Low Bitrate Audio Encoding/Decoding Scheme with Common Preprocessing

• US 20110200198A1
• Filed: 01/11/2011
• Published: 08/18/2011
• Est. Priority Date: 07/11/2008
• Status: Active Grant

• Current Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V.
• Original Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V.
• Inventors: Schuller, Gerald, Herre, Juergen, Geiger, Ralf, Neuendorf, Max, Disch, Sascha, Popp, Harald, Nagel, Frederik, Hilpert, Johannes, Grill, Bernhard, Fuchs, Guillaume, Multrus, Markus, Hirschfeld, Jens, Bayer, Stefan, Kraemer, Ulrich, Wabnik, Stefan, Rettelbach, Nikolaus, Geyersberger, Stefan, Yokotani, Yoshikazu, Lecomte, Jeremie
• Granted Patent: US 8,804,970 B2
• Time in Patent Office: Days
• US Class Current: 381/23
• CPC Class Codes: G10L 19/0017 Lossless audio signal coding; Perfect reconstruction of coded audio signal by transmission of coding error G10L19/24 takes precedence
• CPC Class Codes: G10L 19/008 Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
• CPC Class Codes: G10L 19/0212 using orthogonal transformation
• CPC Class Codes: G10L 19/173 Transcoding, i.e. converting between two coded representations avoiding cascaded coding-decoding
• CPC Class Codes: G10L 19/18 Vocoders using multiple modes
• CPC Class Codes: G10L 2019/0008 Algebraic codebooks

Abstract

An audio encoder has a common preprocessing stage, an information sink based encoding branch such as spectral domain encoding branch, a information source based encoding branch such as an LPC-domain encoding branch and a switch for switching between these branches at inputs into these branches or outputs of these branches controlled by a decision stage. An audio decoder has a spectral domain decoding branch, an LPC-domain decoding branch, one or more switches for switching between the branches and a common post-processing stage for post-processing a time-domain audio signal for obtaining a post-processed audio signal.

Claims

1. Audio encoder for generating an encoded audio signal, comprising:

a first encoding branch for encoding an audio intermediate signal in accordance with a first coding algorithm, the first coding algorithm comprising an information sink model and generating, in a first encoding branch output signal, encoded spectral information representing the audio intermediate signal, the first encoding branch comprising a spectral conversion block for converting the audio intermediate signal into a spectral domain and a spectral audio encoder for encoding an output signal of the spectral conversion block to acquire the encoded spectral information;

a second encoding branch for encoding an audio intermediate signal in accordance with a second coding algorithm, the second coding algorithm comprising an information source model and generating, in a second encoding branch output signal, encoded parameters for the information source model representing the audio intermediate signal, the second encoding branch comprising an LPC analyzer for analyzing the audio intermediate signal and for outputting an LPC information signal usable for controlling an LPC synthesis filter and an excitation signal, and an excitation encoder for encoding the excitation signal to acquire the encoded parameters; and

a common pre-processing stage for pre-processing an audio input signal to acquire the audio intermediate signal, wherein the common pre-processing stage is operative to process the audio input signal so that the audio intermediate signal is a compressed version of the audio input signal.

2. Audio encoder in accordance with claim 1, further comprising a switching stage connected between the first encoding branch and the second encoding branch at inputs into the branches or outputs of the branches, the switching stage being controlled by a switching control signal.

3. Audio encoder in accordance with claim 2, further comprising a decision stage for analyzing the audio input signal or the audio intermediate signal or an intermediate signal in the common pre-processing stage in time or frequency in order to find a time or frequency portion of a signal to be transmitted in an encoder output signal either as the encoded output signal generated by the first encoding branch or the encoded output signal generated by the second encoding branch.

4. Audio encoder in accordance with claim 1, in which the common pre-processing stage is operative to calculate common pre-processing parameters for a portion of the audio input signal not comprised in a first and a different second portion of the audio intermediate signal and to introduce an encoded representation of the preprocessing parameters in the encoded output signal, wherein the encoded output signal additionally comprises a first encoding branch output signal for representing a first portion of the audio intermediate signal and a second encoding branch output signal for representing the second portion of the audio intermediate signal.

5. Audio encoder in accordance with claim 1, in which the common pre-processing stage comprises a joint multichannel module, the joint multichannel module comprising:

a downmixer for generating a number of downmixed channels being greater than or equal to 1 and being smaller than a number of channels input into the downmixer; and

a multichannel parameter calculator for calculating multichannel parameters so that, using the multichannel parameters and the number of downmixed channels, a representation of the original channel is performable.

6. Apparatus in accordance with claim 5, in which the multichannel parameters are interchannel level difference parameters, interchannel correlation or coherence parameters, interchannel phase difference parameters, interchannel time difference parameters, audio object parameters or direction or diffuseness parameters.

7. Audio encoder in accordance with claim 1, in which the common pre-processing stage comprises a band width extension analysis stage, comprising:

a band-limiting device for rejecting a high band in an input signal and for generating a low band signal; and

a parameter calculator for calculating band width extension parameters for the high band rejected by the band-limiting device, wherein the parameter calculator is such that using the calculated parameters and the low band signal, a reconstruction of a bandwidth extended input signal is performable.

