Systems, methods, apparatus, and computer program products for wideband speech coding

1. A method of processing an audio signal having frequency content in a low-frequency subband and in a high-frequency subband that is separate from the low-frequency subband, said method comprising:

filtering the audio signal to obtain a narrowband signal and a superhighband signal;

based on information from the narrowband signal, calculating an encoded narrowband excitation signal;

based on information from the encoded narrowband excitation signal, calculating a superhighband excitation signal;

based on information from the superhighband signal, calculating a plurality of filter parameters that characterize a spectral envelope of the high-frequency subband; and

calculating a plurality of gain factors by evaluating a time-varying relation between a signal that is based on the superhighband signal and a signal that is based on the superhighband excitation signal,

wherein the narrowband signal is based on the frequency content in the low-frequency subband, and

wherein the superhighband signal is based on the frequency content in the high-frequency subband, and

wherein a width of the low-frequency subband is at least three kilohertz, and

wherein the low-frequency subband and the high-frequency subband are separated by a distance that is at least equal to half of the width of the low-frequency subband.

2. The method according to claim 1 , wherein the frequency content of the low-frequency subband includes a component having a frequency at least equal to three kilohertz, and

wherein the frequency content of the high-frequency subband includes a component having a frequency not greater than eight kilohertz.

3. The method according to claim 1 , wherein the low-frequency subband and the high-frequency subband are separated by at least twenty-five hundred Hertz.

4. The method according to claim 1 , wherein said plurality of filter parameters includes a plurality FCH of filter coefficients that characterize a spectral envelope of a frame of the high-frequency subband, and

wherein said method includes calculating a plurality FCL of filter coefficients that characterize a spectral envelope of a corresponding frame of the low-frequency subband, and

wherein FCH is less than FCL.

5. The method according to claim 1 , wherein said filtering the audio signal includes:

resampling a signal that is based on the frequency content in the high-frequency subband to obtain a resampled signal; and

performing a spectral reversal operation on a signal that is based on the resampled signal to obtain a spectrally reversed signal,

wherein the superhighband signal is based on the spectrally reversed signal.

6. The method according to claim 1 , wherein said calculating the superhighband excitation signal includes:

upsampling a signal that is based on the information from the encoded narrowband excitation signal to produce an interpolated signal; and

extending the spectrum of a signal that is based on the interpolated signal to produce a spectrally extended signal, and

wherein the superhighband excitation signal is based on the spectrally extended signal.

7. The method according to claim 1 , wherein said encoded narrowband excitation signal includes a fixed codebook index and an adaptive codebook index.

8. The method according to claim 1 , wherein the narrowband signal has a first sampling rate, and

wherein the width of the high-frequency subband is greater than fifty percent of the first sampling rate.

9. The method according to claim 8 , wherein the width of the high-frequency subband is at least equal to seventy-five percent of the first sampling rate.

10. The method according to claim 1 , wherein the width of the high-frequency subband is at least six kilohertz.

11. The method according to claim 1 , wherein the high-frequency subband includes the frequency range of from eight kilohertz (8 kHz) to eighty-five hundred Hertz (8500 Hz), and

wherein the high-frequency subband includes the frequency range of from thirteen kilohertz (13 kHz) to thirteen-and-one-half kilohertz (13,500 Hz).

12. The method according to claim 1 , wherein the audio signal has frequency content in a mid-frequency subband that is different from the low-frequency subband, and

wherein said filtering the audio signal includes obtaining a highband signal that is based on the frequency content in the mid-frequency subband, and

wherein said method includes:

calculating a highband excitation signal based on information from the encoded narrowband excitation signal;

based on information from the highband signal, calculating a plurality of filter parameters that characterize a spectral envelope of the mid-frequency subband; and

calculating a second plurality of gain factors by evaluating a time-varying relation between a signal that is based on the highband signal and a signal that is based on the highband excitation signal.

13. The method according to claim 12 , wherein said calculated plurality of gain factors includes a plurality n of gain factors that describe a relation between (A) a frame of the signal that is based on the superhighband signal and (B) a corresponding frame of the signal that is based on the superhighband excitation signal, and

wherein said second plurality of gain factors includes a plurality m of gain factors that describe a relation between (A) a frame of the signal that is based on the highband signal and (B) a corresponding frame of the signal that is based on the highband excitation signal, wherein n is greater than m.

