Analog to digital conversion
The voice signal is converted to a digital signal using PCM.
Variable rate vocoder
The vocoder (Voice Coder) is used to compress the digital signal from the Codec (Code/Decode). The vocoder, used in a CDMA system, compresses the voice signal into various data rates. The data rate is dynamically determined by the users speech activity.
The vocoders are located at the BSC and in the phone.
The voice is compressed in the vocoder into either one of four rates (Full, 1/2, 1/4, or 1/8 rate). CDMA systems can use either an 8 kbps or 13 kbps vocoder.