8. Audio encoder in accordance with claim 1, in which the common pre-processing stage comprises a joint multichannel module, a bandwidth extension stage, and a switch for switching between the first encoding branch and the second encoding branch,

wherein an output of the joint multichannel stage is connected to an input of the bandwidth extension stage, and an output of the bandwidth extension stage is connected to an input of the switch, a first output of the switch is connected to an input of the first encoding branch and a second output of the switch is connected to an input of the second encoding branch, and outputs of the encoding branches are connected to a bit stream former.

9. Audio encoder in accordance with claim 3, in which the decision stage is operative to analyze a decision stage input signal for searching for portions to be encoded by the first encoding branch with a better signal to noise ratio at a certain bit rate compared to the second encoding branch, wherein the decision stage is operative to analyze based on an open loop algorithm without an encoded and again decoded signal or based on a closed loop algorithm using an encoded and again decoded signal.

10. Audio encoder in accordance with claim 3,

wherein the common pre-processing stage comprises a specific number of functionalities and wherein at least one functionality is adaptable by a decision stage output signal and wherein at least one functionality is non-adaptable.

11. Audio encoder in accordance with claim 1,

in which the first encoding branch comprises a time warper module for calculating a variable warping characteristic dependent on a portion of the audio signal,

in which the first encoding branch comprises a resampler for re-sampling in accordance with a determined warping characteristic, and

in which the first encoding branch comprises a time domain/frequency domain converter and an entropy coder for converting a result of the time domain/frequency domain conversion into an encoded representation,

wherein the variable warping characteristic is comprised in the encoded audio signal.

12. Audio encoder in accordance with claim 1, in which the common pre-processing stage is operative to output at least two intermediate signals, and wherein, for each audio intermediate signal, the first and the second coding branch and a switch for switching between the two branches is provided.

13. Method of audio encoding for generating an encoded audio signal, comprising:

encoding an audio intermediate signal in accordance with a first coding algorithm, the first coding algorithm comprising an information sink model and generating, in a first output signal, encoded spectral information representing the audio signal, the first coding algorithm comprising a spectral conversion step of converting the audio intermediate signal into a spectral domain and a spectral audio encoding step of encoding an output signal of the spectral conversion step to acquire the encoded spectral information;

encoding an audio intermediate signal in accordance with a second coding algorithm, the second coding algorithm comprising an information source model and generating, in a second output signal, encoded parameters for the information source model representing the intermediate signal, the second encoding branch comprising a step of LPC analyzing the audio intermediate signal and outputting an LPC information signal usable for controlling an LPC synthesis filter, and an excitation signal, and a step of excitation encoding the excitation signal to acquire the encoded parameters; and

commonly pre-processing an audio input signal to acquire the audio intermediate signal, wherein, in the step of commonly pre-processing the audio input signal is processed so that the audio intermediate signal is a compressed version of the audio input signal,

wherein the encoded audio signal comprises, for a certain portion of the audio signal either the first output signal or the second output signal.

14. Audio decoder for decoding an encoded audio signal, comprising:

a first decoding branch for decoding an encoded signal encoded in accordance with a first coding algorithm comprising an information sink model, the first decoding branch comprising a spectral audio decoder for spectral audio decoding the encoded signal encoded in accordance with a first coding algorithm comprising an information sink model, and a time-domain converter for converting an output signal of the spectral audio decoder into the time domain;

a second decoding branch for decoding an encoded audio signal encoded in accordance with a second coding algorithm comprising an information source model, the second decoding branch comprising an excitation decoder for decoding the encoded audio signal encoded in accordance with a second coding algorithm to acquire an LPC domain signal, and an LPC synthesis stage for receiving an LPC information signal generated by an LPC analysis stage and for converting the LPC domain signal into the time domain;

a combiner for combining time domain output signals from the time domain converter of the first decoding branch and the LPC synthesis stage of the second decoding branch to acquire a combined signal; and

a common post-processing stage for processing the combined signal so that a decoded output signal of the common post-processing stage is an expanded version of the combined signal.

15. Audio decoder in accordance with claim 14, in which the combiner comprises a switch for switching decoded signals from the first decoding branch and the second decoding branch in accordance with a mode indication explicitly or implicitly comprised in the encoded audio signal so that the combined audio signal is a continuous discrete time domain signal.

16. Audio decoder in accordance with claim 14, in which the combiner comprises a cross fader for cross fading, in case of a switching event, between an output of a decoding branch and an output of the other decoding branch within a time domain cross fading region.

17. Audio decoder in accordance with claim 16, in which the cross fader is operative to weight at least one of the decoding branch output signals within the cross fading region and to add at least one weighted signal to a weighted or unweighted signal from the other encoding branch, wherein weights used for weighting the at least one signal are variable in the cross fading region.

18. Audio decoder in accordance with claim 14, in which the common pre-processing stage comprises at least one of a joint multichannel decoder or a bandwidth extension processor.

