The Enhanced Variable Rate Codec (EVRC) is based upon the RCELP
algorithm, appropriately modified for variable rate operation and for robustness
in the CDMA environment. RCELP is a generalization of the Code-Excited
Linear Prediction (CELP) algorithm. Unlike conventional CELP encoders,
RCELP does not attempt to match the original speech signal exactly. Instead
of attempting to match the original residual signal, RCELP matches a timewarped
version of the original residual that conforms to a simplified pitch
contour. The pitch contour is obtained by estimating the pitch delay once in
each frame and linearly interpolating the pitch from frame to frame. One benefit
of using this simplified pitch representation is that more bits are available in
each packet for the stochastic excitation and for channel impairment protection
than by traditional fractional pitch approach. This results in enhanced error
performance without impacting perceived speech quality in clear channel
conditions.
Features
Operates at 9.6/4.8/1.2 kbps bitrates
Small memory footprint
Low CPU usage
RCELP algorithm
Fully compliant with the TIA-EIA-IS-127 technical requirements
Enhanced error performance
Supports noise suppression
Supports post filtering
Can be easily ported to any DSP or RISC platform
Benefits
Highly optimized code ideal for resource constrained applications
