PCM Overview

Known in Chinese as Pulse Code Modulation (PCM), proposed by A. Reeves in 1937, this concept laid the foundation for digital communications. In the 1960s, it began to be applied to local telephone networks to expand capacity, increasing the transmission capacity of most core pairs in existing audio cables by 24 to 48 times. By the mid-to-late 1970s, countries successively applied PCM successfully to medium- and large-capacity transmission systems such as coaxial cable communications, microwave relay communications, satellite communications, and optical fiber communications. In the early 1980s, PCM was used in local trunk transmission, large-capacity backbone transmission, and digital program-controlled switches, and was adopted in subscriber telephones.

In optical fiber communication systems, binary optical pulses "0" and "1" are transmitted through the fiber, generated by on-off modulation of the light source using binary digital signals. Digital signals are produced by sampling, quantizing, and encoding continuously varying analog signals, known as PCM (Pulse-code modulation). This electrical digital signal is called a digital baseband signal, generated by the PCM electrical terminal. Current digital transmission systems all adopt the Pulse-code modulation system. PCM was initially not intended for transmitting computer data but to enable a trunk line between switches to carry more than just one telephone signal. PCM has two standards (forms): E1 and T1.