A Bipolar Repeater for Pulse Code Modulation Signals

Companion papers 1 - 3 6 propose transmission of 24 voice channels by means of pulse code modulation over 22-gauge paper insulated cable pairs. Seven-digit coding, built-in signaling, and frame synchronization dictate a 1.54-4-mc pulse rate. Very simply, the repeater function is to 25 26 T H E H E L L SYSTEM T E C H N I C A L J O U R N A L , J A N U A R Y 1 ! H > 2 "look" at a received pulse train and emit a " n e w " pulse for each received pulse, a feat t h a t becomes impossible when the transmission bandwidth is sufficiently limited, or the noise environment is sufficiently strong t h a t realizable circuitry cannot distinguish pulse from interference. In practice these parameters are controlled by repeater spacing, and installation of repeaters is greatly convenienced if the nominal spacing is 6000 feet (a repeater then replaces a load coil). Such spacing produces a very difficult interference situation. One of t h e m a n y virtues of P C M is t h a t the pulse train carries its own timing information, and this paper considers only the self-timed repeater. In such repeaters it is essential t h a t the recovered clock be and remain phase locked to the incoming pulse train, even in the presence of severe interference. Interference is mainly intra- or inter-system crosstalk via the nearend crosstalk path ( N E X T ) . Such interference is strongest at high frequencies where timing information must be transmitted. A second important interference source is office impulse noise, which is confined to the vicinity of offices and requires shortened repeater spacing adjacent to offices.