8. Codec - Coder/Decoder

What is the function of Codec?

• Voice transmission is in analog signal, whereas the data network is digital.
  
• So codec is help to convert analog signal into digital form and transport it to the network. There are many standards to sample an analog signal and the process is done through PCM(Pulse Code Modulation or variations.
• How the PCM works? Firstly the analog signal is input to a low pass filter. After that, the filter signal is sampled using one of the codec. Next the waveform is converted into a discrete digital signal. This sample is than represented by a code that indicate the amplitude of the waveform at the instant the sample was taken.

The following show an example of PCM:


Types of codec

• G.711 - It is a 64kps PCM voice encoding method that encode voice that is ready in PCM format for digital voice delivery in the public network or through PBX.
• G.726 - Describe ADPCM coding at 40,32,24,and 16kpbs. It allow user to interchange ADPCM voice between packet voice and public phone or PBX.
• G.728 - A 16kpbs low-delay variation of CELP voice compression.
• G.729 - A CELP compression that enale voice to be coded into 8kpbs streams. There are two types of variation of this standard(A and B) that are differ in computational complexity and both also provide good speech quality.
• G.723.1 - It compresses speech or other audio signal components of multimedia service at low bit rate, as part of the overall H.324 Standards. There are two bit rates associated with this coder : 5.3 and 6.3kpbs.