Helical scan

Helical scan is a method of recording high frequency signals on magnetic tape. It is used in reel-to-reel video tape recorders, video cassette recorders, digital audio tape recorders, and some computer tape drives.

In a fixed tape head recording system, magnetic tape is drawn past the head at a constant speed. The head creates a fluctuating magnetic field in response to the signal to be recorded, and the magnetic particles on the tape are forced to line up with the field at the head. As the tape moves away, the magnetic particles carry an imprint of the signal in their magnetic orientation. If the tape moves too slowly, a high frequency signal will not be imprinted: the particles' polarity will simply oscillate in the vicinity of the head, to be left in a random position. Thus the bandwidth channel capacity of the recorded signal can be seen to be related to tape speed: the faster the speed, the higher the frequency that can be recorded.

Digital video and digital audio need considerably more bandwidth than analog audio, so much so that tape would have to be drawn past the heads at very high speed to capture this signal. This is impractical, since tapes of immense length would be required. The generally adopted solution is to rotate the head against the tape at high speed, so that the relative velocity is high, but the tape itself moves at a slow speed. To accomplish this, the head must be tilted so that at each rotation of the head, a new area of tape is brought into play. Each segment of the signal is recorded as a diagonal stripe across the tape. This is known as a helical scan because the tape wraps around the circular drum at an angle, travelling up like a helix. The difference between the head writing speed and linear tape speed is vast: for example, 13 mph (5,800 mm/s) versus 35 mm/s.

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