Method and apparatus for performing binary equalization in voice-band

Abstract

A phase modulation modem providing switch selectable speeds on voice-band channels utilizes four phase, modified eight phase, and optimized eight phase, two amplitude modulation techniques. A binary equalizer automatically corrects for envelope delay and amplitude distortion of the carrier signal caused by the voice-band channel. Each symbol time, the X-axis error and Y-axis error for the received symbol is determined by comparing the received X and Y components with stored ideal X and Y components for that symbol. These error X and error Y values are used to modify primary and secondary equalization constants. The equalization constants, thus modified during this symbol time, are used on the next received symbol to correct for the phase and amplitude distortion measured during past symbol times. This sequence is repeated for each symbol time. Preselected equalization constants are utilized on the first symbol received. To equalize the line very rapidly at start-up, a phase training signal is sent prior to data transmission. After this 'course' adjustment the equalizer operates on the received data symbols to very accurately correct the phase and amplitude distortions of the received symbols. A symbol quality signal is conveniently generated as a result of the equalizer operation.