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How does video tape make contact with the VCR?
Question Date: 1999-02-25
Answer 1:

It turns out I have a two-year-old at home who just loves to push all the buttons on the VCR -- so I had to learn to fix it after it failed a couple of times...

VCR's use a clever arrangement to get a very fast head to tape speed while letting the tape move only slowly. The basic idea is that 3 or more tape heads are placed on a cylinder which spins around its axis at a few thousand RPM. The tape is then pulled around the cyllinder at a shallow angle. Each head scans a narrow strip on the tape at an angle of about 15 degrees-- Draw a pair of lines about 1 inch apart and then draw several parallel lines about 1/32 of an inch apart, crossing the first two lines at about 15 degrees, so the length of the lines between the first two is about 3.9 inches. In this picture, the first two lines are the tape boundary and the parallel lines are the video signal tracks. (On a real tape, the lines are much closer together!) The idea is to maximize the length of the recording signal, while letting the tape move slowly -- saving the tape. The heads rotate at a constant rate, but the tape can be moved at different speeds. What does this do to the spacing of the tracks? Can you guess why very slow motion reduced the quality of the recording?
You might check out the following web site:

By the way -- there is a very interesting show sometimes on TLC or on PBS from England describing how a wide variety of machines work. I can't remember the title, but they had a show entirely devoted to the VCR. It seems that an American Russian immigrant invented the flying head technology -- but could not get funding until Bing Crosby (the actor-singer) gave him some start-up money. He founded Ampex Corp. which made all the professional VCRs for the TV networks until very recently.

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