Converter and method for converting an audio signal

1. A method for simplifying a model of an acoustic environment, wherein said model comprises a set of transfer functions, each transfer function corresponding to the sound propagation from a sound emitting position to a sound receiving position in said acoustic environment, the method comprising, for each transfer function, the steps of:

calculating a first simulated reverberation of a first stimulus emitted at said sound emitting position as received at said sound receiving position by applying said transfer function to said first stimulus;

selecting a subset of a set of local maxima in a first intensity envelope of the first simulated reverberation; and

calculating a simplified transfer function which, applied to said first stimulus, provides a second simulated reverberation with a second intensity envelope matching said selected subset of local maxima of the first simulated reverberation.

2. A method as claimed in claim 1 , wherein the number of said selected subset of local maxima is not higher than a predetermined maximum.

3. A method as claimed in claim 2 , wherein said selected subset of local maxima is selected from among those above a time-intensity attenuation function.

4. A method as claimed in claim 1 , wherein said selected subset of local maxima is selected from among those above a time-intensity attenuation function.

5. A method as claimed in claim 4 , wherein said attenuation function is an exponential attenuation function.

6. A method as claimed in claim 5 , wherein said output signal comprises an early part and a late part.

7. A method as claimed in claim 6 , wherein only the early part is generated using said simplified transfer function.

8. A method as claimed in claim 7 , wherein the late part is generated by algorithmic reverberation of the input signal.

9. A method as claimed in claim 5 , wherein said simplified transfer function is applied by convolution with said first stimulus.

10. A method as claimed in claim 5 , wherein said simplified transfer function is expressed as a combination of a signal delay and a signal attenuation for each selected local maximum, and is applied to said first stimulus in a time domain operation.

11. A method as claimed in claim 1 , wherein said simplified transfer function is applied by convolution with said first stimulus.

12. A method as claimed in claim 1 , wherein said simplified transfer function is expressed as a combination of a signal delay and a signal attenuation for each selected local maximum, and is applied to said first stimulus in a time domain operation.

13. A method for converting an first audio stream comprising N input channels into a second audio stream comprising M output channels, the method comprising, for each input and output channel, the steps of:

selecting, in a model of an M-channel acoustic environment simplified using a method according to claim 1 , a simplified transfer function associated with said input channel and output channel; and

processing an input signal from said input channel by applying said selected simplified transfer function to generate at least part of an output signal for said output channel.

14. A method as claimed in claim 13 , wherein M>N.

15. A method as claimed in claim 14 , wherein the selected subsets of local maxima matched by the simplified transfer functions are different for at least two of the M output channels, and preferably for all M output channels.

16. A method as claimed in claim 14 , wherein said output signal comprises an early part and a late part.

17. A method according to claim 16 , wherein only the early part is generated using said simplified transfer function and the late part is generated by algorithmic reverberation of the input signal.

18. A method as claimed in claim 13 , wherein the selected subsets of local maxima matched by the simplified transfer functions are different for at least two of the M output channels, and preferably for all M output channels.

19. A converter for converting a first audio signal comprising N input channels into a second audio signal comprising M calculated channels, using a method according to claim 13 , the converter comprising:

N inputs and M outputs; and

a processor for applying said set of simplified transfer functions to an audio signal received on an input channel so as to obtain reverberation components of a calculated output channel; and

a data carrier with at least coefficients associated with said set of simplified transfer functions, for use by said processor.

20. A converter as claimed in claim 19 , wherein said data carrier is a transient computer memory connected to said processor.