Selective post filter

• US 9,595,270 B2
• Filed: 04/27/2016
• Issued: 03/14/2017
• Est. Priority Date: 07/02/2010
• Status: Active Grant

• Current Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Original Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Inventors: Resch, Barbara, Kjrling, Kristofer, Villemoes, Lars
• Primary Examiner(s): ABEBE, DANIEL DEMELASH
• Application Number:
  US15/140,356
• Publication Number:
  US 20160240209A1
• Time in Patent Office: 321 Days
• Field of Search: 704/219
• US Class Current: 1/1
• CPC Class Codes: G10L 19/02 using spectral analysis, e.g. transform vocoders or subband vocoders
• CPC Class Codes: G10L 19/0212 using orthogonal transformation
• CPC Class Codes: G10L 19/032 Quantisation or dequantisation of spectral components
• CPC Class Codes: G10L 19/09 Long term prediction, i.e. removing periodical redundancies, e.g. by using adaptive codebook or pitch predictor
• CPC Class Codes: G10L 19/107 Sparse pulse excitation, e.g. by using algebraic codebook
• 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 19/125 Pitch excitation, e.g. pitch synchronous innovation CELP [PSI-CELP]
• CPC Class Codes: G10L 19/20 using sound class specific coding, hybrid encoders or object based coding
• CPC Class Codes: G10L 19/22 Mode decision, i.e. based on audio signal content versus external parameters
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering
• CPC Class Codes: G10L 19/265 Pre-filtering, e.g. high frequency emphasis prior to encoding
• CPC Class Codes: G10L 21/003 Changing voice quality, e.g. pitch or formants
• CPC Class Codes: G10L 21/007 characterised by the process used
• CPC Class Codes: G10L 21/013 Adapting to target pitch

Abstract

In one aspect, the invention provides an audio encoding method characterized by a decision being made as to whether the device which will decode the resulting bit stream should apply post filtering including attenuation of interharmonic noise. Hence, the decision whether to use the post filter, which is encoded in the bit stream, is taken separately from the decision as to the most suitable coding mode.

In another aspect, there is provided an audio decoding method with a decoding step followed by a post-filtering step, including interharmonic noise attenuation, and being characterized in a step of disabling the post filter in accordance with post filtering information encoded in the bit stream signal. Such a method is well suited for mixed-origin audio signals by virtue of its capability to deactivate the post filter in dependence of the post filtering information only, hence independently of factors such as the current coding mode.

Claims

1. An interharmonic noise attenuation post filter adapted to receive an input signal, which comprises a preliminary audio signal decoded according to one of a plurality of decoding modes and to supply an output audio signal, the interharmonic noise attenuation post filter comprising:

a control section for selectively operating the post filter in one of the following modes;

i) a filtering mode, wherein the post filter filters the preliminary audio signal to obtain a filtered signal and supplies the filtered signal as an output audio signal;

or

ii) a pass-through mode, wherein the post filter supplies the unfiltered preliminary audio signal as an output audio signal,

said control section being adapted to receive a post-filtering signal that is generated in response to a value of a data field associated with a time frame of the preliminary audio signal, and to disable the post filter while the post filter is operating in the pass-through mode in response to the post-filtering signal.

2. The post filter of claim 1, wherein the post-filtering signal is included in the input signal.

3. The post filter of claim 1, further comprising a decision module adapted to estimate a pitch frequency of the preliminary audio signal and to assess at least one of the following criteria:

a) whether the power of spectral components below the pitch frequency exceeds a predetermined threshold;

b) whether spectral components below the pitch frequency are tonal;

c) whether the power of spectral components between harmonics of the pitch frequency exceed a predetermined threshold; and

d) whether spectral components between harmonics of the pitch frequency are tonal;

and, responsive to a positive determination, to take a decision to generate a negative post-filtering signal disabling the post filter.

4. A decoder system for decoding a bit stream signal as an audio time signal, including:

a decoding section for decoding a bit stream signal as a preliminary audio time signal; and

the interharmonic noise attenuation post filter of claim 1.

