VIDEO ON MAGNETIC TAPE
The use of magnetic tape to record and replay video signals is now commonplace, but the techniques involved are not so straightforward as for audio recording. The main problem is the relatively large bandwidth of the video signal, which for a broadcast-standard signal extends from d.c. (about 25 Hz in practice) to 5.5 MHz. For domestic videorecorders a more limited response is adequate – the h.f. roll-off occurs at about 2.5 MHz, permitting a more sparing use of the tape. A magnetic tape system’s output level is proportional to the rate of change of the magnetic flux, so that output is directly geared to frequency. Thus each halving of frequency (octave) halves the output signal, giving the tape/head interface a characteristic 6 dB/octave curve. Because the difference in levels between magnetic saturation of the tape’s coating and the inherent noise of the system is about 1000:1, corresponding to 60 dB, it is plain that the 6 dB/octave will permit a maximum of ten octaves to be fitted between the noise floor and the overload point, so long as massive compensation is provided
to equalise replay levels across the frequency spectrum.
FM MODULATION
A television signal, even the bandwidth-restricted one described above, occupies fifteen or sixteen octaves, and so cannot be directly recorded on tape by any means. Indirect methods of recording are possible, however, and they involve modulation of the video signal onto a carrier. Invariably an f.m. carrier is chosen for this purpose: the use of f.m. increases noise immunity, masks shortfall in signal strength stemming from slight tracking errors and imperfect head/ tape contact, and permits either (a) its use as recording bias for a second signal carrying the chroma information or (b) the facility to drive the tape coating into magnetic saturation on each f.m. carrier cycle to further improve S/N ratio.
The way in which the f.m. carrier technique reduces the octave range is shown in Fig. 13.1. Carrier frequencies are assigned for both extremes of the luminance signal waveform, typically 3.8 MHz for sync tip and 4.8 MHz for peak white. This actually permits the recording of d.c. (zero frequency) video signals since a constant level of white or grey will give a constant f.m. carrier frequency. During each
line sync pulse the carrier falls to 3.8 MHz for its 4.7 μs duration, and during the 52 μs active line period the carrier frequency rapidly deviates between 4.1 and 4.8 MHz to describe the levels of light and shade in the TV picture.
In deviating in this way the f.m. modulator produces sidebands, and the modulation index (the relationship between video and f.m. frequencies) is chosen so that virtually all the sideband energy is confined to the first pair of sidebands above and below the carrier frequency itself. In the VHS system, for instance, enough sideband energy is recovered to properly demodulate the f.m. signal when the record and replay frequency response extends from about 1 MHz to about 7 MHz, which embraces the entire lower sideband and a portion of the upper one – balance is restored by careful shaping of the frequency response of the playback amplifier. An operating range of frequencies between 1 and 7 MHz represents an octave range of less than four – well within the capability of the magnetic tape system.