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Helical scan

Helical Scan, or striping is a method of recording higher bandwidth signals onto magnetic tape than would otherwise be possible at the same tape speed with fixed heads. It is used in video recorders, digital audio tape recorders, and numerous computer secondary storage and backup systems.

In a fixed head system, tape is drawn past the head at a linear 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 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.

Video and digital audio need considerably more bandwidth than analogue audio, so much so that tape would have to be drawn past the heads at very high speed in order to capture this signal. Clearly this is impractical, since tapes of immense length would be required. (However, see VERA for details of a partially-successful linear videotape system.) 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. Because the tape is moving, the diagonal is actually curved, which is why it is known as a helical scan.

There are a number of practical problems to be overcome with this system. The high tape/head speed could lead to rapid wear of the tape, so both head and tape need to be polished extremely smooth, and the head made of a hard wearing material. In addition, supplying signals to a rotating head is problematic - this is usually accomplished by coupling the signal inductively. The transport mechanism is also much more complex than a fixed head system, since tape must be pulled around a rotating drum containing the head so that a complete stripe can be recorded on each revolution. In a VCR for example, the tape must be pulled right out of the cassette case and threaded around the drum, and between the capstan and pinch roller. This leads to complex and potentially unreliable mechanics.