using a pic to write video data to vcr
Mike Hardwick email (remove spam text)
>I understand that perhaps a VCR is not the highest
>speed, best device around - but the cost makes it attractive. So here is my
>idea - given that one frame consists of 512 horizontal lines, repeated 30
>times per second (NTSC), then if you were to encode just 8 bits on say 400
>lines out of 512, there should be enough data - I imagine to do control and
>display. I would not think this would be asking a lot from a VCR setup.
For your purpose, it's probably better to think of video timing as 262.5
lines repeated 60 times per second. V sync is at 60Hz (give or take a
smidge). Of these 262.5 lines, about 240 are "live" video scan lines. If
you encode data reduntantly, possibly using several scan lines per bit, you
should be able to avoid media-related data integrity problems.
The scheme I suggested has considerable redundancy available though. Just
take multiple samples of each bit in the decoding process and discard
outliers (most UARTs do this). VCR dropouts tend to last for only small
fractions of a scan line unless you're pushing the tape past its wear
limit. Hint: Don't use cheap tape. I would also low-pass filter the video
upstream of the data recovery comparator, or perhaps deliberately choose a
comparator with slow response, in order to remove fast glitches. You can
add parity, checksums, etc. to enhance recovered data accuracy. Noisy
channels have been with us for a long time, and there are many techniques
I think it was mainly the convenience factor that killed VCRs for hard disk
backup. It takes a HUGE amount of time to back up your data this way...
Mike Hardwick, for Decade Engineering -- <http://www.decadenet.com>
Manufacturer of the famous BOB-II Serial Video Text Display Module!
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In reply to: <002701c15cdd$8a683ee0$1b9eb3d1@dvflaptop>
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