Speech coding

In all digital cellular systems, speech coding must take into account the reduced bandwidth. The North American CDMA system uses two types of encoder.

The first type is a variable bit rate encoder, with a maximum bit rate of 8 kbit/s. This encoder transmits 8 kbit/s of speech in a 9.6 kbit/s channel, once the redundancy and error correction bits necessary for transmission on the radio channel have been added.

With this encoder, 18 simultaneous traffic channels are available in one 1.23 MHz channel.

This encoder improves the quality of transmitted speech by suppressing background noise. Any constant noise, such as city traffic noise or car engine noise, is eliminated. Constant noise is seen by the vocoder as noise that carries no useful information, and is therefore suppressed as far as possible. As a result, speech...