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. |