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'[OT]:Video Mixing with Keyed Overlay'
2001\06\19@090416 by David Blain

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Hello,

I am trying to find a circuit or device that would allow me to mix baseband
video from my computer (Already has TV Out) with a 2nd video source.  I am
trying to do  Graphics On Screen Display (with existing video in
background), so Keyed Overlay support is needed.

I have found a few scan converters that take VGA and offer Keyed Overlay
onto a video source, but tend to be too expensive ($500 - $3000).

I am hoping to find a simple circuit I can build (good at building, bad at
designing), or even a cheap (<=$200) unit I could purchase, that would do
good quality keyed video mixing. (No need for broadcast quality).

Any help would be appreciated.

David.

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2001\06\19@195628 by Ashley Roll

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Hi David,

I'm no expert in this, but from what I know your going to have a few
problems with a "mixer".

You would have to ensure that the two video signals are synchronised (called
"locked" in the video industry I think - you use equipment that supports
external "gen-lock") so that when you mix the signals you get the correct
bit of it and the timing isn't disturbed causing the image to rip apart..
Basically this is difficult. If your very lucky you video source may have a
gen-lock input so you could derive a lock signal from your computer video to
lock them together..

How complex are the graphics? On screen displays are normally done by
arranging a switch in the video signal and switches between the real signal
and a specific colour level (voltage) at the right time in the video
frame/field/scan line. You could possibly use a PIC or SX to do this :) Look
at http://www.piclist.com/techref/scenix/lib/io/dev/video/index.htm for some
ideas.

These actually generate the entire signal, but you would just switch between
the live video and your set voltage representing a colour. (if you get
really tricky that voltage could be generated from your PIC to give you
several colours)

There are also on-screen display chips that do this all for you..

The "big boys" now all edit digitally, you digitise the video for your
entire movie (at movie resolution of at least 4096x3072 for "low res") to a
big disk array, mess with it with some expensive software and send it our
again.. it is possible to do this is real time, but not cheap.. not cheap at
all..

Some "home" video capture cards should let you do what you want also which
would be a lot cheaper :) You should be able to have them overlay video into
a "keyed" area of your display and you can take the TV out and record it.
All the "gen-locking" is done by digitising the video and copying pixels
normally.

Good Luck.
Ash.


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2001\06\19@204249 by Robert Rolf

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Good advice Ashley.

The quick and dirty way to do what David wants is to put an
ATI 'All-in-Wonder' card into his machine ($180C). The AIW cards
digitize
the incoming video and put it into a variable sized window (including
full screen, even at 1280x1024) for display on your monitor. It also
has a TV out capability (S and composite), so you'd set
your screen to 640x480 resolution
(to match that of your incoming video) and then make the video your
desktop, with whatever graphics you want on top.

They have a SDK for their API so writing your own C app to drive
the card is not too hard.

The AIW cards do ALL the work on-board (by default they use Magenta
to key in the video) so the video stream happily continues even when
your box locks up.

Now if you really want to MIX the signals (as in variable transparency),
the ATI card won't do it since it's strickly a background keyer.

You might also want to look as some of the older Elsa cards.
Their 'Gloria Synergy' card can output any single desktop window
as video, so concievably you could get your video into your system
via any video digitizer, then use the Elsa to output that window,
with graphics overlay, as video.

And then there is the DPS product line of 'personal time base
correctors'. They would allow you to retime your incoming
video to that of your computer, where you can then build a
simple analog mixer to combine them.

Do remember that the bandwidth of NTSC/PAl video is a fraction
of what you have for your desktop (200 line pairs vs the 400 of
a common 800x600 screen) so your graphics have to be constructed
-specifically- for video output.

For more discussion, contact me off-list. I do a fair bit of
computer video in my 'other life'.

Ashley Roll wrote:
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2001\06\20@113120 by Dal Wheeler

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Maybe you can get your hands on a used video toaster --a card from a company
called NewTek that went into the old Amiga 2000, 3000's that handled most of
those video functions.  I always wanted one when I was into Amiga's, but
maybe you can find one cheap now that they're over 10 years old (geez thats
depressing.)

-Dal

{Original Message removed}

2001\06\20@113333 by Andrew Kunz

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>Maybe you can get your hands on a used video toaster --a card from a company
>called NewTek that went into the old Amiga 2000, 3000's that handled most of
>those video functions.  I always wanted one when I was into Amiga's, but
>maybe you can find one cheap now that they're over 10 years old (geez thats
>depressing.)

Don't count on it - they are still making and using Toasters.

Andy

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2001\06\20@114202 by Mark Newland

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From what I heard, they took the heart and soul of an Amiga (the video portion)
and put it into the PC version of the toaster.  All the PC did was control the
video portion of the Amiga that was inside the toaster.  What is the depressing
portion is that the PC version of the toaster was cheaper than the Amiga
version.  Or at least this used to be the case.

