ontal and frame timebases, although it is feasible, using digital interlacing; instead of transmitting each adjacent horizontal line in technology, to provide only one set of pulses from which both rates sequence, alternate lines are scanned first, to form a field, with the can be derived by counting techniques. missing lines traced out subsequently as a second field, the two The way in which 625 line video signals are combined with line fields forming one complete frame. Since the eye hardly perceives sync pulses is shown in Figure 57.3. individual lines of a single frame, the effect is to provide 50 (60) Colour television requires the transmission of additional informa- 'images' per second, although transmitting only 25 (30) frames per tion which, in practice, takes the form of two chrominance signals, second, at the cost only of introducing a small degree of interline which together with the black and white (luminance) signal defines flicker at the 25Hz (30Hz) rate. It has also been shown that with the picture in terms of the three primary colours, red (R), green (G) digital memory in the receiver, it is possible to display an interlaced and blue (B). picture with sequential (progressive) scanning. With interlaced Any colour can be defined by three characteristics: its brightness scanning the final line of the first field and the first line of the second (luminance); its hue (dominant electromagnetic wavelength/fre- Rise and fall time 10-90% Line time pulse amplitude reference point line sync. 250 +50 ns White level line blanking 300 + 100 ns blanking Picture 12.05 us signal 0.7 V +20 mV 250 ns 1.55 us 250 ns front porch Blanking and black level 0.3 V +9 mV 5.8 us +600 ns Sync. back porch signal Sync. level 4.7 us I 100 ns sync. Figure 57.3 The waveform of a typical line showing synchronising signals Television transmission 57/5 Table 57.1 International standards for television broadcasting. (*Amplitude modulated sound.) System line/fields Channel width Video width VSB width Polarity vision Sound/vision (MHz) 