dard. The baseband video waveform of a D-MAC television channels as baseband data symbols and is designed for mini- line is shown in Figure 57.11. mum complexity in its data multiplex, requiring only 15 000 gates in the decoder IC. It is a fixed format system (i.e. not individually addressed 'packets'). C-MAC/packet transmits up to eight digital high quality audio channels directly modulated on to the carrier during the line (Burst amplitude reference symmetrical point about black level) Burst start 0.3 V 5.6 us I 100 ns +9 mV Black level Burst duration 10 I cycles Figure 57.8 The principle of synchronous demodulation (2.25 us +230 ns) of quadrature modulated signals. Two carrier waves are shown of the same frequency but in quadrature. If each is amplitude modulated by different information and if two Line sync. pulse demodulators, one synchronised with each carrier, sample 4.7 us I 100 ns the waves only at the times when one is at a peak and the other at null, then the two sets of information can be separately retrieved at the receiver. The solid line Figure 57.7 The colour burst on a standard level signal waveform is sampled at times to, t1, t4, etc. and the dashed (700mV white level) line curve at times t1, t3, t5, etc. Television transmission 57/9 Each line of luminance information is digitally compressed by a Green factor of 3:2 (52ps to about 35us). the U and V colour difference signals are reduced by a factor of 3:1 (52us to about 17.5 us). This has the effect of increasing the maximum baseband video frequency from about 5.7MHz to about 8.5MHz. The remaining 10us periods Hue indicated carry 20.25Mbit/s duobinary data, which take the form shown in by angular direction Figure 57.12, inserted on to the baseband signal. Each line period Amplitude thus has 206 data bits with a mean data capacity of about 3Mbit/s. denotes saturation Binary Magente Duobinary Figure 57.9 In the NTSC and PAL systems any hue can be represented by a phasor having a specific phase angle Figure 57.12 Comparison of binary and duobinary 