An Analog-to Digital Encoder Employing Negative Resistance Devices

A six-bit analog-to-digital (A/D) encoder for use in multiplexing a number of voice communication channels is described and experimental results presented. The method of encoding employs the two-terminal multistable voltage-current characteristic that can be produced with a number of negative resistance elements connected to a common source. The basic element is a modified transistor flip-flop having a relationship as seen between ground and its emitter terminal tailored in a desired manner. Five such flip-flops connected in parallel and a polarity bit extractor form the encoder proper. The encoding speed measured for an accuracy of 20 per cent of a step was 16 μsecs/character. The complete system that was built and tested includes the associated circuitry for holding and stretching the PAM signal as well as readout and generation of timing pulses. A decoder was built to allow objective as well as subjective tests on the over-all system. A signal to quantizing noise of 6 db/bit and 44.5 db over-all was measured with a 1100-cps sinewave at full load using a spectrum analyzer. Listening tests were also performed and the results discussed.