14. The method according to claim 12 , wherein said calculating the superhighband excitation signal includes extending the spectrum of the encoded narrowband excitation signal into a frequency range occupied by the high-frequency subband, and

wherein said calculating the highband excitation signal includes extending the spectrum of the encoded narrowband excitation signal into a frequency range occupied by the mid-frequency band.

15. The method according to claim 12 , wherein the mid-frequency subband includes frequencies between five kilohertz and six kilohertz, and

wherein the high-frequency subband includes frequencies between ten kilohertz and eleven kilohertz.

16. The method according to claim 12 , wherein the narrowband signal has a first sampling rate, and

wherein the highband signal has a second sampling rate that is less than the first sampling rate.

17. The method according to claim 16 , wherein the superhighband signal has a third sampling rate that is less than the sum of the first and second sampling rates.

18. The method according to claim 12 , wherein said plurality of filter parameters that characterize a spectral envelope of the high-frequency subband includes a plurality FCH of filter coefficients that characterize a spectral envelope of a frame of the high-frequency subband, and

wherein said plurality of filter parameters that characterize a spectral envelope of the mid-frequency subband includes a plurality FCM of filter coefficients that characterize a spectral envelope of a corresponding frame of the mid-frequency subband, and wherein FCM is less than FCH.

19. An apparatus for processing an audio signal having frequency content in a low-frequency subband and in a high-frequency subband that is separate from the low-frequency subband, said apparatus comprising:

means for filtering the audio signal to obtain a narrowband signal and a superhighband signal;

means for calculating an encoded narrowband excitation signal based on information from the narrowband signal;

means for calculating a superhighband excitation signal based on information from the encoded narrowband excitation signal;

means for calculating a plurality of filter parameters, based on information from the superhighband signal, that characterize a spectral envelope of the high-frequency subband; and

means for calculating a plurality of gain factors by evaluating a time-varying relation between a signal that is based on the superhighband signal and a signal that is based on the superhighband excitation signal,

wherein the narrowband signal is based on the frequency content in the low-frequency subband, and

wherein the superhighband signal is based on the frequency content in the high-frequency subband, and

wherein a width of the low-frequency subband is at least three kilohertz, and

wherein the low-frequency subband and the high-frequency subband are separated by a distance that is at least equal to half of the width of the low-frequency subband.

20. The apparatus according to claim 19 , wherein the frequency content of the low-frequency subband includes a component having a frequency at least equal to three kilohertz, and

wherein the frequency content of the high-frequency subband includes a component having a frequency not greater than eight kilohertz.

21. The apparatus according to claim 19 , wherein the low-frequency subband and the high-frequency subband are separated by at least twenty-five hundred Hertz.

22. The apparatus according to claim 19 , wherein said plurality of filter parameters includes a plurality FCH of filter coefficients that characterize a spectral envelope of a frame of the high-frequency subband, and

wherein said apparatus includes means for calculating a plurality FCL of filter coefficients that characterize a spectral envelope of a corresponding frame of the low-frequency subband, and

wherein FCH is less than FCL.

23. The apparatus according to claim 19 , wherein said means for filtering the audio signal includes:

means for resampling a signal that is based on the frequency content in the high-frequency subband to obtain a resampled signal; and

means for performing a spectral reversal operation on a signal that is based on the resampled signal to obtain a spectrally reversed signal,

wherein the superhighband signal is based on the spectrally reversed signal.

24. The apparatus according to claim 19 , wherein said means for calculating the superhighband excitation signal includes:

means for upsampling a signal that is based on the information from the encoded narrowband excitation signal to produce an interpolated signal; and

means for extending the spectrum of a signal that is based on the interpolated signal to produce a spectrally extended signal, and

wherein the superhighband excitation signal is based on the spectrally extended signal.

25. The apparatus according to claim 19 , wherein said encoded narrowband excitation signal includes a fixed codebook index and an adaptive codebook index.

26. The apparatus according to claim 19 , wherein the narrowband signal has a first sampling rate, and

wherein the width of the high-frequency subband is greater than fifty percent of the first sampling rate.

27. The apparatus according to claim 26 , wherein the width of the high-frequency subband is at least equal to seventy-five percent of the first sampling rate.

