#### In-depth

## Voice coding

The most widely used form of source encoding to date is in fact based on these m-law and A-law companding A/D and D/A converters, often referred to as codecs, which form the digital interface for almost every telephone line in the world. At the telephone exchange or switching centre, each analogue signal from the domestic phone is converted using an 8-bit m-law or A-law codec, with a standardized sampling rate of 8000 times per second. (The maximum voice frequency is limited to 3400 Hz, hence the Nyquist criterion is met.) This results in a data rate of 64 kbps for each voice link.

At the exchange, a number of these eight-bit data words from different phone sources are collected (multiplexed) into a frame (32 for E-type systems and 24 for A-type systems), and then sent using either baseband signalling or bandpass signalling methods over the national and international exchange links. In order to keep pace with the codec sampling rate, a new frame must be constructed and sent every 1/8000 seconds.