Audio Encoder and Decoder for Encoding Frames of Sampled Audio Signals

• US 20110173008A1
• Filed: 01/11/2011
• Published: 07/14/2011
• Est. Priority Date: 07/11/2008
• Status: Active Grant

• Current Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V., VoiceAge Corporation
• Original Assignee: Fraunhofer Gesellschaft Zur Foerderung Der Angewandten Forsching E.V., VoiceAge Corporation
• Inventors: Lecomte, Jeremie, Multrus, Markus, Bayer, Stefan, Rettelbach, Nikolaus, Gournay, Philippe
• Granted Patent: US 8,751,246 B2
• Time in Patent Office: Days
• US Class Current: 704/500
• CPC Class Codes: G10L 19/20 using sound class specific coding, hybrid encoders or object based coding

Abstract

An audio encoder adapted for encoding frames of a sampled audio signal to obtain encoded frames, wherein a frame has a number of time domain audio samples, having a predictive coding analysis stage for determining information on coefficients of a synthesis filter and information on a prediction domain frame based on a frame of audio samples. The audio encoder further has a frequency domain transformer for transforming a frame of audio samples to the frequency domain to obtain a frame spectrum and an encoding domain decider for deciding whether encoded data for a frame is based on the information on the coefficients and on the information on the prediction domain frame, or based on the frame spectrum. Moreover, the audio encoder has a controller for determining an information on a switching coefficient when the encoding domain decider decides that encoded data of a current frame is based on the information on the coefficients and the information on the prediction domain frame when encoded data of a previous frame was encoded based on a previous frame spectrum and a redundancy reducing encoder for encoding the information on the prediction domain frame, the information on the coefficients, the information on the switching coefficient and/or the frame spectrum.

Claims

1. An audio encoder adapted for encoding frames of a sampled audio signal to acquire encoded frames, wherein a frame comprises a number of time domain audio samples, comprising:

a predictive coding analysis stage for determining information on coefficients of a synthesis filter and information on a prediction domain frame based on a frame of audio samples;

a frequency domain transformer for transforming a frame of audio samples to the frequency domain to acquire a frame spectrum;

an encoding domain decider for deciding whether encoded data for a frame is based on the information on the coefficients and on the information on the prediction domain frame, or based on the frame spectrum;

a controller for determining information on a switching coefficient when the encoding domain decider decides that encoded data of a current frame is based on the information on the coefficients and the information on the prediction domain frame when encoded data of a previous frame was encoded based on a previous frame spectrum acquired by the frequency domain transformer; and

a redundancy reducing encoder for encoding the information on the prediction domain frame, the information on the coefficients, the information on the switching coefficient and/or the frame spectrum,

wherein the information on the switching coefficient comprises an information enabling an initialization of a predictive synthesis stage, and the controller is adapted for determining the information on the switching coefficient based on an LPC analysis of the previous frame, and

the controller is adapted for determining the information on the switching coefficient based on a high pass filtered version of a decoded frame spectrum of the previous frame.

2. The audio encoder of claim 1, wherein the predictive coding analysis stage is adapted for determining the information on the coefficients of the synthesis filter and the information on the prediction domain frame based on an LPC (LPC=Linear Prediction Coding) analysis and/or wherein the frequency domain transformer is adapted for transforming the frame of audio samples based on a Fast Fourier Transform or a modified discrete cosine transform.

3. The audio encoder of claim 1, wherein the controller is adapted for determining as information on the switching coefficient information on coefficients for a synthesis filter and information on a switching prediction domain frame based on the LPC analysis.

4. The audio encoder of claim 1, wherein the controller is adapted for determining the information on the switching coefficient such that the switching coefficient represent a frame of audio samples overlapping the previous frame.

5. The audio encoder of claim 4, in which the frame of audio samples overlapping the previous frame is centered at the end of the previous frame.

6. A method for encoding frames of a sampled audio signal to acquire encoded frames, wherein a frame comprises a number of time domain audio samples, comprising:

determining information on coefficients of a synthesis filter and information on a prediction domain frame based on a frame of audio samples;

transforming a frame of audio samples to the frequency domain to acquire a frame spectrum;

deciding whether encoded data for a frame is based on the information on the coefficients and on the information on the prediction domain frame, or based on the frame spectrum;

determining information on a switching coefficient when it is decided that encoded data of a current frame is based on the information on the coefficients and the information on the prediction domain frame when encoded data of a previous frame was encoded based on a previous frame spectrum acquired by the frequency domain transformer; and

encoding the information on the prediction domain frame, the information on the coefficients, the information on the switching coefficient and/or the frame spectra,

wherein the information on the switching coefficient comprises an information enabling an initialization of a predictive synthesis stage, and the determination of the information on the switching coefficient is performed based on an LPC analysis of the previous frame, and

the controller is adapted for determining the information on the switching coefficient based on a high pass filtered version of a decoded frame spectrum of the previous frame.

