MODEM FOR RF SUBSCRIBER TELEPHONE SYSTEM

摘要

A modem including a modulator section for converting a bit stream into a phase modulated intermediate frequency (IF) signal and a demodulator section for converting a similarly converted phase modulated IF signal into the bit stream from which the phase modulated IF signal was converted. In the bit stream each given number of successive bits defines a symbol. In the modulator section, each symbol is phase modulated by DPSK conversion. The phase modulated symbols are digitally filtered by a FIR digital filter to provide a filtered signal that, when converted into an analog signal, provides a modulation signal of a modulation frequency that is centered about a predetermined frequency and deviates therefrom in accordance with the value of the phase modulated symbol. The filtered signal is converted into an analog signal and mixed with a steadystate signal at a predetermined frequency to provide a phase modulated IF signal that is a frequency modulated (FM) signal having an IF frequency that is a modulation product of the modulation frequency and the predetermined frequency. In the receiver section, a FIR digital filter for reconstructing the phase modulated symbols from a received digital signal includes a microprocessor for filtering of the received digital signal, and the microprocessor includes a memory for storing FIR filter coefficients for use in the filtering of the received digital signal. The modem is operable in a training mode wherein a sequence of predetermined symbols is provided to the FIR filter in the modulator section and the phase modulated IF signal provided by the modulator section is provided to the demodulator section. The microprocessor then adjusts the FIR filter coefficients stored in the microprocessor memory until the reconstructed phase modulated symbols correspond to the sequence of predetermined symbols provided to the FIR filter in the modulator section.