Filtering speech

1. A method of filtering a speech signal for speech encoding in a communications network, the method comprising:

determining, by a computing device, a cut off frequency for a filter, wherein a component of the speech signal in a frequency range less than the cut off frequency is to be attenuated by the filter;

receiving, at the computing device, the speech signal at the filter;

determining, by the computing device, at least one parameter of the received speech signal, the at least one parameter providing an indication of an energy of the component of the received speech signal that is to be attenuated, and the at least one parameter comprising at least a pitch frequency of the speech signal; and

adjusting, by the computing device, the cut off frequency based on the at least one parameter, thereby adjusting the frequency range to be attenuated, the adjusting comprising adjusting the cut off frequency to be less than or equal to the pitch frequency.

2. The method of claim 1 wherein the at least one parameter further comprises a signal to noise ratio of the speech signal.

3. The method of claim 2 , further comprising:

calculating a signal quality measure using the signal to noise ratio; and

adjusting the pitch frequency based on the signal quality measure.

4. The method of claim 1 further comprising smoothing the pitch frequency over a plurality of received frames of the speech signal.

5. The method of claim 4 wherein a pitch lag of the received speech signal is used to determine the pitch frequency, the method further comprising determining a pitch correlation value by correlating a first frame of the speech signal with a second frame of the speech signal delayed by the pitch lag, wherein frames for which the correlation value is below a threshold value are classified as unvoiced frames and frames for which the correlation value is at least the threshold value are classified as voiced frames, and wherein the smoothing of the pitch frequency is performed for voiced frames whilst the smoothed pitch frequency is kept constant for unvoiced frames.

6. The method of claim 2 wherein the adjusting further comprises decreasing the cut off frequency as the signal to noise ratio increases.

7. The method of claim 2 wherein the speech signal is split into frequency subbands and the signal to noise ratio is a signal to noise ratio of the lowest frequency subband.

8. The method of claim 1 wherein the at least one parameter is determined dynamically and the cut off frequency is adjusted dynamically.

9. The method of claim 1 wherein the at least one parameter is determined at least once per frame of the received speech signal and the cut off frequency is adjusted at least once per frame of the received speech signal.

10. The method of claim 1 wherein the component of the received speech signal that is to be attenuated is a speech component of the speech signal containing speech.

11. A filter for filtering a speech signal for speech encoding in a communications network, the filter having:

a cut off frequency, wherein a component of the speech signal in a frequency range less than the cut off frequency is to be attenuated by the filter;

means for determining at least one parameter of the received speech signal, the at least one parameter providing an indication of energy of the component of the received speech signal that is to be attenuated, and the at least one parameter comprising at least a signal to noise ratio of the speech signal; and

means for adjusting the cut off frequency based on the at least one parameter, thereby adjusting the frequency range to be attenuated, the means for adjusting the cut off frequency configured to decrease the cut off frequency as the signal to noise ratio increases.

12. The filter of claim 11 wherein the at least one parameter further comprises a pitch frequency of the speech signal.

13. The filter of claim 11 wherein the at least one parameter further comprises a pitch lag of the speech signal.

14. The filter of claim 13 , further comprising:

means for calculating a signal quality measure using the signal to noise ratio; and

means for adjusting the determined pitch frequency based on the signal quality measure.

15. A computer storage device having computer-executable instructions stored on that, when executed by a processor, perform a method of filtering a speech signal for speech encoding in a communications network, the method comprising:

determining a cut off frequency for a filter, wherein a component of the speech signal in a frequency range less than the cut off frequency is to be attenuated by the filter;

receiving the speech signal at the filter;

determining at least one parameter of the received speech signal, the at least one parameter providing an indication of the energy of the component of the received speech signal that is to be attenuated, and the at least one parameter comprising at least a pitch frequency of the speech signal; and

adjusting the cut off frequency in dependence on the at least one parameter, thereby adjusting the frequency range to be attenuated, the adjusting comprising adjusting the cut off frequency to be less than or equal to the pitch frequency.

16. The computer storage device of claim 15 , wherein the at least one parameter further comprises a signal to noise ratio of the speech signal.

17. The computer storage device of claim 16 , wherein the computer-executable instructions, when executed by the processor, perform a method further comprising:

calculating a signal quality measure using the signal to noise ratio; and

adjusting the pitch frequency based on the signal quality measure.

18. The computer storage device of claim 15 , wherein the computer-executable instructions, when executed by the processor, perform a method further comprising smoothing the pitch frequency over a plurality of received frames of the speech signal.

19. The computer storage device of claim 18 , wherein a pitch lag of the received speech signal is used to determine the pitch frequency, and wherein the computer-executable instructions, when executed by the processor, perform a method further comprising determining a pitch correlation value by correlating a first frame of the speech signal with a second frame of the speech signal delayed by the pitch lag, wherein frames for which the correlation value is below a threshold value are classified as unvoiced frames and frames for which the correlation value is at least the threshold value are classified as voiced frames, and wherein the smoothing of the pitch frequency is performed for voiced frames whilst the smoothed pitch frequency is kept constant for unvoiced frames.

20. The computer storage device of claim 16 , wherein the adjusting further comprises decreasing the cut off frequency as the signal to noise ratio increases.