Adaptive transition frequency between noise fill and bandwidth extension

• US 9,269,372 B2
• Filed: 08/26/2008
• Issued: 02/23/2016
• Est. Priority Date: 08/27/2007
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Ullberg, Gustaf, Taleb, Anisse, Briand, Manuel
• Primary Examiner(s): Lerner, Martin
• Application Number:
  US12/674,341
• Publication Number:
  US 20110264454A1
• Time in Patent Office: 2,737 Days
• Field of Search: 704/200.1, 704/205, 704/206, 704/500, 704/501
• US Class Current: 1/1
• 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. A method for spectrum recovery in spectral decoding of an audio signal, the audio signal comprising a set of frames including a first frame and a second frame, the method comprising the steps of:

an audio decoding apparatus obtaining a first set of spectral coefficients representing only said first frame of said audio signal;

the audio decoding apparatus determining a first transition frequency (f_t) for said first frame of said audio signal;

the audio decoding apparatus determining a second transition frequency (f_t2) for said second frame of said audio signal;

the audio decoding apparatus noise filling of spectral holes in said first set of spectral coefficients below said first transition frequency (f_t); and

the audio decoding apparatus bandwidth extending said first set of spectral coefficients above said transition frequency (f_t), wherein

determining said first transition frequency (f_t) comprises selecting said first transition frequency (f_t) based on a distribution of spectral holes in said first set of spectral coefficients representing only said first frame of said audio signal.

2. The method according to claim 1, wherein said step of selecting said transition frequency (f_t) comprises the steps of:

dividing said spectral coefficients of said first set of spectral coefficients into a plurality of frequency bands, including a highest frequency band;

determining the number of coded spectral coefficients or groups within the highest frequency band;

dividing the determined number by a determined total number of spectral coefficients to produce a proportion value; and

determining whether the proportion value is not less than a threshold.

3. The method according to claim 2, wherein said frequency bands have a constant frequency width.

4. The method according to claim 2, wherein at least two of said frequency bands have different frequency widths.

5. The method according to claim 2, wherein said step of selecting said transition frequency (f_t) further comprises:

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

6. The method according to claim 5, 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.

7. The method according to claim 1, wherein said step of determining said first transition frequency (f_t) comprises selecting said first transition frequency (f_t) based on i) the distribution of spectral holes in said first set of spectral coefficients and ii) a previously used transition frequency for a frame of the audio signal that is previous to said first frame of the audio signal.

8. The method according to claim 7, wherein said transition frequency (f_t) is prohibited to change more than a predetermined absolute or relative amount between two consecutive frames.

9. The method according to claim 1, wherein said step of determining said first transition frequency (f_t) comprises selecting said first transition frequency (f_t) based on the distribution of spectral holes in said first set of spectral coefficients, a first previously used transition frequency for a first previous frame that is previous to said first frame, and a second previously used transition frequency for a second previous frame that is previous to said first previous frame.

10. The method of claim 1, further comprising:

an audio decoding apparatus obtaining a second set of spectral coefficients representing only said second frame of said audio signal;

the audio decoding apparatus noise filling of spectral holes in said second set of spectral coefficients below said second transition frequency (f_t2); and

the audio decoding apparatus bandwidth extending said second set of spectral coefficients above said transition frequency (f_t2).

11. A method for use in spectral coding of an audio signal, the audio signal comprising a set of frames including a first frame and a second frame, the method comprising:

an encoding apparatus receiving the first frame of the audio signal;

the encoding apparatus receiving the second frame of the audio signal;

the encoding apparatus producing a binary flux comprising first data representing the first frame of the audio signal and second data representing the second frame of the audio signal; and

transmitting the binary flux towards a decoder apparatus, wherein

producing the binary flux comprises;

i) the encoding apparatus producing a first set of spectral coefficients representing only said first frame of said audio signal, said first set of spectral coefficients having a first set of spectral holes;

ii) the encoding apparatus determining a distribution of spectral holes in said first set of spectral coefficients;

iii) the encoding apparatus determining a first transition frequency (f_t1) for the first set of spectral coefficients based on the distribution of the spectral holes in said first set of spectral holes, iv) the encoding apparatus producing a second set of spectral coefficients representing only said second frame of said audio signal, said second set of spectral coefficients having a second set of spectral holes;

v) the encoding apparatus determining a distribution of spectral holes in said second set of spectral coefficients;

vi) the encoding apparatus determining a second transition frequency (f_t2) for the second set of s spectral coefficients based on the distribution of the spectral holes in said second set of spectral holes,

said first transition frequency (f_t1) defines 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, and

said second transition frequency (f_t2) defines 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.

