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Name: Christine
Status: Student
Age: 17
Location: N/A
Country: N/A
Date: May 2004


Question:
Why is it that when we forward or rewind on the VCR, the TV screen creates these scrunched up lines on the screen?



Replies:
I assume you are referring to "scan forward" and "scan backward" on the VCR, for which the picture is fuzzier and sometimes horizontal bands appear. The way that the picture and sound are written on the videotape is a technological marvel. The videotape moves past a fast rotating drum or cylinder inside the VCR. Electromagnetic heads embedded into the drum record video signal into the tape. The drum is angled to the tape, and as a result, the signal is written onto the tape in a helical fashion in thin strips. These strips are thinner than a human hair and several inches long.

This is very different than an audio tape recorder. When the tape is moving at the normal speed, the heads in the drum read the strips correctly. The precision required to do this is pretty amazing. When the tape moves faster (or even backward) then the strips are not aligned correctly. The VCR is actually reading more than one strip, and the picture is fuzzy. The bands on the screen are the result of the drum heads reading a "guard band" that is placed between the strips to reduce interference. In SLP mode, guard bands are not used. Expensive VCR machines have sophisticated ways of correctly reading the tape, even at unusual playing speeds.

Bob Erck



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