28. The apparatus according to claim 19 , wherein the width of the high-frequency subband is at least six kilohertz.

29. The apparatus according to claim 19 , wherein the high-frequency subband includes the frequency range of from eight kilohertz (8 kHz) to eighty-five hundred Hertz (8500 Hz), and

wherein the high-frequency subband includes the frequency range of from thirteen kilohertz (13 kHz) to thirteen-and-one-half kilohertz (13,500 Hz).

30. The apparatus according to claim 19 , wherein the audio signal has frequency content in a mid-frequency subband that is different from the low-frequency subband, and

wherein said means for filtering the audio signal includes means for obtaining a highband signal that is based on the frequency content in the mid-frequency subband, and

wherein said apparatus includes:

means for calculating a highband excitation signal based on information from the encoded narrowband excitation signal;

means for calculating a plurality of filter parameters, based on information from the highband signal, that characterize a spectral envelope of the mid-frequency subband; and

means for calculating a second plurality of gain factors by evaluating a time-varying relation between a signal that is based on the highband signal and a signal that is based on the highband excitation signal.

31. The apparatus according to claim 30 , wherein said calculated plurality of gain factors includes a plurality n of gain factors that describe a relation between (A) a frame of the signal that is based on the superhighband signal and (B) a corresponding frame of the signal that is based on the superhighband excitation signal, and

wherein said second plurality of gain factors includes a plurality m of gain factors that describe a relation between (A) a frame of the signal that is based on the highband signal and (B) a corresponding frame of the signal that is based on the highband excitation signal, wherein n is greater than m.

32. The apparatus according to claim 30 , wherein said means for calculating the superhighband excitation signal includes extending the spectrum of the encoded narrowband excitation signal into a frequency range occupied by the high-frequency subband, and

wherein said means for calculating the highband excitation signal includes extending the spectrum of the encoded narrowband excitation signal into a frequency range occupied by the mid-frequency band.

33. The apparatus according to claim 30 , wherein the mid-frequency subband includes frequencies between five kilohertz and six kilohertz, and

wherein the high-frequency subband includes frequencies between ten kilohertz and eleven kilohertz.

34. The apparatus according to claim 30 , wherein the narrowband signal has a first sampling rate, and

wherein the highband signal has a second sampling rate that is less than the first sampling rate.

35. The apparatus according to claim 34 , wherein the superhighband signal has a third sampling rate that is less than the sum of the first and second sampling rates.

36. The apparatus according to claim 30 , wherein said plurality of filter parameters that characterize a spectral envelope of the high-frequency subband includes a plurality FCH of filter coefficients that characterize a spectral envelope of a frame of the high-frequency subband, and

wherein said plurality of filter parameters that characterize a spectral envelope of the mid-frequency subband includes a plurality FCM of filter coefficients that characterize a spectral envelope of a corresponding frame of the mid-frequency subband, and wherein FCM is less than FCH.

37. An apparatus for processing an audio signal having frequency content in a low-frequency subband and in a high-frequency subband that is separate from the low-frequency subband, said apparatus comprising:

a memory; a processor;

a filter bank configured to filter the audio signal to obtain a narrowband signal and a superhighband signal;

a narrowband encoder configured to calculate an encoded narrowband excitation signal based on information from the narrowband signal; and

a superhighband encoder configured (A) to calculate a superhighband excitation signal based on information from the encoded narrowband excitation signal, (B) to calculate a plurality of filter parameters, based on information from the superhighband signal, that characterize a spectral envelope of the high-frequency subband, and (C) to calculate a plurality of gain factors by evaluating a time-varying relation between a signal that is based on the superhighband signal and a signal that is based on the superhighband excitation signal,

wherein the narrowband signal is based on the frequency content in the low-frequency subband, and

wherein the superhighband signal is based on the frequency content in the high-frequency subband, and

wherein a width of the low-frequency subband is at least three kilohertz, and

wherein the low-frequency subband and the high-frequency subband are separated by a distance that is at least equal to half of the width of the low-frequency subband.

38. The apparatus according to claim 37 , wherein the frequency content of the low-frequency subband includes a component having a frequency at least equal to three kilohertz, and

wherein the frequency content of the high-frequency subband includes a component having a frequency not greater than eight kilohertz.

