An instantaneous compressor and an instantaneous expandor both of which use the logarithmic characteristics of diodes to provide compression and expansion, respectively, of uncompressed analog PAM signals and compressed analog PAM signals. Both the compressor and expandor also include circuitry having a temperature dependent gain characteristic. The signal at the compressor output is a temperature independent compressed analog signal which follows the compressor's logarithmic encoding characteristic. The signal at the output of the expandor is a temperature independent uncompressed analog signal which is substantially identical to the analog signal at the compressor input.
To encode an analog to digital signal, a comparator compares an input analog signal with a reconstructed negative feedback analog signal and provides a comparison output digital bit for each comparison. A number of output digital bits is stored in a clocked shift register. A coincidence detector is coupled to the register and provides an output pulse when all the bits in the register are the same. A proportional coincidence integrator proportionally amplifies and integrates the coincidence detector output pulses. A slope polarity switch is coupled to the output of the integrator and to the first bit in the shift register and applies a polarity sign to the integrator output according to the bit value from the register. The output of the switch is integrated to form the reconstructed negative feedback analog signal which is coupled to the comparator for comparison with the input analog signal. The digital output is taken from the shift register first bit. To decode a digital to analog signal, the above circuit is used. The digital signal is applied directly to the input of the shift register, the comparator not being used for decoding. The output analog signal is the integrated output from the polarity switch.
A digital carrier telephony system for delivering voice and data telephony services to subscribers in rural locations having low population densities. A Central Office processes telephony signals having subscriber channels. A Terminal (COT), which is connected to the CO and to one end of a digital carrier line, can transmit outgoing digital carrier line signals on the digital carrier line in response to multiplexing the subscriber channels of telephony signals received on a wired link from the CO. The COT also can transmit telephony signals to the CO in response to demultiplexing subscriber channels of incoming digital carrier line signals received on the digital carrier line. At subscriber locations, multiple remote drop units are typically connected to the digital carrier line. The remote drop unit can insert one of the subscriber channels into the multiplexed subscriber channels of one of the incoming digital carrier line signals. In addition, the remote drop unit can retrieve one of the subscriber channels from the multiplexed subscriber channels of one of the outgoing digital carrier line signals. The remote drop unit also can pass those incoming and outgoing carrier line signals carried on the digital carrier line not intended for communication to the subscribers associated with that unit. Each remote drop unit can be powered in the field by coupling span power generated by the COT and carried on the digital carrier line.
