Digitalization of the Signal and Pulse Code Modulation

In general, the process of digitizing the signal creates an adequate digital model for a non-digital process. The challenge is trying to represent a real-life scenario as an algorithm. In some cases, the task appears to be much easier because of the existence of repeated samples.

Human voice transfer requires a 4-kHz frequency band. Based on the Nyquist sampling theorem, digital sampling must take place at least twice the highest frequency to reconstruct the analog signal accurately. As a result, the sampling theorem requires that, given that fa equals a 4-kHz frequency band , fs is greater or equal to 4000 x 2 = 8000 Hz.

Digitalization of voice as an analog signal includes several steps:

Step 1.	Sampling of the analog waves. The process of sampling involves adequately transforming the analog wave in a way that after recreating the signal from the sample, there is an accurate waveform of the signal. Internationally, the sampling requires 8000 times per second, or 8 kHz.
Step 2.	Quantization of the analog waves. The waves need to be quantized by adequately assigning 1s or 0s to the samples. Each input sample is assigned to a quantization interval that is closest to the amplitude height of the analog wave. A standard of 8 bits per sample is accepted as an international standard. NOTE The choice of 8 bits per sample, given the fact that even the existing microprocessors use a 32- and 64-bit architecture, seems outdated .
Step 3.	Coding of the bits. The bits must be coded in forms or frames to be transmitted over the links. All standards must support at least 64 kbps because 8000 samples x 8 bits / sample = 64 kbps.

A unit called a channel bank can perform sampling, quantization, and coding.

A coder -decoder (codec) device performs the first two functions of pulse code modulation (PCM), and the coding is performed by a data service unit/channel service unit (DSU/CSU). The process is effective and is much less vulnerable to noise, but it is more expensive. The digitalized voice is called PCM voice.