HEAD GAP AND WRITING SPEED
Although the octave range has necessarily been reduced by using an f.m. 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 ns, 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 micron (=5 × 10–7m) the tape-to-head speed needs to be around 5 metres/ second: this is called writing speed.
The achievement of such a high writing speed is very difficult in a ‘direct’ transport system, where extremely high spool and capstan speeds would be required. A solution is to rotate the heads themselves against the tape; the heads are mounted on a spinning head-drum, and protrude beyond its surface to make intimate contact with the tape ribbon which itself is wrapped around the drum. For domestic (and professional) video formats the arrangement takes the form of a helical-scan set-up, illustrated in basic form in Fig. 13.2.