A PCM system consists of a PCM encoder (transmitter) and a PCM decoder (receiver). The essential operations in the PCM transmitter are sampling, quantizing and encoding. All the operations are usually performed in the same circuit called as analog-to digital convert Early electrical communications started to sample signals in order to multiplex samples from multiple telegraphy sources and to convey them over a single telegraph cable. The American inventor Moses G. Farmer conveyed telegraph time-division multiplexing (TDM) as early as 1853. Electrical engineer W. M. Miner, in 1903, used an electro-mechanical commutator for time-division multiplexing multiple telegraph signals; he also applied this technology to telephony.
