Qualcomm code-excited linear prediction (QCELP), also known as Qualcomm PureVoice, is a speech codec developed in 1994 by Qualcomm to increase the speech quality of the IS-96A codec earlier used in CDMA networks.
It was later replaced with EVRC since it provides better speech quality with fewer bits.
In CDMA systems, a QCELP vocoder converts a sound signal into a signal transmissible within a circuit.
A QCELP vocoder with variable rates can reduce the rate enough to fit a wireless system by coding the information more efficiently.
In particular, it can change its own coding rates based on the speaker's volume or pitch; a louder or higher-pitched voice requires a higher rate.