19. Audio decoder in accordance with claim 18, in which the joint multichannel decoder comprises a parameter decoder and an upmixer controlled by a parameter decoder output.

20. Audio decoder in accordance with claim 19,

in which the bandwidth extension processor comprises a patcher for creating a high band signal, an adjuster for adjusting the high band signal, and a combiner for combining the adjusted high band signal and a low band signal to acquire a bandwidth extended signal.

21. Audio decoder in accordance with claim 14, in which the first decoding branch comprises a frequency domain audio decoder, and the second decoding branch comprises a time domain speech decoder.

22. Audio decoder in accordance with claim 14, in which the first decoding branch comprises a frequency domain audio decoder, and the second decoding branch comprises a LPC-based decoder.

23. Audio decoder in accordance with claim 14,

wherein the common post-processing stage comprises a specific number of functionalities and wherein at least one functionality is adaptable by a mode detection function and wherein at least one functionality is non-adaptable.

24. Method of audio decoding an encoded audio signal, comprising:

decoding an encoded signal encoded in accordance with a first coding algorithm comprising an information sink model, comprising spectral audio decoding the encoded signal encoded in accordance with a first coding algorithm comprising an information sink model, and time domain converting an output signal of the spectral audio decoding step into the time domain;

decoding an encoded audio signal encoded in accordance with a second coding algorithm comprising an information source model, comprising excitation decoding the encoded audio signal encoded in accordance with a second coding algorithm to acquire an LPC domain signal, an for receiving an LPC information signal generated by an LPC analysis stage and LPC synthesizing to convert the LPC domain signal into the time domain;

combining time domain output signals from the step of time domain converting and the step of LPC synthesizing to acquire a combined signal; and

commonly processing the combined signal so that a decoded output signal of the common post-processing stage is an expanded version of the combined signal.

25. Computer program for performing, when running on a computer, the method of audio encoding for generating an encoded audio signal, comprising:

encoding an audio intermediate signal in accordance with a first coding algorithm, the first coding algorithm comprising an information sink model and generating, in a first output signal, encoded spectral information representing the audio signal, the first coding algorithm comprising a spectral conversion step of converting the audio intermediate signal into a spectral domain and a spectral audio encoding step of encoding an output signal of the spectral conversion step to acquire the encoded spectral information;

encoding an audio intermediate signal in accordance with a second coding algorithm, the second coding algorithm comprising an information source model and generating, in a second output signal, encoded parameters for the information source model representing the intermediate signal, the second encoding branch comprising a step of LPC analyzing the audio intermediate signal and outputting an LPC information signal usable for controlling an LPC synthesis filter, and an excitation signal, and a step of excitation encoding the excitation signal to acquire the encoded parameters; and

commonly pre-processing an audio input signal to acquire the audio intermediate signal, wherein, in the step of commonly pre-processing the audio input signal is processed so that the audio intermediate signal is a compressed version of the audio input signal,

wherein the encoded audio signal comprises, for a certain portion of the audio signal either the first output signal or the second output signal.

26. Computer program for performing, when running on a computer, the method of audio decoding an encoded audio signal, comprising:

decoding an encoded signal encoded in accordance with a first coding algorithm comprising an information sink model, comprising spectral audio decoding the encoded signal encoded in accordance with a first coding algorithm comprising an information sink model, and time domain converting an output signal of the spectral audio decoding step into the time domain;

decoding an encoded audio signal encoded in accordance with a second coding algorithm comprising an information source model, comprising excitation decoding the encoded audio signal encoded in accordance with a second coding algorithm to acquire an LPC domain signal, an for receiving an LPC information signal generated by an LPC analysis stage and LPC synthesizing to convert the LPC domain signal into the time domain;

combining time domain output signals from the step of time domain converting and the step of LPC synthesizing to acquire a combined signal; and

commonly processing the combined signal so that a decoded output signal of the common post-processing stage is an expanded version of the combined signal.

27. Encoded audio signal comprising:

a first encoding branch output signal representing a first portion of an audio signal encoded in accordance with a first coding algorithm, the first coding algorithm comprising an information sink model, the first encoding branch output signal comprising encoded spectral information representing the audio signal, the first encoding branch comprising a spectral conversion block for converting the audio intermediate signal into a spectral domain and a spectral audio encoder for encoding an output signal of the spectral conversion block to acquire the encoded spectral information;

a second encoding branch output signal representing a second portion of an audio signal, which is different from the first portion of the output signal, the second portion being encoded in accordance with a second coding algorithm, the second coding algorithm comprising an information source model, the second encoding branch output signal comprising encoded parameters for the information source model representing the intermediate signal, the second encoding branch comprising an LPC analyzer for analyzing the audio intermediate signal and for outputting an LPC information signal usable for controlling an LPC synthesis filter and an excitation signal, and an excitation encoder for encoding the excitation signal to acquire the encoded parameters; and

common pre-processing parameters representing differences between the audio signal and an expanded version of the audio signal.