PULSE FEEDS
Many sections of the receiver or monitor require pulse feeds at line rate for gating, clamping and keying. In the luminance stage the signal must be black-level clamped; the line flywheel sync circuit needs a timing reference pulse; the decoder requires gating pulses for extraction of the colour burst and for triggering the PAL switch; some i.f.
a.g.c. systems are keyed by line-rate pulses; the scanning spot must
be extinguished during flyback; some switch-mode power supply units work synchronously with the line timebase and must be triggered; and so on. Most of these pulses are derived from the line oscillator chip in late designs, but the l.o.p.t. itself may be a source of reference and timing pulses.
In Fig. 10.8 the junction of C416 and C417 provides feedback to the second phase-locked-loop in the line generator section of the jungle IC.
SUPPLY VOLTAGE STABILISATION
The amplitude of the line and field scanning waveform is directly related to the supply voltage to the timebase. While a degree of internal stabilisation is possible within the negative feedback loop of a field timebase, and via the E-W correction circuit of a line time- base, close stabilisation of supply voltage is necessary. This is the func- tion of the power supply circuit, which can be integrated into the line output stage. More commonly, a quite separate PSU (power sup- ply unit) is provided to cope with the greatly varying load presented by a line timebase, whose energy demand depends largely on tube beam current, which in turn depends from moment to moment on picture content. It may vary in a large-screen colour set from 40 W at zero beam current to 65 W when a very bright contrasty picture is being displayed. Where the operating voltage for the field timebase is obtained from the l.o.p.t. it will be indirectly stabilised by the action of the main PSU.