 this modest degree of reduction can be implemented. For example, an significant bit of each 9 bit word should be transmitted first. The experimental system developed by the IBA investigated two main signal should be conveyed in NRZ (non return to zero) form with approaches. The first involved discarding alternate samples with the bit stream carried on either coaxial cable or fibre optic bearer. sub Nyquist sampling and the second used differential pulse code Peak to peak signal amplitude should lie between 400mV and modulation (DPCM). Variations and combinations of these two 700m measured across a 750 resistive load connected directly to basic techniques were simulated. the output terminals without any transmission line. In sub Nyquist sampling, advantage is taken of the two dimen- sional sampling nature of the digitized video signal to maintain 57.2.6 Digital transmission (links) horizontal and vertical spatial frequency response at the expense of some small loss of resolution and the introduction of some aliasing Digital transmission of video and/or audio programme channels in diagonal frequencies. DPCM removes some of the redundancy in over well engineered links is capable of offering a performance that a typical TV signal by transmitting only difference information is virtually independent of distance or of the number of regenera- between samples within some areas of the picture. ting, multiplexing or switching stages involved. Overall, quality is Excellent results were achieved with hybrid (H) DPCM, in which determined by the parameters chosen for the encoding of the signal every fourth word in a line quincunx structure remained an an and the performance of the encoders/decoders (codecs). Multi- unmodified 8 bit linear PCM sample. The pattern of these data plexed digital signals can be transmitted without significant mutual words was then used as the basis of two dimensional predictor/in- interference (crosstalk). Digital links offer flexibility in their ability terpolators for the intervening words, which 