Method for reduction of aliasing introduced by spectral envelope adjustment in real-valued filterbanks
The present invention proposes a new method for improving the performance of a real-valued filterbank based spectral envelope adjuster. By adaptively locking the gain values for adjacent channels dependent on the sign of the channels, as defined in the application, reduced aliasing is achieved. Furthermore, the grouping of the channels during gain-calculation, gives an improved energy estimate of the real valued subband signals in the filterbank.
1. Apparatus for reducing or eliminating intermodulation introduced by aliasing in a real-valued filterbank used for spectral envelope adjustment, comprising:
means for analysing an input signal to obtain information; and
means for using the obtained information to restrain envelope adjustment capabilities of the real-valued filterbank by grouping gain-values of adjacent channels.
2. The apparatus in accordance with claim 1 , in which the real-valued filterbank is configured for generating a plurality of subband signals generated by filtering a signal using an analysis filterbank, the real-valued filterbank having subband filters, adjacent subband filters of the filterbank having transition bands overlapping in an overlapping range.
3. The apparatus of claim 2 , in which the means for analysing comprises means for examining a subband signal originating from a subband filter and an adjacent subband signal originating from an adjacent subband filter to determine, whether the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range to obtain grouped subband signals in response to a specific result of the examination, the specific result of the examination indicating that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range.
4. The apparatus of claim 3 , in which the means for using comprises means for calculating a first gain adjustment value and a second gain adjustment value for the grouped adjacent subband signals, wherein the means for calculating is operative
to determine an indication of a reference energy for the grouped subband signals and an energy estimate for an energy in the grouped adjacent subband signals, and
to calculate the gain adjustment values for the grouped subband signals based on the indication of the reference energy and the energy estimate.
5. The apparatus in accordance with claim 4 , in which the means for calculating is operative to calculate the first and the second gain adjustment values such that they differ by less than a predetermined threshold or are equal to each other.
6. The apparatus in accordance with claim 4 , in which the means for examining is operative to calculate signs of subband signals based on coefficients of prediction polynomials for the subband signal and the adjacent subband signal and to indicate a positive result, when the signs have a predetermined relationship to each other.
7. The apparatus in accordance with claim 6 , in which the means for examining is operative to apply an auto-correlation method or a co-variance method.
8. The apparatus in accordance with claim 6 , in which the prediction polynomial is a low order polynomial having a first order coefficient, wherein the order of the low order polynomial is smaller than 4 and in which the means for examining is operative to use the first order coefficient for calculating the signs of the subband signals.
9. The apparatus in accordance with claim 6 , in which the predetermined relationship is defined such that the subband signal having associated therewith the channel number k has a first sign and the adjacent subband signal having associated therewith the channel number k−1 has a second sign, which is opposite to the first sign.
10. The apparatus in accordance with claim 1 , in which an energy estimate for a subband signal is calculated by summing of squared subband samples of the subband signal.