Enhanced Variable Rate Codec
Encyclopedia
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.
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itself has been replaced by the new CDMA2000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.
EVRC can be also used in 3GPP2 container file format - 3G2
.
QCELP
Qualcomm code-excited linear prediction , also known as Qualcomm PureVoice, is a speech codec developed in 1994 by Qualcomm to increase the speech quality of the IS-96A codec earlier used in CDMA networks. It was later replaced with EVRC since it provides better speech quality with fewer bits...
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
RCELP
Relaxed code-excited linear prediction is a method used in some advanced speech codecs. The RCELP algorithm does not attempt to match the original signal exactly. Instead, it matches a time-warped version of this original signal that conforms to a simplified pitch contour....
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
EVRC-B
Enhanced Variable Rate Codec B is a speech codec used by CDMA networks. EVRC-B is an enhancement to EVRC and compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speechinput into output frames of one of the four different sizes:...
.
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.
EVRC was replaced by SMV
SMV
Selectable Mode Vocoder is variable bitrate speech coding standard used in CDMA2000 networks. SMV provides multiple modes of operation that are selected based on input speech characteristics....
which retained the speech quality and at the same time improved network capacity.
Recently, however, SMV
SMV
Selectable Mode Vocoder is variable bitrate speech coding standard used in CDMA2000 networks. SMV provides multiple modes of operation that are selected based on input speech characteristics....
itself has been replaced by the new CDMA2000
CDMA2000
CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
4GV codecs. 4GV
4GV
Qualcomm’s Fourth Generation Vocoder is a suite of voice speech codecs used by CDMA networks that allows the network operators to dynamically prioritize voice quality to increase network capacity while maintaining voice quality. Currently, the 4GV suite offers EVRC-B and EVRC-WB....
is the next generation 3GPP2 standards-based EVRC-B
EVRC-B
Enhanced Variable Rate Codec B is a speech codec used by CDMA networks. EVRC-B is an enhancement to EVRC and compresses each 20 milliseconds of 8000 Hz, 16-bit sampled speechinput into output frames of one of the four different sizes:...
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
3GP
3GP is a multimedia container format defined by the Third Generation Partnership Project for 3G UMTS multimedia services. It is used on 3G mobile phones but can also be played on some 2G and 4G phones....
.
External links
- 3GPP2 specification
- EVRC – The Savior of CDMA?
- RFC 4788 - Enhancements to RTP Payload Formats for EVRC Family Codecs