Enhanced Variable Rate Codec
Enhanced Variable Rate CODEC (EVRC) is a speech codec used in CDMA networks. It was developed in 1995 to replace the QCELP vocoder which used more bandwidth on the carrier's network, thus EVRC's primary goal was to offer the mobile carriers more capacity on their networks while not increasing the amount of bandwidth or wireless spectrum needed. EVRC uses RCELP technology, which Qualcomm claims improves speech quality with lower bit rates.
EVRC compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speech input into output frames of one of three different sizes: full rate – 171 bits (8.55 kbit/s), half rate – 80 bits (4.0 kbit/s), eighth rate – 16 bits (0.8 kbit/s). 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 kbit/s.
Recently, however, SMV itself has been replaced by the new CDMA2000 4GV codecs. 4GV is the next generation 3GPP2 standards-based EVRC-B codec. 4GV is designed to allow service providers to dynamically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.
EVRC can be also used in 3GPP2 container file format - 3G2.
- 3GPP2 specification
- EVRC – The Savior of CDMA?
- RFC 4788 - Enhancements to RTP Payload Formats for EVRC Family Codecs
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