulses "slew". In the two TV station case,
the sync is so stable, the drifting might gain one line per 15 minutes!!)
If you'd like to build a TBC, you'll need to build some really
macho dual ported RAM, and a system for writing in dirty video (using
a clock recovered from the VCR colour burst) and writing out clean
video (using a clock recovered from some reference source). You also
need some way of forcing all the incoming TBC video sources to be
SC-H phased because the output certainly will be! And, if you only
build two field storage, you'll have to break out the delay lines
and analog switches to maintain the NTSC four field sequence.
Finally, the colour video coming from a PC is flat to at least 8 mHz
even at NTSC rates (one of those cases where horizontal resolution can
exceed vertical resolution); colour response of VCR's is not given,
but I can assure you that home VCR's have very poor colour response
past 0.3 to 0.5 mHz. (The detail of an NTSC image is always transmitted
in monochrome. In the best of all ideal worlds, the I-channel of colour
might get to 1.5 mHz). Your VCR also combs and cores the living s**t
out of the video because colour-under systems are notorious for crosstalk
between chrominance and luminance.
Your question wasn't silly at all, though. I can see consumer TBC's
being available within 4-7 years so that video freaks can mix two or
more nonsynchronous sources. Then, the home video freak can do wipes,
dissolves, digital video effects (!) right from his own little TBC. Right
now, though, the Tektronix TBC (which is obstensibly the best) costs
$12,600 - this was last summer, when they had a big sale on them after
the Olympics) and is probably $ 18k new. The Intermetall people have totally
missed their market with digital TV chips - we NEED a TBC chip set, not
a TV receiver chip set !
(And goollyeee gee, standards converters by the time I'm 40? Phew!)
David Anthony
DataSpan, Inc (The Southeast's largest consumer of TRW a/d flash converters!)les@kitc.UUCP (Les Johnson) (09/09/85)
In article <127@butler.UUCP> olivier@butler.UUCP (Charles Olivier) writes: >Does anybody know of any cheap method of doing video overlay with >an IBM PC. I have an IBM PC and a VCR, and I would like to over >text (& graphics data from my pc). Can this be done cheaply? The Image Capture Board from Electronic Photography and Image Center (EPIC) at AT&T Consumer Products should be able to do this at a reasonable cost. Contact Alan Wlasuk at 317-352-6124 for more info. Les Johnson @ ihnp4!kitc!les