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Consumer Electronics by S. P. Bali

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HEAD GAP AND WRITING SPEED

Although the octave range has necessarily been reduced using an FM carrier system the maximum frequency required to be recorded has greatly increased. Peak white occurs at 4.8 MHz, and the upper sideband signal— most extensive when sharply-defined detail is being recorded—extends towards 8 MHz. The head/tape transfer system must be capable of passing such frequencies and the magnetic surface of the tape capable of retaining them. The period of an 8 MHz signal is 125 μs and during this short time enough tape must traverse the video head gap to adequately imprint the entire cycle as a magnetic pattern in the tape coating. With a typical head gap of 0.5 microns (5 × 10–7m) the tape-to-head speed needs to be 5 m/s; this ...

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