Method and device for low delay processing

Abstract

Adaptive processing of an input signal is achieved by offline analysis, with inline processing comprising an adaptive filter. The method comprises passing the input signal through an adaptive time domain filter to produce an output signal. The input signal and/or output signal is used as an offline analysis signal. The analysis signal is transformed into a transform domain (eg frequency domain) to produce a transformed analysis signal. The transformed analysis signal is analyzed, for example by ADRO, to produce a plurality of desired gains each corresponding to a respective transform domain sub-band. A time domain filter characteristic is synthesized to at least approach the desired gains. The adaptive filter is updated with the synthesized filter characteristic. Minimum phase adaptive filter techniques are found to possess particular benefits in this scheme.

1 . A method of adaptively processing an input signal, the method comprising:
 passing the input signal through an adaptive time domain filter to produce an output signal;   taking at least one of the input signal and the output signal as an analysis signal;   transforming the analysis signal into a transform domain to produce a transformed analysis signal;   analyzing the transformed analysis signal to produce a plurality of desired gains each corresponding to a respective transform domain sub-band;   synthesizing a minimum phase time domain filter characteristic which approaches the desired gains; and   updating the adaptive filter with the synthesized filter characteristic.   
   
   
       2 . The method of  claim 1  further comprising limiting phase non-linearity of the adaptive filter below a perception threshold. 
   
   
       3 . The method of  claim 1  wherein the adaptive filter is a FIR filter, and wherein synthesis of the minimum phase filter characteristic comprises:
 expressing a characteristic polynomial of the FIR filter;   factoring the characteristic polynomial to locate poles and zeros of the FIR filter; and   revising the characteristic polynomial by reciprocating each pole and zero outside the unit circle into the unit circle, to obtain a minimum phase FIR filter.   
   
   
       4 . The method of  claim 1  further comprising applying preventative measures to reduce the rate of gain variation between the desired gains, such that the synthesized filter is more likely to be able to replicate a response defined by the desired gains. 
   
   
       5 . The method of  claim 1  further comprising applying compensatory measures to measure the extent of gain errors in the output signal, and to apply appropriate feedback for ongoing iterations of generating desired gains and synthesizing the filter, so as to reduce such errors. 
   
   
       6 . The method of  claim 1  wherein the adaptive time domain filter is selected from a single FIR filter, a mix of filters, whether FIR or IIR, a non-linear filter and a neural network. 
   
   
       7 . The method of  claim 1  wherein updating the adaptive filter to at least approach the synthesized filter characteristic comprises varying the adaptive filter by no more than a pre-defined amount, in order to avoid inappropriate user perceptions. 
   
   
       8 . A computer program for adaptively processing an input signal, the computer program comprising:
 code for passing the input signal through an adaptive time domain filter to produce an output signal;   code for taking at least one of the input signal and the output signal as an analysis signal;   code for transforming the analysis signal into a transform domain to produce a transformed analysis signal;   code for analyzing the transformed analysis signal to produce a plurality of desired gains each corresponding to a respective transform domain sub-band;   code for synthesizing a minimum phase time domain filter characteristic which approaches the desired gains; and   code for updating the adaptive filter with the synthesized filter characteristic.   
   
   
       9 . The computer program of  claim 8  comprising code for limiting phase non-linearity of the adaptive filter below a perception threshold. 
   
   
       10 . A device for adaptively processing an input signal, the device comprising:
 an adaptive time domain filter for producing an output signal from an input signal;   a transform block for taking an analysis signal from at least one of the input signal and the output signal, and for transforming the analysis signal into a transform domain to produce a transformed analysis signal;   a processor for analyzing the transformed analysis signal to produce a plurality of desired gains each corresponding to a respective transform domain sub-band;   a filter synthesizer for synthesizing a minimum phase time domain filter characteristic which approaches the desired gains, and for updating the adaptive filter with the synthesized filter characteristic.   
   
   
       11 . The device of  claim 10  wherein the filter synthesizer is for limiting phase non-linearity of the adaptive filter below a perception threshold.