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It is theoretically shown that the capacity of a voiceband channel operating through a digital network is at least 56 kbps (for a 3.5 kHz baseband bandwidth). The capacity may actually be higher but how close one can come to 64 kbps, the limit imposed by the digital network itself, is unknown. The transmission method is based on generating nonuniformly pulsed PAM (pulse amplitude modulation) signals which at the network sampling instants go through one of the slicing levels of the A/D converter of the network. To accomplish this, both the slicing levels and the sampling clock times must be known. This is a hard practical problem and one can only conjecture if a reasonable solution will be possible. The signal may be recovered using a Viterbi detector to reduce intersymbol interference effects, but at present, the number of states required for this detector is very large. >

In modern voiceband data communication using T1 carrier systems, the received signal is subject to nonlinear quantization noise due to companding. In order to design better signal constellations for this environment, it is of interest to characterize this noise. An analytical expression for the probability density function (PDF) of the nonlinear quantization noise and a numerical result for the PDF of the filtered noise, which is the actual noise at the slicer input of the linear receiver, are obtained. The results are compared to a Gaussian PDF. It is concluded that a Gaussian approximation, although not entirely accurate, is not unreasonable. >