Apparatus and Method for Encoding a Multi-Channel Audio Signal

• US 20110085671A1
• Filed: 09/09/2008
• Published: 04/14/2011
• Est. Priority Date: 09/25/2007
• Status: Active Grant

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Gibbs, Jonathan A.
• Granted Patent: US 8,577,045 B2
• Time in Patent Office: Days
• US Class Current: 381/23
• CPC Class Codes: G10L 19/00 Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis in musical instruments G10H
• 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 19/032 Quantisation or dequantisation of spectral components
• CPC Class Codes: G10L 19/06 Determination or coding of the spectral characteristics, e.g. of the short-term prediction coefficients
• CPC Class Codes: G10L 19/087 using mixed excitation models, e.g. MELP, MBE, split band LPC or HVXC
• 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 19/24 Variable rate codecs, e.g. for generating different qualities using a scalable representation such as hierarchical encoding or layered encoding
• CPC Class Codes: G10L 25/06 the extracted parameters being correlation coefficients
• CPC Class Codes: G10L 25/12 the extracted parameters being prediction coefficients
• CPC Class Codes: G10L 25/18 the extracted parameters being spectral information of each sub-band
• CPC Class Codes: G10L 25/21 the extracted parameters being power information
• CPC Class Codes: G10L 25/24 the extracted parameters being the cepstrum
• CPC Class Codes: H03M 3/466 Multiplexed conversion systems
• CPC Class Codes: H04S 2400/01 Multi-channel, i.e. more than two input channels, sound reproduction with two speakers wherein the multi-channel information is substantially preserved
• CPC Class Codes: H04S 2420/03 Application of parametric coding in stereophonic audio systems

Abstract

An encoding apparatus comprises a frame processor (105) which receives a multi channel audio signal comprising at least a first audio signal from a first microphone (101) and a second audio signal from a second microphone (103). An ITD processor 107 then determines an inter time difference between the first audio signal and the second audio signal and a set of delays (109, 111) generates a compensated multi channel audio signal from the multi channel audio signal by delaying at least one of the first and second audio signals in response to the inter time difference signal. A combiner (113) then generates a mono signal by combining channels of the compensated multi channel audio signal and a mono signal encoder (115) encodes the mono signal. The inter time difference may specifically be determined by an algorithm based on determining cross correlations between the first and second audio signals.

Claims

1. An apparatus for encoding a multi channel audio signal, the apparatus comprising:

a receiver for receiving the multi channel audio signal comprising at least a first audio signal from a first microphone and a second audio signal from a second microphone;

a time difference unit for determining an inter time difference between the first audio signal and the second audio signal;

a delay unit for generating a compensated multi channel audio signal from the multi channel audio signal by delaying at least one of the first audio signal and the second audio signal in response to the inter time difference signal;

a mono unit for generating a mono signal by combining channels of the compensated multi channel audio signal; and

a mono signal encoder for encoding the mono signal.

2. The apparatus of claim 1 wherein the time difference unit is arranged to determine cross correlations between the first audio signal and the second audio signal for a plurality of time offsets, and to determine the inter time difference in response to the cross correlations.

3. The apparatus of claim 2 wherein the time difference unit is arranged to low pass filter the first audio signal and the second audio signal prior to the cross correlation.

4. The apparatus of claim 2 wherein the time difference unit is arranged to decimate the first audio signal and the second audio signal prior to the cross correlation.

5. The apparatus of claim 2 wherein the delay unit is arranged to compensate the inter time difference for a decimation factor of the decimation in order to determine a delay for at least one of the first audio signal and the second audio signal.

6. The apparatus of claim 2 wherein the time difference unit is arranged to apply a spectral whitening to the first audio signal and the second audio signal prior to the correlation.

7. The apparatus of claim 2 wherein the time difference unit is arranged to perform windowing of the first audio signal and the second audio signal prior to the cross correlation.

8. The apparatus of claim 2 wherein the time difference unit comprises:

a trellis state machine having a plurality of states, each of the plurality of states corresponding to a time offset of the plurality of time offsets;

a path unit for determining path metrics for states of the trellis state machine in response to the cross correlations;

a probability unit for determining state probability metrics for the states in response to path metrics associated with paths from previous states to current states; and

a unit for determining the inter time difference in response to the state probability metrics.

9. The apparatus of claim 1 wherein the delay unit is arranged to transition from a first delay to a second delay by generating a first compensated multi channel audio signal in response to the first delay and a second compensated multi channel audio signal in response to the second delay and to combine the first compensated multi channel audio signal and second compensated multi channel audio signal to generate the compensated multi channel audio signal.

11. The apparatus of claim 6 wherein the spectral whitening comprises computing linear predictive coefficients for the first audio signal and the second audio signal and to filter the first audio signal and the second audio signal in response to the linear predictive coefficients.

12. The apparatus of claim 2 wherein the time difference unit is arranged to normalise the cross correlations.

13. The apparatus of claim 8 wherein the time difference unit further comprises:

a unit for dividing the first audio signal and the second audio signal into a plurality of frames; and

the time difference unit is arranged to, for each state of the states of the trellis state machine, determining a new state probability metric in response to a previous state probability metric for the state and correlation coefficients for a subset of time offsets.

14. The apparatus of claim 13 wherein the subset of time offsets for a first state includes only the time offset associated with the first state and a pair of time offsets being adjacent to the time offset associated with the first state.

15. The apparatus of claim 14 wherein the time difference unit is arranged to weigh a cross correlation for the time offset associated with the first state at least five times higher than cross correlations for the pair of time offsets.

16. The apparatus of claim 14 wherein the unit for determining the inter time difference is arranged to determine the inter time difference from a time offset associated with a state having a highest state probability metric.

17. The apparatus of claim 9 wherein the delay unit is arranged to gradually reduce a contribution to the compensated multi channel audio signal from the first compensated multi channel audio signal and to gradually increase a contribution to the compensated multi channel audio signal from the second compensated multi channel audio signal.

18. The apparatus of claim 9 wherein the delay unit is arranged to generate the compensated multi channel audio signal as a weighted summation of the first compensated multi channel audio signal and the second compensated multi channel audio signal.

19. The apparatus of claim 1 wherein the mono encoder is a speech encoder.

20. The apparatus of claim 1 wherein the mono encoder is a Code Excited Linear Prediction coder.

10. A method of encoding a multi channel audio signal, the method comprising:

receiving the multi channel audio signal comprising at least a first audio signal from a first microphone and a second audio signal from a second microphone;

determining an inter time difference between the first audio signal and the second audio signal;

generating a compensated multi channel audio signal from the multi channel audio signal by delaying at least one of the first audio signal and the second audio signal in response to the inter time difference signal;

generating a mono signal by combining channels of the compensated multi channel audio signal; and

encoding the mono signal in a mono signal encoder.