Adaptive Transition Frequency Between Noise Fill and Bandwidth Extension

• US 20110264454A1
• Filed: 08/26/2008
• Published: 10/27/2011
• Est. Priority Date: 08/27/2007
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Ullberg, Gustaf, Briand, Manuel, Taleb, Anisse
• Granted Patent: US 9,269,372 B2
• Time in Patent Office: Days
• US Class Current: 704/500
• CPC Class Codes: G10L 19/0204 using subband decomposition
• CPC Class Codes: G10L 19/028 Noise substitution, i.e. substituting non-tonal spectral components by noisy source comfort noise for discontinuous speech transmission G10L19/012
• CPC Class Codes: G10L 19/032 Quantisation or dequantisation of spectral components
• CPC Class Codes: G10L 19/035 Scalar quantisation
• CPC Class Codes: G10L 21/038 using band spreading techniques

Abstract

A method for spectrum recovery in spectral decoding of an audio signal, comprises obtaining (210) of an initial set of spectral coefficients representing the audio signal, and determining (212) a transition frequency. The transition frequency is adapted to a spectral content of the audio signal. Spectral holes in the initial set of spectral coefficients below the transition frequency are noise filled (214) and the initial set of spectral coefficients are bandwidth extended (216) above the transition frequency. Decoders and encoders being arranged for performing part of or the entire method are also illustrated.

Claims

1. Method for spectrum recovery in spectral decoding of an audio signal, comprising the steps of:

obtaining (210) an initial set (42) of spectral coefficients representing said audio signal;

determining (212) a transition frequency (f_t);

noise filling (214) of spectral holes in said initial set (42) of spectral coefficients below said transition frequency (f_t); and

bandwidth extending (216) said initial set (42) of spectral coefficients above said transition frequency (f_t);

said transition frequency (f_t) being adapted to a spectral content of said audio signal.

2. Method according to claim 1, wherein said transition frequency (f_t) is adaptively dependent on a distribution of spectral holes in said initial set (42) of spectral coefficients.

3. Method according to claim 2, wherein said step of determining said transition frequency (f_t) in turn comprises the steps of:

dividing said spectral coefficients of said initial set (42) of spectral coefficients into a plurality of frequency bands (74); and

selecting said transition frequency (f_t) dependent on a proportion of spectral holes in said frequency bands (74).

4. Method according to claim 3, wherein said frequency bands (74) have a constant frequency width.

5. Method according to claim 3, wherein at least two of said frequency bands (74) have different frequency widths.

6. Method according to any of the claims 3 to 5, wherein said step of selecting said transition frequency (f_t) comprises:

finding a transition frequency band, being a highest frequency band in which said proportion is lower than a first threshold.

7. Method according to claim 6, wherein said step of selecting said transition frequency (f_t) further comprises:

setting said transition frequency (f_t) dependent on an upper frequency limit of said transition frequency band.

8. Method according to claim 6 or 7, wherein said step of setting said transition frequency (f_t) is further dependent on a previously used transition frequency.

9. Method according to claim 8, wherein said step of setting said transition frequency (f_t) is further dependent on more than one previously used transition frequency.

10. Method according to claim 8 or 9, wherein said transition frequency (f_t) is prohibited to change more than a predetermined absolute or relative amount between two consecutive frames.

11. Method for use in spectral coding of an audio signal, comprising:

determining (212) a transition frequency (f_t) for an initial set (24;

42) of spectral coefficients representing said audio signal;

said transition frequency (f_t) defining a border between a frequency range, intended to be a subject for noise filling of spectral holes, and a frequency range, intended to be a subject for bandwidth extension;

said transition frequency (f_t) being adapted to a spectral content of said audio signal.

12. Decoder (40) for spectral decoding of an audio signal, comprising:

input for obtaining an initial set (42) of spectral coefficients representing said audio signal;

transition determining circuitry (60) arranged for determining a transition frequency (f_t);

a noise filler (50) for noise filling of spectral holes in said initial set (42) of spectral coefficients below said transition frequency (f_t); and

a bandwidth extender (55) arranged for bandwidth extending said initial set (42) of spectral coefficients above said transition frequency (f_t);

said transition frequency (f_t) being adapted to a spectral content of said audio signal.

13. Decoder according to claim 12, wherein said transition determining circuitry (60) is arranged for adaptively determining said transition frequency (f_t) dependent on a distribution of spectral holes in said initial set (42) of spectral coefficients.

14. Decoder according to claim 13, wherein said transition determining circuitry (60) is further arranged for dividing said spectral coefficients of said initial set of spectral coefficients into a plurality of frequency bands (74), and for selecting said transition frequency (f_t) dependent on a proportion of spectral holes in said frequency bands (74).

15. Decoder according to claim 14 wherein said frequency bands (74) have a constant frequency width.

16. Decoder according to claim 14, wherein at least two of said frequency bands (74) have different frequency widths.

17. Decoder according to any of the claims 14 to 16, wherein said transition determining circuitry (60) is further arranged for

finding a transition frequency band, being a highest frequency band in which said proportion is lower than a first threshold.

18. Decoder according to claim 17, wherein said transition determining circuitry (60) is further arranged for

setting said transition frequency (f_t) dependent on an upper frequency limit of said transition frequency band.

19. Encoder (20) for spectral coding of an audio signal, comprising:

transition determining circuitry (60) arranged for determining a transition frequency (f_t) for an initial set (24) of spectral coefficients representing said audio signal;

said transition frequency (f_t) defining a border between a frequency range, intended to be a subject for noise filling of spectral holes, and a frequency range, intended to be a subject for bandwidth extension;

said transition frequency (f_t) being adapted to a spectral content of said audio signal.