Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, Compact Discs, digital telephony and other digital audio applications.

In a PCM stream, the amplitude of the analog signal is sampled regularly at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps.

Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. It is used when the data rate of the transmission medium exceeds that of signal to be transmitted.

When a large number of PCM signals are to be transmitted over a common channel, multiplexing of these PCM signals is required.

Figure shows the basic time division multiplexing scheme, called as the $T_1$ digital system.

This system is used to convey multiple signals over telephone lines using wideband coaxial cable.

Operation of the $T_1$ system.

The operation of the PCM-TDM system is as follows

• This system has been designed to accommodate 24 voice channels marked $S_1$ to $S_{24}$ .

• Each signal is bandlimited to 3.3kHz, and the sampling is done at a standard rate of 8 kHz.

• This is higher than the Nyquist rate.

• The sampling is done by the commutator switch ${SW}_1$ .

• These voice signals are selected one by one and connected to a PCM transmitter by the commutator switch $SW_1$.

  • These voice signals are selected one by one and connected to a PCM transmitter by the commutator switch $SW_1$.

• Each sampled signal is then applied to the PCM transmitter which converts it into a digital signal by the process of A to D conversion and companding, as explained earlier.

• The resulting digital waveform is transmitted over a co-axial cable.

• Periodically, after every 6000 ft, the PCM-TDM signal is regenerated by amplifiers called “repeaters”.

• They eliminate the distortion introduced by the channel and remove the superimposed noise and regenerate a clean PCM-TDM signal at their output.

• This ensures that the received signal is free from the distortions and noise.

• At the destination the signal is companded, decoded and demultiplexed, using a PCM receiver.

• The PCM receiver output is connected to different low pass filters via commutator switch $SW_2$ .

• Synchronization between the transmitter and receiver commutators $SW_1$ and $SW_2$ is essential in order to ensure proper communication.