Qualcomm code-excited linear prediction

Qualcomm code-excited linear prediction, usually shortened to QCELP and also sold as Qualcomm PureVoice, is a speech codec that Qualcomm developed in 1994 to improve on the speech quality of the IS-96A codec that CDMA networks had used before. It was later replaced by EVRC, which gave better quality with fewer bits. Two versions exist, QCELP8 and QCELP13, which run at 8 and 13 kilobits per second. The problem it addresses is cost on a wireless link. Wired telephone systems sample voice at 8 kHz with 8-bit samples, which needs 64 kilobits per second, an expensive rate for a cellular network. A QCELP vocoder converts the sound signal into a much smaller stream, and because it varies its coding rate with the speaker's volume or pitch, a louder or higher-pitched voice is given a higher rate while quieter speech uses less. The 13 kilobit version is also the data format that the QCP file format of RFC 3625 was defined to store.