SSG AND TIMEBASE GENERATORS
The timing waveforms for use throughout the camera are generated in an LSI (Large Scale Integration) IC, with the fundamental clock pulses coming from a very stable quartz crystal reference running at a multiple of fsc. Use of a quadruple-frequency clock signal in conjunction with digital counters permits direct derivation of all required phases of subcarrier at 4.43 MHz; no recourse to drift- prone analogue (i.e. LC) phase-shift circuits is required. Further counting of fsc (set to exactly 4.433619 MHz by a trimming capacitor at the crystal) renders not only the line and field drive (sync) pulses at 15.625 kHz and 50 kHz respectively, but a half-line pulse at 7.8 kHz (ident) and the required blanking, clamping, gating and burst-flag pulses. A complex pulse train is required (Fig. 2.4) during the field sync interval, and a four-field-repeat sequence of burst phase and burst suppression during field blanking (Bruch blanking) is necessary to conform fully with the PAL specification. All these are gener- ated within the SSG chip and distributed as required to all parts of the camera.
Genlock
Where the camera is required to operate with others in a ‘studio’ situation involving vision mixers or faders, the scan generators of all cameras involved must be synchronised so that their line and field scans are time-coincident. To ensure correct colour reproduction their colour subcarriers must also be locked together. Professional camera types have facilities for this, in which the SSG master crystal becomes part of a VXO (Voltage-controlled crystal Oscillator) under the influ- ence of a phase-lock-loop (PLL). This can be slaved to an incoming CVBS or burst-plus-syncs signal from an external SSG, which may be part of another camera. Variations in the phase response of electrical circuits, and in camera lead lengths usually necessitate a phase control preset at some point – it is trimmed to match the hue between cameras.
The advent of field-store memories in consumer equipment has opened the way to a form of genlock which does not depend on the SSGs of the cameras running in synchronism. The video output from one camera is digitised, one field at a time, then written into a large RAM (Random Access Memory). It is read out in accordance with the scan timings of the second camera to achieve the same result as true genlock. This system has the advantage of working with autonomous picture sources like VCRs and TV broadcasts.
CAMERA POWER SUPPLIES
A great deal of the design effort for a domestic TV camera is concerned with minimising its power consumption, since it is required to run (in conjunction with its videorecorder section) from a small rechargeable battery. In general the voltage requirement is 4.5 V, 6 V or 7.2 V. Inside the machine, d.c−d.c. converters (see Chapter 11) furnish stabilised supply lines for the various electrical, control, and mechanical sections.