Modulator having improved bandwidth and power requirements

摘要

A modulator is disclosed for use with digitized multiplexed voice channels in which service bits are transmitted using QPSK modulation while the audio information is transmitted using amplitude and phase modulation. The audio information in each channel is digitized and compressed using a mapping technique in which zero crossings of the audio signal are represented by binary O. The digitized channels are then multiplexed and coupled to the address inputs of a read-only memory. In the memory are stored digital numbers which represent in quadrature the amplitude and phase of the pulse to be transmitted corresponding to the digital data sample or service word then being presented at the address inputs of the memory. The digital numbers for the control words represent a constant amplitude while those corresponding to the data words represent an amplitude dependent upon the level of the original audio signal. No output carrier is produced for zero-value audio signals. A greater number of audio channels may be multiplexed within a given bandwidth than with previous systems while greatly reducing the required transmitter power.