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Helical Scan

Why is that shiny thing inside the VCR crooked?

Helical Scan

The head drum of a six-head NTSC (Hi-Fi) VHS VCR / Atlant via Wikimedia

Did you ever look into the open door of a VCR (if you were old enough to have access to one years ago) and wonder why that shiny round component inside looked crooked or ‘lopsided’? I remember thinking that as a kid. It looked like it hadn’t been mounted correctly.

It turns out, that there is a very good reason that the tape head in a VCR is intentionally mounted at that precise angle, and it is an example of a very clever solution to an engineering problem as explained in this wikipedia article.

The amount of signal data that can be encoded onto a tape depends on the speed of the tape moving across the read head. The faster a tape moves across the head, the more signal it can capture. For audio cassettes (if you’re old enough to remember those too), a fixed head works well enough as the tape can move at a speed that results in a reasonable length of tape to pack into the audio cassette casing, while capturing all of the data necessary on the tape to record and playback an audio signal.

However, video data is much more complex and requires greater bandwidth to capture the audio and video components. One solution is to simply run the tape a lot faster across a fixed head. This works in theory, but the problem is that the tape would have to be incredibly long - much longer than what could be packed into the casing of something the size of a VHS tape.

Another solution to achieve a high relative read/record ‘speed’ is to move the head across the tape very quickly, by spinning the head. However, unless the tape is moving as fast as the head, the head would ‘re-scan’ the same areas of the tape repeatedly, negating the relative speed of the spinning head (and probably causing distorted recording/playback as well).

But, if you tilt the head at an angle and record/read in diagonal tracks across the tape, then voila, problem solved! You can attain a very high relative scan speed—and thus high storage capacity—while maintaining a tape speed slow enough to result in a reasonable length. Very clever!

Helical scan tape recording method. Notice how the 'tracks' are read diagonally by the spinning head as the tape moves past it.

Helical scan tape recording method. Notice how the ‘tracks’ are read diagonally by the spinning head as the tape moves past it. / via Wikimedia

I love clever solutions like this! If you can’t run the tape fast across the head, run the head fast across the tape!

So there you have it—the reason why the big shiny read head in a VCR is ‘crooked’.