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.