NTSC/PAL via audio

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NTSC/PAL via audio

Postby LuTELLO » Tue Mar 04, 2014 11:00 am

Howz everyone, been a while!
Crazy question: What kind of picture information if any can be recovered by digitizing a composite NTSC or PAL signal using 44/16 or 96/24 audio hardware? Silhouettes at best, I assume! I'd like to use this for a glitch art project.

Edit: Here's what my capture hardware got.
http://imgur.com/vis0rsu

I was actually able to get some horizontal lines corresponding to some test patterns I tried, but the vertical is too out of range.
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Postby gary » Tue Mar 04, 2014 5:13 pm

Hmmm let's see - 625 lines at 25 fps = 15625 lines per second (64 uS).

So... if you used 96k sampling you would achieve a massive 96000/15625 = ~6 pixels per line (less than 3 if 48k - NB *never* use 44.1k).

In truth if you wanted to try it anyway you would be better off choosing a sample rate that is an integer multiple of the line frequency (say 93.750) that way there is an even number of samples per line.

A better approach would be to use the parallel port of an old PC (new ones don't tend to have them) which would give you a sampling rate of around 700kHz (from memory).

This is what a PAL picture looks like when played OUT through a parallel port:
http://www.youtube.com/watch?v=Sa04GhS99pw

Now capturing a frame is pretty much doing the reverse, in fact there is software available on the net that actually does just that (although for NTSC I think). I could probably find it if you are interested.

This shows how the parallel port is turned into a DAC, and turning one into an ADC would be similar.

http://www.youtube.com/watch?v=tW822Yv6b4s
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Postby LuTELLO » Thu Mar 06, 2014 3:17 pm

That makes sense. Results I got today are a little like the results of my bare bones 32 line viewer. (In which I think I drilled the holes to large.)

http://imgur.com/dV9QMpw

Ok, 32 line looked better!

Didn't know parallel ports could do frequencies like that. I'll check it out if it's easy for you to find.
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Postby gary » Thu Mar 06, 2014 6:24 pm

Yep I think 32 lines should be quite a bit better than that - but hey well done on getting that far! I'm impressed!

Ok, it's always hard to find but I'll get back to you - I know I have the software somewhere on my PC but I always forget what it's called.
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Postby gary » Fri Mar 07, 2014 2:55 pm

Ok, if you google "Dirt Cheap Frame Grabber" you will get to that parallel port NTSC capture article I mentioned. The original link is broken but there are some clones out there - worse comes to worse it's in the wayback machine database.

I attach an example frame FYI
Attachments
dcfg.jpg
dcfg.jpg (7.71 KiB) Viewed 13423 times
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Postby LuTELLO » Fri Mar 07, 2014 3:59 pm

Found it!
Pretty cool, how about NTSC output?
Not sure if I'll get around to it, need to get my brain together to make a proper 32line viewer. Maybe I should try this first.
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Re: NTSC/PAL via audio

Postby Lowtone » Wed May 14, 2014 3:53 am

how much pixels will it be with PAL at 192kHz ?
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Re: NTSC/PAL via audio

Postby gary » Wed May 14, 2014 9:03 am

Lowtone wrote:how much pixels will it be with PAL at 192kHz ?


Twice the number calculated above for 96kHz.
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Re: NTSC/PAL via audio

Postby Lowtone » Fri May 16, 2014 6:13 pm

Long long i tried to record PAL video on an audio cassette.
It was impossible to play back the picture, but the sound was different, it was possible to identify the moment i zoomed or moved the camera.
If i found this cassette, is this possible to recover the signal, or don't you think it's not worth it ?
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Re: NTSC/PAL via audio

Postby AncientBrit » Sat May 17, 2014 12:37 am

Hi Lowtone,

The bandwidth of a typical domestic audio cassette recorder would probably be 100Hz to 7kHz or so.

What you heard on playback is the very low frequencies of the video signal which would normally extend from 25Hz to 5MHz, so the upper frequencies would not be present on playback.

So no, it's not worth it.
The video signal is just not there for you to view.

Cheers,

Graham

PS I tried this on a Philips reel to reel machine many moons ago, around 1960 I think.
Nothing there to view I'm afraid

pps also see http://www.totalrewind.org/cameras/C_PXL2.htm
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Re: NTSC/PAL via audio

Postby Lowtone » Sat May 17, 2014 3:51 am

ah yes the pxl2000! very rare to find at a good price nowadays

Thanks for your answer :)
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