TWO-HEAD SCANNING
Because the tape wrap only covers half the circumference of the head drum, and signal transfer must take place all the time, conventional domestic head drums are fitted with two video heads. As one leaves the tape wrap at the end of a scan, another (diametrically opposite) begins a new scan of the tape. The signals from each head are routed in turn into the replay amplifiers by a switch during playback – the switch is synchronised by a head-position sensor associated with the head drum. During record both heads are driven with writing signals, though only the one traversing the tape is recording at any one moment.
Rotating transformer
The transfer of video signals to and from the spinning video heads is accomplished by a rotating transformer whose ferrite core is arranged as two shallow discs concentric with the head drum itself. One half is stationary and the other rotates with the head disc, magnetic coupling between the two taking place via a very small air- gap. One pair of windings is provided for each head; in multi-head machines three or four pairs of windings are required. In a later chapter the use of separate rotary heads for hi-fi sound signal transfer is described – for these a separate rotary transformer assembly is usu- ally provided.