5. The decoder system of claim 4,

wherein the post filter has variable gain determining the interharmonic attenuation and the control section includes a gain controller operable to set the absolute value of the gain below a predetermined threshold, whereby the post filter is disabled.

6. The decoder system of claim 4,

wherein the post filter is adapted to attenuate only such spectral components which are located below a predetermined cut-off frequency.

7. The decoder system of claim 4,

the decoding section further comprising a code-excited linear prediction, CELP, decoding module and a transform-coded excitation, TCX, decoding module for decoding a bit stream signal as an audio time signal,

the control section being adapted operate the decoder system in at least the following modes:

a) the TCX module is enabled and the post filter is disabled;

b) the CELP module and the post filter are enabled; and

c) the CELP module is enabled and the post filter is disabled, wherein the preliminary audio time signal and the audio time signal coincide.

8. The decoder system of claim 6,

the decoding section further comprising an Advanced Audio Coding, AAC, decoding module for decoding a bit stream signal as an audio time signal,

the control section being adapted to operate the decoder also in the following mode:

d) the AAC module is enabled and the post filter is disabled.

9. The decoder system of claim 4,

wherein the bit stream signal is a Moving Pictures Experts Group, MPEG, bit stream and is segmented into time frames and the control section is adapted to disable an entire time frame or a sequence of entire time frames.

10. A method of decoding a bit stream signal as an audio time signal, comprising:

decoding a bit stream signal as a preliminary audio time signal in one of a plurality of decoding modes, the bit stream signal being segmented into time frames; and

post-filtering, by a post filter, the preliminary audio time signal by attenuating interharmonic noise, thereby obtaining an audio time signal,

characterized in that the post-filtering step is selectively omitted responsive to post-filtering information encoded in the bit stream signal, and wherein the post filter is adapted to receive, for each time frame, a data field associated with the time frame, and responsive to a value of the data field, enable or disable the post filter for post-filtering the preliminary audio time signal.

11. A non-transitory, computer-readable storage medium storing instructions for performing the method of claim 10.

12. An encoder system for encoding an audio time signal as a bit stream signal, including an encoding section operable in several coding modes, for encoding an audio time signal as a bit stream signal, the encoder system comprising:

a decision section adapted to decide whether post filtering, which includes attenuation of interharmonic noise, is to be disabled at a decoder operable for decoding of the bit stream signal separately from deciding on the coding mode and to encode this decision in the bit stream signal as post filtering information, wherein the bit stream signal is segmented into time frames and the encoding includes adding a value to a data field of each time frame to enable or disable a post filter in the decoder that is operable for post filtering the decoded bit stream signal.

13. The encoder system of claim 12,

the decision section being adapted to:

detect a co-presence of a signal component with dominant fundamental frequency and a signal component located below the fundamental frequency and, optionally, between its harmonics; and

responsive to a positive determination, to take a decision to disable.

14. The encoder system of claim 12,

further comprising a code-excited linear prediction, CELP, encoding module,

said encoding section further including a transform-coded excitation, TCX, encoding module,

wherein the decision section is adapted to select one of the following coding modes, preferably on the basis of a rate-distortion optimization:

a) TCX coding;

b) CELP coding with post filtering; and

c) CELP coding without post filtering,

the encoder system further comprising a coding selector adapted to select one of the following super-modes;

i) Advanced Audio Coding, AAC coding, wherein the decision section is disabled; and

ii) TCX/CELP coding, wherein the decision section is enabled to select one of coding modes a), b) and c).

15. A method of encoding an audio time signal as a bit stream signal, the method comprising:

encoding an audio time signal as a bit stream signal in one of several coding modes, wherein the bit stream signal is segmented into time frames and the encoding includes adding a value to a data field of each time frame to enable or disable a post filter in a decoder that is operable for post filtering the decoded bit stream signal; and

deciding whether post filtering, which includes attenuation of interharmonic noise, is to be disabled at the decoder operable for decoding of the bit stream signal separately from deciding on the coding mode and encoding this decision in the bit stream signal as post filtering information.