The control of noise and crosstalk on N1 carrier systems

THE 12-channel N1 carrier system is designed primarily for short- and medium-haul service and to make use of existing, and new, cable plant with a minimum of special treatment. As compared to other types of carriers, the lower transmission levels of the N1 system tend to make it more susceptive to interference from external sources. The use of higher line frequencies with the corresponding decrease in coupling loss between line conductors tends to increase the amount of crosstalk between carrier systems. While the technical features of the N1 system have already been described,1 it may be well to review some of the characteristics that are of interest in connection with the control of noise and crosstalk on message circuits. These include: 1. Transmission of both side bands and the carrier. 2. A compandor per channel. At the transmitting end, a compressor raises the average talker volume by about 10 decibels (db) and reduces by one-half the normal range of talker volumes. At the receiving end, a complementary expandor restores the talker volumes to their normal range. 3. Transmission in the two directions in different frequency ranges on separate pairs in the same cable. Low-group frequency range, 44 to 140 kc; high-group range, 164 to 260 kc; frequency inversion at each repeater by modulating the signal with a 304-kc carrier. 4. In each repeater section the carriers at the high-frequency end of the band are transmitted at higher level than those at the low-frequency end. Because of the increase in attenuation with frequency, the relative levels at the receiving end are reversed.