39. The apparatus according to claim 37 , wherein the low-frequency subband and the high-frequency subband are separated by at least twenty-five hundred Hertz.

40. The apparatus according to claim 37 , wherein said plurality of filter parameters includes a plurality FCH of filter coefficients that characterize a spectral envelope of a frame of the high-frequency subband, and

wherein said narrowband encoder is configured to calculate a plurality FCL of filter coefficients that characterize a spectral envelope of a corresponding frame of the low-frequency subband, and

herein FCH is less than FCL.

41. The apparatus according to claim 37 , wherein said filter bank includes:

a resampler configured to resample a signal that is based on the frequency content in the high-frequency subband to obtain a resampled signal; and

a spectral reversal module configured to perform a spectral reversal operation on a signal that is based on the resampled signal to obtain a spectrally reversed signal,

wherein the superhighband signal is based on the spectrally reversed signal.

42. The apparatus according to claim 37 , wherein said superhighband encoder includes:

an upsampler configured to upsample a signal that is based on the information from the encoded narrowband excitation signal to produce an interpolated signal; and

a spectrum extender configured to extend the spectrum of a signal that is based on the interpolated signal to produce a spectrally extended signal, and

wherein the superhighband excitation signal is based on the spectrally extended signal.

43. The apparatus according to claim 37 , wherein the narrowband signal has a first sampling rate, and

wherein the width of the high-frequency subband is greater than fifty percent of the first sampling rate.

44. The apparatus according to claim 43 , wherein the width of the high-frequency subband is at least equal to seventy-five percent of the first sampling rate.

45. The apparatus according to claim 37 , wherein the width of the high-frequency subband is at least six kilohertz.

46. The apparatus according to claim 37 , wherein the high-frequency subband includes the frequency range of from eight kilohertz (8 kHz) to eighty-five hundred Hertz (8500 Hz), and

wherein the high-frequency subband includes the frequency range of from thirteen kilohertz (13 kHz) to thirteen-and-one-half kilohertz (13,500 Hz).

47. The apparatus according to claim 37 , wherein the audio signal has frequency content in a mid-frequency subband that is different from the low-frequency subband, and

wherein said filter bank is configured to obtain a highband signal that is based on the frequency content in the mid-frequency subband, and

wherein said apparatus includes:

a highband encoder configured (A) to calculate a highband excitation signal based on information from the encoded narrowband excitation signal, (B) to calculate a plurality of filter parameters, based on information from the highband signal, that characterize a spectral envelope of the mid-frequency subband, and (C) to calculate a second plurality of gain factors by evaluating a time-varying relation between a signal that is based on the highband signal and a signal that is based on the highband excitation signal.

48. The apparatus according to claim 47 , wherein said calculated plurality of gain factors includes a plurality n of gain factors that describe a relation between (A) a frame of the signal that is based on the superhighband signal and (B) a corresponding frame of the signal that is based on the superhighband excitation signal, and

wherein said second plurality of gain factors includes a plurality m of gain factors that describe a relation between (A) a frame of the signal that is based on the highband signal and (B) a corresponding frame of the signal that is based on the highband excitation signal, wherein n is greater than m.

49. A non-transitory computer-readable storage medium having tangible features that cause a machine reading the features to perform the following acts to process an audio signal having frequency content in a low-frequency subband and in a high-frequency subband that is separate from the low-frequency subband:

filter the audio signal to obtain a narrowband signal and a superhighband signal;

based on information from the narrowband signal, calculate an encoded narrowband excitation signal;

based on information from the encoded narrowband excitation signal, calculate a superhighband excitation signal;

based on information from the superhighband signal, calculate a plurality of filter parameters that characterize a spectral envelope of the high-frequency subband; and

calculate a plurality of gain factors by evaluating a time-varying relation between a signal that is based on the superhighband signal and a signal that is based on the superhighband excitation signal,

wherein the narrowband signal is based on the frequency content in the low-frequency subband, and

wherein the superhighband signal is based on the frequency content in the high-frequency subband, and

wherein a width of the low-frequency subband is at least three kilohertz, and

wherein the low-frequency subband and the high-frequency subband are separated by a distance that is at least equal to half of the width of the low-frequency subband.