7. An audio decoder for decoding encoded frames to acquire frames of a sampled audio signal, wherein a frame comprises a number of time domain audio samples, comprising:

a redundancy retrieving decoder for decoding the encoded frames to acquire information on a prediction domain frame, information on coefficients for a synthesis filter and/or a frame spectrum;

a predictive synthesis stage for determining a predicted frame of audio samples based on the information on the coefficients for the synthesis filter and the information on the prediction domain frame;

a time domain transformer for transforming the frame spectrum to the time domain to acquire a transformed frame from the frame spectrum;

a combiner for combining the transformed frame and the predicted frame to acquire the frames of the sampled audio signal; and

a controller for controlling a switch-over process, the switch-over process being effected when a previous frame is based on a transformed frame and a current frame is based on a predicted frame, the controller being configured for providing a switching coefficient to the predictive synthesis stage for initialization of the predictive synthesis stage based on an LPC analysis of the previous frame so that the predictive synthesis stage is initialized when the switch-over process is effected.

8. The audio decoder of claim 7, wherein the redundancy reducing decoder is adapted for decoding an information on the switching coefficient from the encoded frames.

9. The audio decoder of claim 7, wherein the predictive synthesis stage is adapted for determining the predictive frame based on an LPC synthesis and/or wherein the time domain transformer is adapted for transforming the frame spectrum to the time domain based on an inverse FFT or an inverse MDCT.

10. The audio decoder of claim 7, wherein the controller is adapted for analyzing the previous frame to acquire a previous frame information on coefficients for a synthesis filter and a previous frame information on a prediction domain frame and wherein the controller is adapted for providing the previous frame information on coefficients to the predictive synthesis stage as switching coefficient and/or wherein the controller is adapted for further providing the previous frame information on the prediction domain frame to the predictive synthesis stage for training.

11. The audio decoder of claim 7, wherein the predictive synthesis stage is adapted for determining a switch-over prediction frame which is centered at the end of the previous frame.

12. The audio decoder of claim 7, wherein the controller is adapted for analyzing a high-pass filtered version of the previous frame.

13. A method for decoding encoded frames to acquire frames of a sampled audio signal, wherein a frame comprises a number of time domain audio samples, comprising:

decoding the encoded frames to acquire information on a prediction domain frame, and information on coefficients for a synthesis filter and/or a frame spectrum;

determining a predicted frame of audio samples based on the information of the coefficients for the synthesis filter and the information on the prediction domain frame;

transforming the frame spectrum to the time domain to acquire a transformed frame from the frame spectrum;

combining the transformed frame and the predicted frame to acquire the frames of the sampled audio signal; and

controlling a switch-over process, the switch-over process being effected when a previous frame is based on the transformed frame, and a current frame is based on the predicted frame;

providing a switching coefficient for initialization based on an LPC analysis of the previous frame so that a predictive synthesis stage is initialized when the switch-over process is effected.

14. A computer program comprising a program code for performing, when a computer program runs on a computer or processor, the method for encoding frames of a sampled audio signal to acquire encoded frames, wherein a frame comprises a number of time domain audio samples, comprising:

determining information on coefficients of a synthesis filter and information on a prediction domain frame based on a frame of audio samples;

transforming a frame of audio samples to the frequency domain to acquire a frame spectrum;

deciding whether encoded data for a frame is based on the information on the coefficients and on the information on the prediction domain frame, or based on the frame spectrum;

determining information on a switching coefficient when it is decided that encoded data of a current frame is based on the information on the coefficients and the information on the prediction domain frame when encoded data of a previous frame was encoded based on a previous frame spectrum acquired by the frequency domain transformer; and

encoding the information on the prediction domain frame, the information on the coefficients, the information on the switching coefficient and/or the frame spectra,

wherein the information on the switching coefficient comprises an information enabling an initialization of a predictive synthesis stage, and the determination of the information on the switching coefficient is performed based on an LPC analysis of the previous frame, and

the controller is adapted for determining the information on the switching coefficient based on a high pass filtered version of a decoded frame spectrum of the previous frame.

15. A computer program comprising a program code for performing, when a computer program runs on a computer or processor, the method for decoding encoded frames to acquire frames of a sampled audio signal, wherein a frame comprises a number of time domain audio samples, comprising:

decoding the encoded frames to acquire information on a prediction domain frame, and information on coefficients for a synthesis filter and/or a frame spectrum;

determining a predicted frame of audio samples based on the information of the coefficients for the synthesis filter and the information on the prediction domain frame;

transforming the frame spectrum to the time domain to acquire a transformed frame from the frame spectrum;

combining the transformed frame and the predicted frame to acquire the frames of the sampled audio signal; and

controlling a switch-over process, the switch-over process being effected when a previous frame is based on the transformed frame, and a current frame is based on thr predicted frame;

providing a switching coefficient for initialization based on an LPC analysis of the previous frame so that a predictive synthesis stage is initialized when the switch-over process is effected.