High-Speed Data Transmission Over a Group Link Telephone Channel

The studies carried out by the Centre National d´Etudes des Télécommunications had for its object the determination of the maximum modulation rate which is possible over a group link telephone channel of a modern carrier system. The data modulator-demodulator which has been realized for a modulation rate of about 3700 bauds makes use of vestigial sideband amplitude modulation. One particular point is the direct translation of the signals from the baseband into the 4-kc to 8-kc band at the transmitting end and inversely from the 4-kc to 8-kc band into the baseband at the receiving end. For this reason it is necessary to modify the first stages of the modulating chain and the last stages of the demodulating chain of the carrier system. Measurements of binary error rates have been made for a speed of 3700 bauds with addition of white noise at different levels (from 20,000 pw up to 180,000 pw, referred to zero transmission level). A typical result is an error rate of 10^-6for a noise power of 50,000 pw. Two applications are considered: providing special private lines for high-speed data transmission and designing time-division multichannel telegraphy.