Apparatus and method for encoding a multi channel audio signal

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.

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. 
     
     
         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.