5 Functions on the transmit (TX) side
3GPP46.062Comfort noise aspects for Enhanced Full Rate (EFR) speech traffic channelsRelease 17TS
The comfort noise evaluation algorithm uses the following parameters of the GSM Enhanced Full Rate speech encoder, defined in GSM 06.60 [3]:
– the unquantized and quantized Linear Prediction (LP) parameters, using the Line Spectral Pair (LSP) representation, where the unquantized Line Spectral Frequency (LSF) vector is given by 
, the quantized LSF vector is given by 
, and the two sets of unquantized and quantized LSF vectors (one for each half of a frame) are given by 
, 
, 
and 
, respectively;
– the quantized fixed-codebook gain 
.
The algorithm also computes the following parameters to assist in comfort noise generation:
– the reference LSF parameter vector 
(average of the quantized LSF parameters of the hangover period);
– the averaged LSF parameter vector 
(average of the LSF parameters of the eight most recent frames);
– the reference fixed codebook gain 
(average of the quantized fixed codebook gain values of the hangover period);
– the averaged fixed codebook gain 
(average of the fixed codebook gain values of the eight most recent frames);
– the unquantized fixed codebook gain 
.
These parameters give information on the level (
, 
, 
, 
) and the spectrum (
, 
, 
, 
, 
, 
) of the background noise.
Two of the evaluated comfort noise parameters (
and 
) are encoded into a special frame, called a Silence Descriptor (SID) frame, for transmission to the RX side. Since the reference LSF parameter vector 
and the reference fixed codebook gain 
can be evaluated in the same way in the encoder and decoder, as given in clause 5.1, no transmission of these parameters is necessary.
The averaged LSF parameter and fixed codebook gain values, 
and 
, are computed in the encoder using both quantized and unquantized parameter values if the period of the eight most recent frames (the SID averaging period) is overlapping with the hangover period (the parameters from the frames overlapping with the hangover period have quantized values, while the parameters of the more recent frames of the SID averaging period have unquantized values). If the period of the eight most recent frames is non-overlapping with the hangover period, the averaged LSF parameter and fixed codebook gain values are computed using only unquantized parameter values.
The SID frame also serves to initiate the comfort noise generation on the receive side, as a SID frame is always sent at the end of a speech burst, i.e., before the radio transmission is terminated.
The scheduling of SID or speech frames on the radio path is described in GSM 06.81 [5].