and an amplitude representing the degree of saturation coding. After low pass filtering, the three level duobinary data signal is time division multiplexed with the time compressed vision signal Line 625 carries only data, with duobinary data inserted onto the baseband signal with a capacity of 1296 bits. The use of duobinary coding means that both vision and data signals are contained within a baseband of about 8.5MHz. The digital data are organized into 'packets' of 751 bits with 164 packets in each 625 line frame (4100 packets/s). The basic structure of a D-MAC packet frame is illus- trated on Figure 57.13, having, for example, up to eight 15kHz audio channels with 32kHz sampling and NICAM companding of 14 bits/sample to 10 bits/sample, or up to sixteen 7kHz 'commentary' audio channels. The total data capacity is optionally available for Figure 57.10 Principle of PAL automatic line correction 64 us (one line) by averaging: (a) hue A transmitted with a positive (R-Y) component; recieved as B because of a phase shift; (b) 164 packets of 751 bits same hue transmitted with negative (R-Y) component A'; Field-blanking interval received as B' due to a phase error equivalent to that at (a); Sound (c) after reversing the polarity of (b) the two received signals B and B' represent B and B" which when averaged Colour Luminance give the correct hue A difference lines 20.25 Mbit/s duobinary data Colour (not to scale) difference Luminance 0.8 V p-p Zero-level of colour difference White level Colour Line 625: Synchronization and service identification Luminance difference 1V p-p 1 V p-p Figure 57.13 Simplified structure of D-MAC packet frame Black level showing the distribution of vision and 20.25Mbit/s Time compressed Time compressed duobinary data. Organising the data into two sub-frames by 3:1 by 3:2 allows easy transcoding of one sub-frame into D2-MAC 64 us any mix of teletext data, high quality stereo audio, mono/stereo Figure 57.11 Baseband video waveform of a D-MAC 'commentary' channels, or for utilisation in part for