What is PCM

Pulse code modulation (PCM) is used to transform an analogue wave form into digital that is compatible with a digital communication system, the following steps are taken.

Sampling- Digital signal transmission is done by having samples from analog signals. Sampling we obtain samples of an analogue signal within a certain time period. The analogue signal will sample at every sampling interval. According to the Nyquest theorem sampling frequency should be equal or greater than to the maximum frequency of the sound wave.

Quantization- Sampling gives a series of pulses with varying amplitude. Then the receiver can not identify the every finite value in the amplitude. So without sending the real sample we put the sample signals amplitude in to the recognized number of levels.  We ought to approximate the amplitude of the sample to the nearest quantization level. The heights between two levels are called a zone.

       

Encoding - Encoding means converting quantized levels into binary formats to suit for digital transmission. Encoding represent the sign, segment number and the sample

Compander in Pulse Code Modulation

Compander is a Signal processing technique which uses both compression and expansion to improve dynamic range and signal-to-noise ratio.

            Before the conversion of analog to digital in PCM, the compander boost up the low amplitudes of the voice.

In leaner quantization we give a low signal to distortion ratio and high ratio for high amplitudes. This is not useful, because if we get a low signal to distortion ratio, it let know the distortion is higher, compare to signal’s amplitude. Therefore we use compander for overcome that problem. What compander do is it only boost up the low amplitudes of the voice.

And also receiving end compander (expander) reduce the amplitude value of the boosted up signal.

But when we use nonlinear quantization there is a solution for this matter. It has lower gaps in quantized levels for low amplitudes and higher gaps in quantized levels for high amplitudes. Therefore we can achieve high signal to distortion ratio in both cases. And it is useful.

Analog over Digital

               The fundamental transmission concept is to transmit the minimum possible frequency or the bandwidth with maximum possible number of channels or the bit rate.

When we consider the analog transmission we can’t transmit much distance than digital transmission. Therefore we need to use small bandwidth. Using that small bandwidth can transmit much distance. Therefore we use modulation techniques to reduce the bandwidth. FDM multiplexing system is combining number of channels to convey over common transmission medium. Major advantage of analogue transmission is occupied band width is low When we use frequency modulation in low pass filter there are two, out come upper side bandwidth and lower side bandwidth. By choosing the lower side bandwidth we can reduce bandwidth of the signal.

             In digital transmission we can have more than solutions compare to analog.

Theoretically we know Nyquist Theorem, it tells us relationship between bandwidth and channel capacity.

                             C = 2Blog₂M

C- Channel capacity   ,    B – Bandwidth       ,    M – signal levels

If we want increase channel capacity for a value of bandwidth need to increase signal levels.

              In digital transmission we can use time division multiplexing (TDM).using this technique we can reduce bandwidth. In this TDM large numbers of channels are put into one line and transmit. . In the digital transmission can use the available bandwidth of the channel much more efficiently as compared to analogue system.