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Thread: using a pic to write video data to vcr
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face BY : Mike Hardwick email (remove spam text)



David,

I've done this, so I'll try to shed a little light...

Circuit Cellar published an article on a VCR data storage adapter several
years back. It might be worth looking up, or perhaps someone else on the
List can say which issue it was in. I recall that fairly elaborate
processing was used to correct for data corruption, much like a CD.

Video Demystified by Keith Jack is a good book on video.

Basically, you must generate the composite sync part of a video signal,
then add data modulation in the 'unblanked' portion of video scan lines. By
unblanked, I mean the portion that normally carries image modulation. You
need composite sync to fool the VCR into working normally, but most VCRs
don't require technically perfect color sync with all of the serrations and
equalizing pulses. Video head drum rotation is very tightly phase-locked to
vertical sync. Video head switching occurs 3~4 lines prior to vertical
blanking, so it's a bad idea to use scan lines in that area for data.

The Closed Caption system puts low-rate data on line 21 only, which is the
last line of the vertical blanking interval (VBI). Line 21 wasn't
previously used to carry any information. If you want high data bandwidth,
you might also take a look at Teletext system standards, e.g. WST. Among
other things, they add a clock preamble to each line of data, to circumvent
VCR playback timing jitter.

For simple applications, consider encoding just one big fat (up to 50uS)
data bit immediately after each horizontal blanking interval outside the
VBI. Net data rate is around 15Kbps. This scheme requires minimal data
encoding and recovery circuitry. Your data recovery micro has to get H and
V sync, and the output of a comparator that slices active video levels.
Comparator input video has to be DC-restored, of course, but cheap
comparators like the 311 are entirely adequate. Your code has to find V
sync, count H sync pulses to the first active line, then march in data bits
at some loosely defined time after each subsequent H sync pulse up 'til VBI
start.

Watch out for the VCR's dropout compensator (DOC) if you try this. The DOC
"fixes" video playback glitches by substituting good video from the
previous scan line (from a 1H delay line). Some VCRs have a switch to
defeat the DOC.

Mike Hardwick
Decade Engineering


>I am working on a conceptual project of using a VCR as a data storage
>medium.  The idea is to use the audio channels to carry music and the video
>channel to carry data that would be used to control things.  I did a search
>on the Internet and found that there are data storage units available to run
>off a PC, but I would rather have a data format of my own choosing.  I admit
>up front that I do not know very much about video signals.  Has anyone else
>developed such an idea, or does anyone have URL's available that I could
>look at to get a better understanding of what I need to do?

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