Loop Plant Electronics: Analog Loop Carrier Systems

The analog carrier systems used in the loop plant fall into two distinct groups: the single-channel systems that piggyback a carrier channel on top of an existing voice channel and the multichannel systems where four to eight voice channels are multiplexed onto a single pair of wires. The single-channel systems piggyback a carrier channel on top of an existing voice channel without disturbing the baseband channel in any appreciable manner. These systems are limited in their use to urban and suburban areas where no intermediate repeaters or pair conditioning, like removal of load coils, is necessary. In their range of application, they provide the most economic or frequently the most expeditious pair gain technique. The multichannel systems, on the other hand, tend to be used on longer routes with repeaters. These systems, because of having to 1109 deload the line for carrier frequency transmission and difficulties in handling baseband signals at the repeaters, do not attempt to preserve the normal baseband transmission on the wire pair and all channels are derived via carrier techniques. In both the single and multichannel systems double-sideband AM with transmitted carrier is popular for several reasons. Wideband FM,1 even though practical for single-channel systems with some advantages over DSB-AM, has limited use for multichannel systems because of the large bandwidth requirements. The desirable compatibility between single and multichannel systems used in the same cable usually forces both systems to use the same or similar modulation technique and consequently wideband FM is not common for single channel systems.