12. The method of claim 11, wherein determining the transition frequency (f_t) for the first set of spectral coefficients based on the distribution of the spectral holes comprises:

dividing said spectral coefficients of said first set of spectral coefficients into a plurality of frequency bands, including a highest frequency band;

determining the number of coded spectral coefficients or groups within the highest frequency band;

dividing the determined number by a determined total number of spectral coefficients to produce a proportion value; and

determining whether the proportion value is not less than a threshold.

13. The method of claim 11, wherein the encoding apparatus determines the transition frequency (f_t) for the first set of spectral coefficients based not only on the distribution of the spectral holes but also on a previously determined transition frequency for a previous frame of the audio signal that is previous to said first frame of the audio signal.

14. A decoder apparatus for spectral decoding of an audio signal, the audio signal comprising a set of frames including a first frame and a second frame, the decoder apparatus comprising:

input for obtaining a first set of spectral coefficients representing only said first frame of said audio signal;

transition determining circuitry arranged for determining a first transition frequency (f_t1) for said first frame of said audio signal and a second transition frequency (f_t2) for said second frame of said audio signal;

a noise filler for noise filling of spectral holes in said first set of spectral coefficients below said first transition frequency (f_t1); and

a bandwidth extender arranged for bandwidth extending said first set of spectral coefficients above said first transition frequency (f_t1), wherein

said transition determining circuitry is arranged to determine said first transition frequency (f_t1) dependent on a distribution of spectral holes in said first set of spectral coefficients representing only said first frame of said audio signal.

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

16. The decoder apparatus according to claim 15, wherein said frequency bands have a constant frequency width.

17. The decoder apparatus according to claim 15, wherein at least two of said frequency bands have different frequency widths.

18. The decoder apparatus according to claim 15, wherein said transition determining circuitry is further arranged for

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

19. The decoder apparatus according to claim 18, wherein said transition determining circuitry is further arranged for setting said transition frequency (f_t) dependent on an upper frequency limit of said transition frequency band.

20. An encoder apparatus for spectral coding of an audio signal, the audio signal comprising a set of frames including a first frame and a second frame, the encoder apparatus comprising:

transition determining circuitry arranged for determining;

i) a first transition frequency (f_t1) for a first set of spectral coefficients representing only said first frame of said audio signal, and ii) a second transition frequency (f_t2) for a second set of spectral coefficients representing only said second frame of said audio signal; and

a transmitter for transmitting encoded data and data identifying the determined transition frequencies,

said first transition frequency (f_t1) 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; and

said transition determining circuitry is arranged for;

i) determining said first transition frequency (f_t1) dependent on a distribution of spectral holes in said first set of spectral coefficients representing only said first frame of said audio signal and ii) determining said second transition frequency (f_t2) dependent on a distribution of spectral holes in said second set of spectral coefficients representing frame of said audio signal.

21. The encoder apparatus of claim 20, wherein said transition determining circuitry is arranged for determining said transition frequency (f_t) dependent on not only the distribution of spectral holes in said first set of spectral coefficients but also on a previously determined transition frequency for a previous frame of the audio signal that is previous to said first frame of the audio signal.

22. The encoder apparatus of claim 20, wherein said transition determining circuitry is configured to:

divide said spectral coefficients of said first set of spectral coefficients into a plurality of frequency bands, including a highest frequency band;

determine the number of coded spectral coefficients or groups within the highest frequency band;

divide the determined number by a determined total number of spectral coefficients to produce a proportion value; and

determine whether the proportion value is not less than a threshold.