Enhanced Variable Rate Codec
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Enhanced Variable Rate CODEC (EVRC) is a speech codec used in CDMA networks. It was developed in 1995 to replace the QCELP codec which was of worse quality. EVRC uses RCELP technology, which improves speech quality with lower bit rates.
EVRC compresses each 20 milliseconds of 8000Hz, 16-bit sampled speech input into output frames of one of three different sizes: full rate – 171 bits (8.55 kbps), half rate – 80 bits (4.0 kbps), eighth rate – 16 bits (0.8 kbps). A quarter rate was not included in the original EVRC specification and eventually became part of EVRC-B.
A zero length frame is included in the specification for 'null' and erasure frames. The eighth (1/8) rate is not intended for speech signals, but instead for background noise. Due to the source controlled nature of the codec the average bit rate varies based on network conditions, but is usually around 6 kbps.[citation needed]
EVRC was replaced by SMV which retained the speech quality and at the same time improved network capacity.
Recently, however, SMV itself has been replaced by the new CDMA2000 4GV codecs. 4GV is the next generation 3GPP2 standards-based EVRC-B codec. 4GV is designed to allow service providers to dynamically prioritize voice capacity on their network as required.
