Modulator and demodulator for data transmission systems

摘要

A MODEM has a delay equalizer for receiving a trellis-coded test signal transmitted from an opposite MODEM and compensating for delay distortion of this test signal on the transmission path. The output of this delay equalizer and the input test signal are selectively entered into a demodulator. The output of this demodulator is assigned to predetermined signal point coordinates by a decision circuit. A Viterbi decoder computes a branch metric representing the distance between each assigned point and receive signal point from the output signals of the decision circuit and the demodulator, and figures out the pass metrics of accumulated values based on the branch metrics so computed. A difference signal represents the difference between the maximum and minimum values of the pass metrics from the Viterbi decoder. A first difference signal corresponds to the pass metrics when the delay equalizer is not set for the transmission path, and a second difference signal corresponds to the pass metrics when the delay equalizer is set for the transmission path. A control circuit compares the first and second difference signals and sets the delay equalizer out of the transmission path if the first difference signal is greater than the second, or sets the delay equalizer in the transmission path if the second difference signal is greater than the first. Thus the MODEM automatically decides whether the delay equalizer is to be inserted in the transmission path.