To be able to record a video signal embracing 18 octaves or more, it is necessary to modulate the signal onto a carrier and ensure that the octave range of the carrier is within the capabilities of the tape recording system. While the carrier could be amplitude modulation (AM), the frequency modulation (FM) system has been adopted because it confers other advantages, particularly in the realm of noise performance. An FM signal can be recorded at constant level regardless of the modulating signal amplitude, so that head losses and effects of imperfect head-to-tape contact are less troublesome. To achieve a picture replay with no perceptible background noise (snow), the S/N ratio needs to be over 40 dB, and this can be achieved by an FM recording system in a domestic VCR. Professional and broadcast machines can do much better than this!

An FM system, familiar to us in VHF sound broadcasts, starts with a continuous wave ( CW) oscillator to generate the basic carrier frequency. The frequency of the oscillator is made to vary in sympathy with the modulating signal, audio for VHF sound transmitters, video for VCR recording systems and satellite broadcasts. For any FM system, the deviation (the distance that the carrier frequency can be pulled by the modulating signal) is specified. In VHF sound broadcasting, it is 75 kHz, giving a total frequency swing of the carrier of 150 kHz. In a VCR, carrier frequencies are specified for zero video signal amplitude represented by...