Device and method for reducing quantization noise in a time-domain decoder

• US 9,384,755 B2
• Filed: 03/04/2014
• Issued: 07/05/2016
• Est. Priority Date: 03/04/2013
• Status: Active Grant

• Current Assignee: VoiceAge EVS LLC (SoftBank Group Corp.)
• Original Assignee: VoiceAge Corporation
• Inventors: Vaillancourt, Tommy, Jelinek, Milan
• Primary Examiner(s): Godbold, Douglas
• Application Number:
  US14/196,585
• Publication Number:
  US 20140249807A1
• Time in Patent Office: 854 Days
• US Class Current: 1/1
• CPC Class Codes: G10L 19/08 Determination or coding of the excitation function; Determination or coding of the long-term prediction parameters
• CPC Class Codes: G10L 19/12 the excitation function being a code excitation, e.g. in code excited linear prediction [CELP] vocoders
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering
• CPC Class Codes: G10L 21/0208 Noise filtering
• CPC Class Codes: G10L 21/0224 Processing in the time domain
• CPC Class Codes: G10L 21/0232 Processing in the frequency domain
• CPC Class Codes: G10L 25/21 the extracted parameters being power information
• CPC Class Codes: G10L 25/78 Detection of presence or absence of voice signals switching of direction of transmission by voice frequency in two-way loud-speaking telephone systems H04M9/10
• CPC Class Codes: G10L 25/93 Discriminating between voiced and unvoiced parts of speech signals G10L25/90 takes precedence

Abstract

The present disclosure relates to a device and method for reducing quantization noise in a signal contained in a time-domain excitation decoded by a time-domain decoder. The decoded time-domain excitation is converted into a frequency-domain excitation. A weighting mask is produced for retrieving spectral information lost in the quantization noise. The frequency-domain excitation is modified to increase spectral dynamics by application of the weighting mask. The modified frequency-domain excitation is converted into a modified time-domain excitation. The method and device can be used for improving music content rendering of linear-prediction (LP) based codecs. Optionally, a synthesis of the decoded time-domain excitation may be classified into one of a first set of excitation categories and a second set of excitation categories, the second set including INACTIVE or UNVOICED categories, the first set including an OTHER category.

Claims

1. A device implemented in a CELP decoder for reducing quantization noise in a sound signal contained in a decoded CELP time-domain excitation to be processed through a LP synthesis filter to produce a synthesis thereof, the device comprising:

at least one processor; and

a memory coupled to the processor and comprising non-transitory code instructions that when executed cause the processor to implement;

a converter of the decoded CELP time-domain excitation, before synthesis, into a frequency-domain excitation;

a mask builder responsive to the frequency-domain excitation to produce a weighting mask for retrieving spectral information lost in the quantization noise;

a modifier of the frequency-domain excitation to increase spectral dynamics by application of the weighting mask to the frequency-domain excitation; and

a converter of the modified frequency-domain excitation into a modified CELP time-domain excitation containing a quantization noise-reduced version of the sound signal.

2. A device according to claim 1, comprising:

the LP synthesis filter to produce the synthesis of the decoded CELP time-domain excitation; and

a classifier of the synthesis of the decoded CELP time-domain excitation into one of a first set of excitation categories and a second set of excitation categories;

wherein the second set of excitation categories comprises INACTIVE or UNVOICED categories and the first set of excitation categories comprises an OTHER category.

3. A device according to claim 2, wherein the converter of the decoded CELP time-domain excitation into a frequency-domain excitation applies to the decoded CELP time-domain excitation when the synthesis of the decoded CELP time-domain excitation is classified in the first set of excitation categories.

4. A device according to claim 2, wherein the classifier of the synthesis of the decoded CELP time-domain excitation into one of a first set of excitation categories and a second set of excitation categories uses classification information transmitted from an encoder to the CELP decoder and retrieved at the CELP decoder from a decoded bitstream.

5. A device according to claim 2, comprising a second LP synthesis filter to produce a synthesis of the modified CELP time-domain excitation.

6. A device according to claim 5, comprising a de-emphasizing filter and resampler to generate the sound signal from one of the synthesis of the decoded CELP time-domain excitation and of the synthesis of the modified CELP time-domain excitation.

7. A device according to claim 5, comprising a two-stage classifier for selecting an output synthesis as:

the synthesis of the decoded CELP time-domain excitation when the synthesis of the decoded CELP time-domain excitation is classified in the second set of excitation categories; and

the synthesis of the modified CELP time-domain excitation when the synthesis of the decoded CELP time-domain excitation is classified in the first set of excitation categories.

8. A device according to claim 1, comprising an analyzer of the frequency-domain excitation to determine whether the frequency-domain excitation contains music.

9. A device according to claim 8, wherein the analyzer of the frequency-domain excitation determines that the frequency-domain excitation contains music by comparing a statistical deviation of spectral energy differences of the frequency-domain excitation with a threshold.

10. A device according to claim 1, comprising an excitation extrapolator to evaluate an excitation of future frames, whereby conversion of the modified frequency-domain excitation into a modified CELP time-domain excitation is delay-less.

11. A device according to claim 10, comprising an excitation concatenator of past frame, current frame and extrapolated future frame time-domain excitations supplied to the converter of the decoded CELP time-domain excitation into the frequency-domain excitation.

12. A device according to claim 1, wherein the mask builder produces the weighting mask using time averaging or frequency averaging, or a combination of time and frequency averaging.

13. A device according to claim 1, comprising a noise reductor to estimate a signal to noise ratio in a selected band of the decoded CELP time-domain excitation and to perform a frequency-domain noise reduction based on the signal to noise ratio.

