Sandcastle pulse
Some sync separator processing systems incorporate the line and field pulses in a single multi-level pulse known as a sandcastle pulse. A typical three-level sandcastle pulse is shown in Figure 13.17. The highest level, 7.5 V, provides the narrow burst gating pulse whose average duration is 4 j.Ls. It is generated by level detection of the line sawtooth signal. The intermediate level, 4.5 V, is derived from the line flyback and has duration of 12 j.Ls. At the lowest level, 2.5 V, we have the field blanking pulse with duration of 21 lines. A level detector or slicer may be used to extract the required pulse from the sandcastle combination as and when required.
Field timebase
In the case of a CRT display, the function of the field timebase is to deflect the electron beam relatively slowly (cycle time 20 ms for PAL and 16.7 ms for NTSC) from top to bottom of the viewing screen, then rush them quickly back to the top during the field blanking interval. During the downward stroke, the line timebase draws over 300 scanning lines on the screen, and the timing is such that they are distributed evenly over the viewing area. The lines of one field are traced out in the spaces between
those of the previous field to satisfy the interlace requirement. The trig- gering of the field timebase by the broadcast sync pulses is critical for good interlacing performance.
Figure 13.18 is a representative block diagram of a field timebase sys- tem. It starts with a free-running oscillator capable of being triggered by the separated broadcast field sync pulses at 50 Hz (or 60 Hz for NTSC) rate. The timing pulse produced by the oscillator triggers a precision ramp generator whose output is a sawtooth waveform with a period of 20 ms (or 17.6 ms for NTSC) and excellent linearity. It is amplified by a driver stage for application to the power output stage and thence to the field coils in the deflection yoke. The a.c. and d.c. feedback are used respectively for linearity correction and stabilisation of the working conditions.