Post-quantization gain correction in audio coding
A gain adjustment apparatus ( 60 ) for use in decoding of audio that has been encoded with separate gain and shape representations includes an accuracy meter ( 62 ) configured to estimate an accuracy measure (A(b)) of the shape representation (Ñ(b)), and to determine a gain correction (g c (b)) based on the estimated accuracy measure (A(b)). It also includes an envelope adjuster ( 64 ) configured to adjust the gain representation (Ê(b)) based on the determined gain correction.
1. A method of decoding an encoded audio signal comprising:
receiving an encoded audio signal comprising a set of gain values and a corresponding set of shape vectors, each gain value representing the energy of a frequency sub-band in a frequency transform of an input audio signal, and each corresponding shape vector representing a fine structure of the frequency transform in the frequency sub-band;
determining an accuracy measure for each shape vector as a function of a quantization resolution the shape vector, the accuracy measure reflecting how accurately the shape vector represents the fine structure of the frequency transform in the frequency sub-band corresponding to the shape vector, and wherein higher quantization resolutions correspond to higher accuracy and lower quantization resolutions correspond to lower accuracy;
obtaining a set of corrected gain values by scaling each gain value as a function of the accuracy measure calculated for the corresponding shape vector;
synthesizing an audio signal from the set of corrected gain values and the corresponding set of shape vectors; and
outputting the synthesized audio signal.
2. The method of claim 1 , wherein each shape vector comprises a pulse vector and wherein calculating the accuracy measure for the shape vector comprises calculating the accuracy measure as a function of the number of pulses allocated to the pulse vector, as said quantization resolution, and a maximum pulse height for the pulse vector, and wherein greater pulse allocations correspond to higher accuracy and smaller pulse allocations correspond to lower accuracy.
3. The method of claim 2 , further comprising determining the accuracy measure for each shape vector as a function of the number of pulses allocated to the pulse vector in relation to a bandwidth of the frequency sub-band corresponding to the shape vector.
4. The method of claim 1 , wherein scaling each gain value as a function of the corresponding accuracy measure comprises obtaining a gain correction factor from a stored table of gain correction factors indexed as a function of accuracy measures, and applying the gain correction factor to the gain value.
5. The method of claim 1 , wherein determining the accuracy measure for each shape vector comprises obtaining the accuracy measure from a stored table of accuracy measures indexed as a function of quantization resolution.
6. The method of claim 1 , wherein determining the accuracy measure for each shape vector comprises determining the accuracy measure as a linear function of an allocated bit rate.
7. The method of claim 1 , wherein scaling each gain value as said function of the accuracy measure calculated for the corresponding shape vector further includes adapting the scaling applied to the set of gain values in dependence on whether the encoded audio signal represents encoded speech or encoded music.
8. An apparatus configured to decode encoded audio signals and comprising:
input circuitry configured to receive an encoded audio signal comprising a set of gain values and a corresponding set of shape vectors, each gain value representing the energy of a frequency sub-band in a frequency transform of an input audio signal, and each corresponding shape vector representing a fine structure of the frequency transform in the frequency sub-band; and
gain correction circuitry configured to:
determine an accuracy measure for each shape vector as a function of a quantization resolution the shape vector, the accuracy measure reflecting how accurately the shape vector represents the fine structure of the frequency transform in the frequency sub-band corresponding to the shape vector, and wherein higher quantization resolutions correspond to higher accuracy and lower quantization resolutions correspond to lower accuracy; and
obtain a set of corrected gain values by scaling each gain value as a function of the accuracy measure calculated for the corresponding shape vector; and
output circuitry configured to:
synthesizing an audio signal from the set of corrected gain values and the corresponding set of shape vectors; and
output the synthesized audio signal.
9. The apparatus of claim 8 , wherein each shape vector comprises a pulse vector and wherein the gain correction circuitry is configured to calculate the accuracy measure for the shape vector by calculating the accuracy measure as a function of the number of pulses allocated to the pulse vector, as said quantization resolution, and a maximum pulse height for the pulse vector, and wherein greater pulse allocations correspond to higher accuracy and smaller pulse allocations correspond to lower accuracy.
10. The apparatus of claim 9 , wherein the gain correction circuitry is further configured to determine the accuracy measure for each shape vector as a function of the number of pulses allocated to the pulse vector in relation to a bandwidth of the frequency sub-band corresponding to the shape vector.
11. The apparatus of claim 8 , wherein the gain correction circuitry is configured to scale each gain value as a function of the corresponding accuracy measure by obtaining a gain correction factor from a stored table of gain correction factors indexed as a function of accuracy measures, and applying the gain correction factor to the gain value.
12. The apparatus of claim 8 , wherein the gain correction circuitry is configured to determine the accuracy measure for each shape vector by obtaining the accuracy measure from a stored table of accuracy measures indexed as a function of quantization resolution.
13. The apparatus of claim 8 , wherein the gain correction circuitry is configured to determine the accuracy measure for each shape vector as a linear function of an allocated bit rate.
14. The apparatus of claim 8 , wherein the gain correction circuitry is further configured to adapt the scaling applied to the set of gain values in dependence on whether the encoded audio signal represents encoded speech or encoded music.