Qualcomm code-excited linear prediction (QCELP), also known as Qualcomm PureVoice, is a
speech codec
Speech coding is an application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic d ...
developed in 1994 by
Qualcomm to increase the speech quality of the
IS-96A codec earlier used in
CDMA
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
networks. It was later replaced with
EVRC since it provides better speech quality with fewer bits. The two versions, ''QCELP8'' and ''QCELP13'', operate at 8 and 13
kilobits per second
In telecommunications, data-transfer rate is the average number of bits ( bitrate), characters or symbols ( baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are mul ...
(Kbit/s) respectively.
In
CDMA
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
systems, a QCELP
vocoder
A vocoder (, a portmanteau of ''voice'' and ''encoder'') is a category of speech coding that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption or voice transformation.
The vocoder was ...
converts a sound signal into a signal transmissible within a circuit. In wired systems, voice signals are generally sampled at 8 kHz (that is, 8,000 sample values per second) and then encoded by 8-bit
quantization for each sample value. Such a system transmits at 64 kbit/s, an expensive rate in a wireless system. 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.
References
* https://web.archive.org/web/20130129041829/http://www.rfdh.com/ez/system/db/lib_jnl/upload/2489/
SD9904QCELP_Vocoders_in_CDMA_Systems_Design.pdf
Archived PureVoice encoder and player downloads
{{DEFAULTSORT:Qcelp
Speech codecs
Qualcomm