5.2.4 Bass post-filter gain quantization

26.4453GPPCodec for Enhanced Voice Services (EVS)Detailed algorithmic descriptionRelease 15TS

At 16.4 and 24.4 kbps, the bass post-filter gain is quantized on 2 bits. First the signal is reconstructed as defined in subclause 6.1.3. The bass post-filter is applied and the enhancement signal, is computed as described in subclause 6.1.4.2.

The residual signal is then low pass filtered in time domain by a convolution with the impulse response , impulse response simulating the filtering done in CDLFB at the decoder side. Moreover the signal is adjusted by an estimated gain  corresponding to the attenuation factor of the anti-harmonic components. It is estimated as follows;

(1)

The optimal gain adjustment is computed as:

(2)

The optimal gain adjustment is quantized on 2 bits as follows:

(3)

The quantization is adjusted in case the delta SNR provided by the estimated gain is detected as positive:

(4)

The index k is then modified in the following way:

(5)

The again adjustment is decoded as follows

(6)