Andrew Kunz wrote:

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2001\06\20@154746 by Peter L. Peres

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David, you can do this using a simple keyer and a TBC. The TBC needs to
have ext. sync input (aka genlock) (some don't have this ?!). You should
be able to buy one for under $100 used.

The TBC will sync the TV signal to the computer signal. The signal at the
output of the TBC can be keyed directly using a simplistic keyer (this is
also known as titling or downstream keying).

If you are really into video you could spend the money on a used video
mixer like a Panasonic WJ-MX5, 7 or 12 or a Videonics or other product.
These have proper keying and titling facilities.

The last option would be to examine the manuals of the video card with TV
output for clues on whether it can be genlocked or whether it can do the
keying itself. I think that Amigas used to be able to do this. Maybe you
can buy a used Amiga computer and use it as is.

Peter

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2001\06\20@173603 by Robert.Rolf

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www.dps.com/
http://www.dps.com/dpsweb.nsf/BHP?OpenFrameSet
Microsync 8 looks like what you need (unless your source is a home VCR).

Amiga's could genlock with an external module.
One could also genlock any VGA card with a 'feature' connector
by using the ancient Motorola genlock chip (MC9584??)
to create an external clock that is 'adjusted' to get synchronization.

The problem is that VGA video out is not really NTSC. Only close.
By tweaking the registers on the mc6845 emulation most cards have,
one can get a nearly perfect G/L with nothing more than a RF VCO and
phase compartors.

I once used an RF generator as the 'external clock' on an ancient
ATI card so that I could get the 'sync bars' commonly seen
on video cameras shooting monitors to disappear during
a pro video shoot. The display was running 800x600, but at a vertical
rate of EXACTLY 59.9400Hz (for NTSC). 14.31818Mhz/4/227.5/262.5



"Peter L. Peres" wrote:
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2001\06\22@021049 by Peter L. Peres

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Robert, that was a daring move with the RF generator to drive the VGA
clock. Did you ever try to properly genlock it or something like it ? I
have been toying with the idea for some time now.

Most SVGA cards have a highly programmable clock generator on board. It
should be possible to add a varicap to the crystal and build a compound
analog/digital (partly software) PLL that could genlock to an external
signal or some harmonic thereof. It could even use a PIC for interface. If
this was derived from a sync separator and color subcarrier regenerator it
would genlock the SVGA to the external signal.

Peter

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2001\06\22@034434 by Robert Rolf

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"Peter L. Peres" wrote:
>
> Robert, that was a daring move with the RF generator to drive the VGA
> clock. Did you ever try to properly genlock it or something like it ? I

No. It was sufficient that the noise bar was moved into the retrace.
Since the monitor was running 800x600 it would have been impossible
to 'genlock' in the usual sense since the line timing would be all
wrong.

> have been toying with the idea for some time now.

It does work -IF- you can program the VGA timing registers so that
you get interlace and correct line counts. With today's complex chip
sets, the user no longer gets that capability. If the driver doesn't
support the exact resolution you need, you're out of luck.

> Most SVGA cards have a highly programmable clock generator on board. It

No programmable enough, in my experience.

> should be possible to add a varicap to the crystal and build a compound
> analog/digital (partly software) PLL that could genlock to an external
> signal or some harmonic thereof. It could even use a PIC for interface. If

Yep. I already looked at doing just that. It turns out to be MUCH
simpler to use a VCO and overide the crystal by simply connecting
to it. I had developed a cicuit that would genlock a VGA at 640x480
quite nicely (by diddling the CRTC registers), but then 800c600
became the popular standard, and then feature connectors disappeared,
and then TV out became common, so I never brought it to market.

If there really was a need to genlock the newer SVGA cards (at
VGA resolutions of course) I might reconsider/resurrect.

> this was derived from a sync separator and color subcarrier regenerator it
> would genlock the SVGA to the external signal.

You still have to encode the RGB signal, and yes, there are chips
that do that now.

Today I would take a TV out VGA card and diddle
the clock. HOWEVER, most cards DO NOT PRODUCE NTSC, only something
close to it. A TV may be happy with 61Hz 15800kHz, but a color
video camera would not mix with that. Also, the ATI cards
go out of their way to NOT interlace the color subcarrier (it
reduces the 'dot crawl' effect), which makes them useless
for overlays.

Unfortunately the demand for genlockable VGA cards is too small
to drive down the price, even though it only costs a couple
dollars to add the circuitry necessary to genlock.

And do realize that locking to a home VCR is 2 orders of magnitude
more difficult to do that locking to a stable camera signal.
To see what I mean power line lock your scope, and then look at the
hugh amount of jitter in the video coming from a hetrodyne
VCR.


Robert

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