A low-cost 4800 bit/s modem is described which is applicable to limited distance, direct-copper channels. The modulation technique is eight-phase PSK and both the transmitter and receiver algorithms are aimed at economy of implementation in digital circuitry. The transmitter uses digitally stored waveform elements, and the detection and clock recovery techniques are based on received signal polarity. The implementation of the transmitter and receiver algorithms in a 6800 microprocessor is described. Some experimental results are given.

Two quantization rules, optimum (minimum mean-square error) and logarithmic companding, are compared for application to digital speech transmission. Comparison is made on the basis of signal-to-quantizing noise ratio (S/N), subjective quality judgments, idle channel noise, and dynamic range. These quantizers are considered in both PCM and differential PCM configurations. A computer algorithm is described that yields the optimum quantizer levels for a given speech record. It has been found that the improvement in S/N which optimum quantization affords over conventional logarithmic quantization is offset by the greater idle channel noise and smaller dynamic range (range of talker volumes handled with a lower limit on S/N) of the optimum law.

The performance of differential PCM encoders used for both speech and data signals is investigated. Primarily concerned with data signals, this paper shows how differential PCM systems designed for speech perform with differential-phase-modulation, single sideband and partial response data signals operating at data rates of 1200-9600 bits/s. The seven signals considered in this study are shown in Table I. Also discussed are compromise differential PCM systems, which operate well for either speech or data but which are not optimum for either. These results show the advantages and limitations of using differential PCM on switched telecommunication networks carrying speech and data. This paper contains results generated by theoretical studies, computer simulation, and experiments with hardware encoders and modems.