Method and apparatus for enhancing loudness of a speech signal
A speech filter ( 108 ) enhances the loudness of a speech signal by expanding the formant regions of the speech signal beyond a natural bandwidth of the formant regions. The energy level of the speech signal is maintained so that the filtered speech signal contains the same energy as the pre-filtered signal. By expanding the formant regions of the speech signal on a critical band scale corresponding to human hearing, the listener of the speech signal perceives it to be louder even though the signal contains the same energy.
1. A method of increasing the perceived loudness of a processed speech signal, the processed speech signal corresponding to a natural speech signal and having formant regions and non-formant regions and a natural energy level, the method comprising:
expanding the formant regions of the processed speech signal beyond a natural bandwidth by way of a warped linear prediction pole displacement model; and
restoring an energy level of the processed speech signal to the natural energy level;
wherein restoring the energy level occurs upon expanding the formant regions in accordance with a critical band scale set by a single warping factor.
2. A method of increasing the perceived loudness as defined in claim 1 , wherein the expanding and restoring are performed on a frame by frame basis of the processed speech signal using a warped finite impulse response (WFIR) and a warped infinite impulse response filter (WIIR) sharing a common warped delay line.
3. A method of increasing the perceived loudness as defined in claim 2 , wherein the expanding and restoring are selectively performed on the processed speech signal when the frame contains substantial vowelic content.
4. A method of increasing the perceived loudness as defined in claim 3 , wherein the vowelic content is determined by a voicing level.
5. A method of increasing the perceived loudness as defined in claim 4 , wherein the voicing level is indicated by a spectral flatness of the speech signal.
6. A method of increasing the perceived loudness as defined in claim 2 , wherein expanding the formant regions is performed to a degree, and wherein the degree depends on a voicing level of a present frame of the processed speech signal.
7. A method of increasing the perceived loudness as defined in claim 1 , wherein expanding and restoring are performed according to a non-linear frequency scale.
8. A method of increasing the perceived loudness as defined in claim 7 , wherein the non-linear scale is a critical band scale.
9. A speech filter, comprising,
an analysis portion having a set of filter coefficients determined by warped linear prediction analysis including pole displacement, the analysis portion having unit delay elements;
a synthesis portion having a set of filter coefficients determined by warped linear prediction synthesis including pole displacement, the synthesis portion having unit delay elements; and
a locally recurrent feedback element having a scaling value coupled to the unit delay elements of the analysis and synthesis portions thereby producing non-linear frequency resolution.
10. A speech filter as defined in claim 9 , wherein the scaling value of the locally recurrent feedback element is selected such that the non-linear frequency resolution correspond to a critical band scale.
11. A speech filter as defined in claim 9 , wherein the pole displacement of the synthesis and analysis portions is determined by voicing level analysis.
12. A method of processing a speech signal comprising:
expanding formant regions of the speech signal on a critical band scale using a warped pole displacement filter;
performing an auto-correlation analysis on portions of the speech signal to generate an auto-correlation sequence;
applying an all-pass transformation to the auto-correlation sequence to generate warped linear prediction coefficients;
performing a linear transform on the warped linear prediction coefficients to generate a sequence of bandwidth expanded warped linear prediction coefficients; and
filtering the speech signal with the bandwidth expanded warped linear prediction coefficients to expand formant bandwidths of the speech signal on a critical band scale.
13. The method of claim 12 , wherein the step of performing a linear transformation on the warped linear prediction coefficients includes binomial expansion.
14. The method of claim 13 , wherein the binomial expansion includes a warping factor that increases higher frequency formants by more than it expands lower frequency formants in accordance with a critical band scale established by the warping factor.
15. The method of claim 12 , wherein the step of filtering the speech signal uses a collapsed delay Direct Form II filter.