Multichannel decorrelation in spatial audio coding
Each of N audio signals are filtered with a unique decorrelating filter ( 38 ) characteristic, the characteristic being a causal linear time-invariant characteristic in the time domain or the equivalent thereof in the frequency domain, and, for each decorrelating filter characteristic, combining ( 40, 44, 46 ), in a time and frequency varying manner, its input (Zi) and output (Z-i) signals to provide a set of N processed signals (X i). The set of decorrelation filter characteristics are designed so that all of the input and output signals are approximately mutually decorrelated. The set of N audio signals may be synthesized from M audio signals by upmixing ( 36 ), where M is one or more and N is greater than M.
1. A method for processing a set of N audio signals, comprising filtering each of the N audio signals with a unique decorrelating filter characteristic, the characteristic being a causal linear time-invariant characteristic in the time domain or the equivalent thereof in the frequency domain, and, for each decorrelating filter characteristic, combining, in a time and frequency varying manner, its input and output signals to provide a set of N processed signals, wherein said set of N audio signals are synthesized from M audio signals, where M is one or more and N is greater than M, further comprising upmixing the M audio signals to N audio signals prior to filtering each of the N audio signals with a unique decorrelating filter characteristic.
2. A method according to claim 1 wherein each unique decorrelating filter characteristic is selected such that the output signal of each filter characteristic has less correlation with every one of the N audio signals than the corresponding input signal of each filter characteristic has with every one of the N audio signals and such that each output signal has less correlation with every other output signal than the corresponding input signal of each filter characteristic has with every other one of the N audio signals.
3. A method according to claim 1 further comprising receiving parameters describing desired spatial relationships among said N synthesized audio signals, and wherein said upmixing operates with the help of received parameters.
4. A method according to claim 2 further comprising receiving parameters describing desired spatial relationships among said N synthesized audio signals, and wherein said upmixing operates with the help of received parameters.
5. A method according to any one of claims 1 , 2 , 3 or 4 wherein each decorrelating filter characteristic is characterized by a model with multiple degrees of freedom.
6. A method according to claim 5 wherein each decorrelating filter characteristic has a response in the form of a frequency varying delay where the delay decreases monotonically with increasing frequency.
7. A method according to any ones of claims 1 , 2 , 3 or 4 wherein each decorrelating filter characteristic has a response in the form of a frequency varying delay where the delay decreases monotonically with increasing frequency.
8. A method according to claim 2 wherein the impulse response of each filter characteristic is specified by a sinusoidal sequence of finite duration whose instantaneous frequency decreases monotonically.
9. A method according to claim 8 wherein a noise sequence is added to the instantaneous phase of the sinusoidal sequence.
10. A method according to claim 1 , wherein said combining is a linear combining.
11. A method according to claim 1 , wherein the degree of combining by said combining operates with the help of received parameters.
12. A method according to claim 1 , further comprising receiving parameters describing desired spatial relationships among said N processed signals, and wherein the degree of combining by said combining operates with the help of received parameters.
13. A method according to claim 11 or claim 12 wherein each of the N audio signals represent channels and the received parameters helping the combining operation are parameters relating to interchannel cross-correlation.
14. A method according to claim 13 wherein other received parameters include parameters relating to one or more of interchannel amplitude differences and interchannel time or phase differences.
15. Apparatus adapted to perform the methods of any one of claims 1 , 2 , 3 or 4 .
16. A computer program, stored on a non-transitory computer-readable medium, for causing a computer to perform the methods of any one of claims 1 , 2 , 3 or 4 .
17. Apparatus for processing a set of N audio signals, comprising
means for filtering each of the N audio signals with a unique decorrelating filter characteristic, the characteristic being a causal linear time-invariant characteristic in the time domain or the equivalent thereof in the frequency domain,
for each decorrelating filter characteristic, means for combining, in a time and frequency varying manner, its input and output signals to provide a set of N processed signals, and
wherein said set of N audio signals are synthesized from M audio signals, where M is one or more and N is greater than M, further comprising an upmixer that upmixes the M audio signals to N audio signals prior to filtering each of the N audio signals with a unique decorrelating filter characteristic.