Pulse Code Modulation (PCM)

PCM encoding is from the waveform encoding family. Speech is sampled at 8KHz. Each sample is then quantized either by linear or non-linear (logarithmically) quantization. For linear quantization, a 12 bits is used to encode each sample that gives 96 Kbps. On the other hand, when using non-linear quantization, only 8 bits are sufficient to encode each sample giving a bit rate of 64 Kbps. The quality of the encoded signal, in the latter, is approximately the same as the original one (about 4.6 on the 5-point ITU subjective quality scale). There are two variant of PCM codecs: A-law and u-law codecs, which are used in America and Europe respectively. They are also referred as the ITU-T G711 codec. They are widely used because of their simplicity and the very good quality they offer without significant complexity. It is easy to use them in software encoder as they do not demand much CPU power. Hence they are widely used in real-time Internet audio conferencing applications. The only disadvantage of PCM codecs is the resulting bit rate, which is sometimes high with respect to other codecs. Thus it is not suitable for archiving too long speech or audio material.