5.2 Modification of the speech encoding algorithm during SID frame generation

3GPP46.062Comfort noise aspects for Enhanced Full Rate (EFR) speech traffic channelsRelease 17TS

When the SP flag is equal to "0" the speech encoding algorithm is modified in the following way:

– The non-averaged LP parameters which are used to derive the filter coefficients of the filters and of the speech encoder are not quantized.

– The open loop pitch lag search is performed, but the closed loop pitch lag search is inactivated. The adaptive codebook gain is set to zero.

– No fixed codebook search is made. In each subframe the pulse positions and signs of the fixed codebook excitation are locally generated using uniformly distributed pseudo random numbers. The excitation pulses take values of +1 and ‑1 when comfort noise is generated. The fixed codebook comfort noise excitation generation algorithm is defined in clause 6.2.

– The memory of weighting filter is set to zero, i.e., the memory of is not updated.

– The ordinary LP parameter quantization algorithm is inactive. At the end of the hangover period the reference LSF parameter vector is calculated as defined in clause 5.1. For the rest of the comfort noise insertion period is frozen. The averaged LSF parameter vector is calculated each time a new SID frame is to be sent to the Radio Subsystem. This parameter vector is encoded into the SID frame as defined in clause 5.1.

– The ordinary fixed codebook gain quantization algorithm is inactive. At the end of the hangover period the reference fixed codebook gain is calculated as defined in clause 5.1. For the rest of the comfort noise insertion period is frozen. The averaged fixed codebook gain value is calculated each time a new SID frame is to be sent to the Radio Subsystem. This gain value is encoded into the SID frame as defined in clause 5.1.

– The predictor memories of the ordinary LP parameter quantization and fixed codebook gain quantization algorithms are reset when SP flag = "0", so that the quantizers start from their initial states when the speech activity begins again.

– The computation of the unquantized fixed codebook gain is performed based on the energy of the LP residual signal.