A Companded Coder for an Experimental PCM Terminal

The heart of the terminal for an experimental PCM system developed at Bell Laboratories is the “companded coder,” which consists of a logarithmic instantaneous compandor plus a linear (equal-step) coder. The companded coder plays a major role in determining the over-all performance of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11 2</tex> megabit system. This paper includes a discussion of the fundamental design concepts, the practical realization and the performance of the all-semiconductor equipment that performs the analog-to-digital and digital-to-analog conversions for the PCM terminal. The instantaneous compandor employs matched semiconductor diodes to obtain a logarithmic gain characteristic. A 7-digit network type coder performs the conversion of the signal from analog to digital form and vice versa.