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Injecting digits into a analog video feed?

M

Martin Griffith

Jan 1, 1970
0
I need to put some digital iinfo, from a fast uart, possibly,maybe
biohase mark, into a few lines of video blanking, on a standard 75R
coax. 2 or 3 bytes per line will be ok

I was thinking of just using a simple PNP current source, from a
5Vish rail, fed from a micro, 8051 or similar.

I'll have an Elantec sync stripper doing a lot of the hard work/
timing, somewhere in the system.

Any gotchas on feeding a coax this way?


Martin
 
F

Fred Bloggs

Jan 1, 1970
0
I need to put some digital iinfo, from a fast uart, possibly,maybe
biohase mark, into a few lines of video blanking, on a standard 75R
coax. 2 or 3 bytes per line will be ok

I was thinking of just using a simple PNP current source, from a
5Vish rail, fed from a micro, 8051 or similar.

I'll have an Elantec sync stripper doing a lot of the hard work/
timing, somewhere in the system.

Any gotchas on feeding a coax this way?

None other than possibly the transmitter and/or receiver may not like
the DC component, or the DC is blocked and the threshold shifts.
 
J

Jim Thompson

Jan 1, 1970
0
None other than possibly the transmitter and/or receiver may not like
the DC component, or the DC is blocked and the threshold shifts.

Surely there's some kind of buffer driving the cable? It'd probably
be better to insert the encoding there.

...Jim Thompson
 
T

Tam/WB2TT

Jan 1, 1970
0
Martin Griffith said:
I need to put some digital iinfo, from a fast uart, possibly,maybe
biohase mark, into a few lines of video blanking, on a standard 75R
coax. 2 or 3 bytes per line will be ok

I was thinking of just using a simple PNP current source, from a
5Vish rail, fed from a micro, 8051 or similar.

I'll have an Elantec sync stripper doing a lot of the hard work/
timing, somewhere in the system.

Any gotchas on feeding a coax this way?


Martin
Look at how broadcast stations insert the time of day information into the
video.

Tam
 
M

Martin Griffith

Jan 1, 1970
0
Surely there's some kind of buffer driving the cable? It'd probably
be better to insert the encoding there.

...Jim Thompson
I agree, it would be so easy to add the signal to the video buffer,
but it's basically an add on, to add telemetry, by using a bnc T piece
to insert the signal as a switched constant current, and my Tek/grass
valley manuals are long lost


Martin
 
J

John B

Jan 1, 1970
0
I need to put some digital iinfo, from a fast uart, possibly,maybe
biohase mark, into a few lines of video blanking, on a standard 75R
coax. 2 or 3 bytes per line will be ok

I was thinking of just using a simple PNP current source, from a
5Vish rail, fed from a micro, 8051 or similar.

I'll have an Elantec sync stripper doing a lot of the hard work/
timing, somewhere in the system.

Any gotchas on feeding a coax this way?


Martin

No, that method was used in a lot of the cheaper analog broadcast
equipment. There's a lot of good stuff about video here:

http://www.cq-tv.com/index.htm

You can download past editions free, but unfortunately there's no
article index on the website. There might be one on the DVD.
 
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