14. A method implemented in a CELP decoder for reducing quantization noise in a sound signal contained in a decoded CELP time-domain excitation to be processed through a LP synthesis filter to produce a synthesis thereof, the method comprising:

converting, using a time-domain to frequency-domain converter, the decoded CELP time-domain excitation, before synthesis, into a frequency-domain excitation;

producing, using a mask builder and in response to the frequency-domain excitation, a weighting mask for retrieving spectral information lost in the quantization noise;

modifying the frequency-domain excitation to increase spectral dynamics by application of the weighting mask to the frequency-domain excitation; and

converting, using a frequency-domain to time-domain converter, the modified frequency-domain excitation into a modified CELP time-domain excitation containing a quantization noise-reduced version of the sound signal.

15. A method according to claim 14, comprising:

processing the decoded CELP time-domain excitation through the LP synthesis filter to produces a synthesis of the decoded CELP time-domain excitation; and

classifying the synthesis of the decoded CELP time-domain excitation into one of a first set of excitation categories and a second set of excitation categories;

wherein the second set of excitation categories comprises INACTIVE or UNVOICED categories and the first set of excitation categories comprises an OTHER category.

16. A method according to claim 15, comprising applying a conversion of the decoded CELP time-domain excitation into a frequency-domain excitation to the decoded CELP time-domain excitation when the synthesis of the decoded CELP time-domain excitation is classified in the first set of excitation categories.

17. A method according to claim 15, comprising using classification information transmitted from an encoder to the CELP decoder and retrieved at the CELP decoder from a decoded bitstream to classify the synthesis of the decoded CELP time-domain excitation into the one of a first set of excitation categories and a second set of excitation categories.

18. A method according to claim 15, comprising producing a synthesis of the modified CELP time-domain excitation.

19. A method according to claim 18, comprising generating the sound signal from one of the synthesis of the decoded CELP time-domain excitation and of the synthesis of the modified CELP time-domain excitation.

20. A method according to claim 18, comprising selecting an output synthesis as:

the synthesis of the decoded CELP time-domain excitation when the synthesis of the decoded CELP time-domain excitation is classified in the second set of excitation categories; and

the synthesis of the modified CELP time-domain excitation when the synthesis of the decoded CELP time-domain excitation is classified in the first set of excitation categories.

21. A method according to claim 14, comprising analyzing the frequency-domain excitation to determine whether the frequency-domain excitation contains music.

22. A method according to claim 21, comprising determining that the frequency-domain excitation contains music by comparing a statistical deviation of spectral energy differences of the frequency-domain excitation with a threshold.

23. A method according to claim 14, comprising evaluating an extrapolated excitation of future frames, whereby conversion of the modified frequency-domain excitation into a modified CELP time-domain excitation is delay-less.

24. A method according to claim 23, comprising concatenating past frame, current frame and extrapolated future frame time-domain excitations for conversion into the frequency-domain excitation.

25. A method according to claim 14, wherein the weighting mask is produced using time averaging or frequency averaging or a combination of time and frequency averaging.

26. A method according to claim 14, comprising:

estimating a signal to noise ratio in a selected band of the decoded CELP time-domain excitation; and

performing a frequency-domain noise reduction based on the estimated signal to noise ratio.

27. A device for reducing quantization noise in a sound signal contained in a time-domain excitation decoded by a time-domain decoder, comprising:

at least one processor; and

a memory coupled to the processor and comprising non-transitory code instructions that when executed cause the processor to implement;

a converter of the decoded time-domain excitation into a frequency-domain excitation;

a mask builder responsive to the frequency-domain excitation to produce a weighting mask for retrieving spectral information lost in the quantization noise;

a modifier of the frequency-domain excitation to increase spectral dynamics by application of the weighting mask to the frequency-domain excitation; and

a converter of the modified frequency-domain excitation into a modified time-domain excitation containing a quantization noise-reduced version of the sound signal;

wherein the mask builder comprises;

a normalizer of a spectral energy of the frequency-domain excitation to produce a scaled energy spectrum;

an averager of the scaled energy spectrum along a frequency axis; and

a smoother of the averaged energy spectrum along a time-domain axis to smooth frequency spectrum values from frame to frame.

28. A device according to claim 27, wherein the normalizer produces a normalized energy spectrum, applies a power value to the normalized energy spectrum to produce the scaled energy spectrum, and limits a value of the scaled energy spectrum to a maximum limit.

29. A method for reducing quantization noise in a sound signal contained in a time-domain excitation decoded by a time-domain decoder, comprising:

converting, using a time-domain to frequency-domain converter, the decoded time-domain excitation into a frequency-domain excitation;

producing, using a mask builder and in response to the frequency-domain excitation, a weighting mask for retrieving spectral information lost in the quantization noise;

modifying the frequency-domain excitation to increase spectral dynamics by application of the weighting mask to the frequency-domain excitation; and

converting, using a frequency-domain to time-domain converter, the modified frequency-domain excitation into a modified time-domain excitation containing a quantization noise-reduced version of the sound signal;

wherein producing a weighting mask comprises;

normalizing a spectral energy of the frequency-domain excitation to produce a scaled energy spectrum;

averaging the scaled energy spectrum along a frequency axis; and

smoothing the averaged energy spectrum along a time-domain axis to smooth frequency spectrum values from frame to frame.

30. A method according to claim 29, wherein normalizing the spectral energy of the frequency-domain excitation comprises producing a normalized energy spectrum, applying a power value to the normalized energy spectrum to produce the scaled energy spectrum, and limiting a value of the scaled energy spectrum to a maximum limit.