CLAMPING AND BLANKING
In a monochrome monitor or TV the route from video input socket or detector to picture-tube includes a low-level video amplifier as an interface between the source and the output stage. Here or at the output stage itself, two services are required. Because the response of the entire video path is unlikely to extend to d.c. and very low frequencies, the video signal must be clamped. At some known reference level in the waveform (i.e. back-porch or line sync pulse tip) the signal is returned to a fixed voltage by a diode or transistor switch. DC restorers operate on a passive basis; driven clamps have better performance, using an electronic switch driven by a suitably timed pulse from the line scan or sync separator stage of the set. Clamping ensures that picture black-level always coincides with the cut-off point of the picture-tube, and remains stable with respect to time, temperature and picture content.
Blanking takes place at line and field scanning rate, and cuts off the video amplifier except when picture information is actually present. Thus spurious images from the flyback action of the scanning spot (during which burst, teletext and other signals are present) are suppressed.
What has been described so far in this chapter, and in Chapter 3, completes the signal processing arrangements for monochrome TV sets and monitors.