COLOUR-UNDER SYSTEM
The basic concept of colour-under video recording is a simple one: to translate the entire encoded chroma signal to a lower frequency around 700 kHz for recording by means of a heterodyne process with a local oscillator; and during replay to translate it back to its correct 4.43 MHz frequency by a second heterodyne process. If the tape were a perfect recording medium this would be all that were necessary. In practice there are three factors which prevent the use of such a simple system: the fact that at low colour-under frequencies the azimuth- offset of the recording heads does little to prevent inter-track crosstalk; an inability with conventional electronics to control replay speed within the very fine limits required to adequately maintain colour subcarrier frequency during replay; and, allied to this, the mechanical imperfections in the deck, transport system and tape rib- bon which impart timing jitter to all video signals recorded and replayed on tape – or, indeed, on disc. Chapter 6 indicated how crucial is the timing and phase of the chroma signal, which must be held to within ±3 ns or so, representing a mere 5° of phase error tolerance. Such a requirement is far beyond the ability of a mechanically driven system, so special artifices are necessary to maintain correct hue and saturation in the reproduced colours.