Converting Voice Signal Into VoIP

For the transmission of the voice signal over IP network, the analog voice signal must be converted to the digital VoIP packets. For that the voice signal must be pass through following steps in order: Sampling, Quantization, Encoding, and optionally Compression. 

Sampling

Sampling is the process of chopping analog signal into regular intervals.Harry Nyquist prove that if the signal is sampled at the rate of twice the highest frequency (2*4000 kHz), the samples will contain sufficient information to accurately reconstruct signal at the receiver end.The figure below shows the sampling of an analog signal.

Sampling

• The human ear can sense sounds from 20 to 20,000 Hz
• Human speech uses frequencies from 200 to 9000 Hz
• The telephone channel was designed to operate at frequencies of 300 to 4000 Hz

Quantization 

Quantization divides  an analog signal sample into a set of discrete steps that are closest in value to the original analog signal.

The amplitude range is divided into 16 segments (0 to 7 positive, and 0 to 7 negative). Starting with segment 0, each segment has less-granular intervals than the previous segment, which reduces the signal-to-noise ratio (SNR) and makes the segment uniform.The sample value ranges from +127 to -127.

Quantization

Encoding

Encoding convert decimal values in the binary expressions of either 1 and 0. After sampling the signal next step to encode samples for the transmission over the telephony network.This process is called pulse-code modulation(PCM).  The PCM process, as shown in Figure below.

Coding

•  The first bit (MSB) identifies polarity
•  Bits two, three, and four identify segment
•  Final four bits quantize the segment

In the United States, Canada, and Japan, mu-law is used. The rest of the world uses a-law.

Compression

Signal compression is used to reduce the bandwidth usage per call. The most common codec algorithms are presented in the table below.

Compression

Summery

These four steps are performed by the DSP {Digital Signal Processor} at the originating gateway.The VoIP packets are then traveled to the destination gateway. DSPs on the destination side decodes the payload and reverse the process performed on the originating gateway.