MadSci Network: Engineering

Re: Is it possible to delay a digital video signal a few seconds before replay?

Date: Fri Oct 8 10:19:02 1999
Posted By: Matthew B. Weyerich, Technical Coordinator,ES&R Dept., CPI Corp.
Area of science: Engineering
ID: 935030434.Eg


Sorry for the "delay" in getting back to you, I've been swamped this week. 
I hope this will prove worth the wait.

I'm not sure what your setup is, but you may want to consider capturing 
your camera output 
into still images on a computer. There are several low-cost capture cards 
available on the market, depending on your camera type. Assuming a fairly 
quick capture time and save time, you could then display your still images 
using an image viewing program's slideshow utility. (I'm thinking of one 
particularly speedy program, but I'm trying not to give anyone any free 
advertising. E-mail me if you can't find what you need.) If you play with 
the display settings you should be able to achieve the delays you seek 
quite easily. (Possibly more "authentically", too, as I believe the 
Sojourner images were downloaded as slow-scan stills.)

If that doesn't work, then high-end video may be the only other convenient 
way to do this, if it can be done at all. 


Thinking you might be limited to an NTSC video and a VCR, I started 
thinking about tape. An image of an audio "tape delay" the Beatles used at 
Abbey Road popped into my head for some reason. Basically, the unit was a 
reel to reel recorder with an upstream "write" head and a "downstream" 
playback head. The distance between the heads could be varied to change the 
delay.(The signals were blended across tracks.) Hmmm…could this work with a 

If you are pretty handy, have access to at least two old VCR's and parts, 
it could be made to work, according to some of my electronics technicians. 
The tape transport is the tough part, especially when you consider the 
following: tape speed on many old VCR's is about 6 inches per second. You'd 
have to have 6" between your "record" head and your "playback" head to 
approximate one second of delay.

Five seconds delay would require 30 inches of space, and thirty seconds 
delay would call for 15 FEET of head separation! (The technicians are still 
rolling in the aisles over that one. They're firmly convinced Rube Goldberg 
is alive and well!) Unless you want tape all over the place, you'd probably 
want to come up with a system of rollers to jam that huge distance into a 
more compact area. (I'd try looping the tape over opposing rollers, so that 
it followed a zig-zag path. Fifteen rollers a foot apart, or 30 spaced six 
inches apart. Of course, keeping the tape ON the rollers would be a pain, 
so you'd probably want idler rollers and guides along the way.) If you 
needed to vary the delay, you'd add, or subtract roller stages.

You'd still probably wind up tangled in tape and loose VCR parts! It might 
just do the trick, though, and it would certainly add a new twist to an 
already fascinating project. Let me know how it turns out, please. Best of 

Your MadSci,
-Matt (a.k.a. "Rube" ;)

P.S. Check out this excellent Sojourner site. If you follow enough links, 
you'll encounter an online "Sojourner Simulator" you might use for 

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