Experimental pulse-code-modulation transmission for local-area telephony

Pulse-code modulation system is described which provides 24 channels, with 8 kc/s sampling, 70 quantization steps and amplitude compression. Speech samples are coded in 7 binary digits, with an 8th digit carrying signalling information and used as two half-digits at 4 kc/s. As the total number of transmitted digits corresponds to 1middot536 Mc/s, repeaters are required at 1 km spacing when 0middot64 mm cable pairs are used, Speech channels are converted into amplitude modulated pulses, compressed, sampled again, stored in a capacitor memory, and then converted into pulse-code signals by means of a battery of amplitude comparators. Regenerators are used for normalizing individual pulses and allocating them to time positions. Repeaters derive their own timing waveform from signals and have current switches as combined timing gates and discriminators. For ease of regeneration a unit-disparity unit-distance code is employed. The operation of the coder and decoder is described in some detail and synchronization problems are discussed. A programme of field tests executed in Madrid is described.