Method and device for low delay processing
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 FIR 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 FIR filter characteristic which approaches the desired gains, the synthesizing comprising:
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 a unit circle into the unit circle, to obtain a minimum phase FIR filter; and
updating the adaptive FIR filter below a perception threshold.
2. The method of claim 1 further comprising limiting phase non-linearity of the adaptive FIR filter below a perception threshold.
3. 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 FIR filter is more likely to be able to replicate a response defined by the desired gains.
4. 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 FIR filter, so as to reduce such errors.
5. The method of claim 1 wherein updating the adaptive FIR filter to at least approach the synthesized filter characteristic comprises varying the adaptive FIR filter by no more than a pre-defined amount, in order to avoid inappropriate user perceptions.
6. A non-transitory computer readable medium for adaptively processing an input signal, comprising instructions which, when executed by one or more processors, causes performance of the following:
passing the input signal through an adaptive time domain FIR 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 FIR filter characteristic which approaches the desired gains, the synthesizing comprising:
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 a unit circle into the unit circle, to obtain a minimum phase FIR filter; and
updating the adaptive FIR filter with the synthesized filter characteristic.
7. The non-transitory computer readable medium of claim 6 comprising instructions which, when executed by one or more processors, causes limiting phase non-linearity of the adaptive FIR filter below a perception threshold.
8. A device for adaptively processing an input signal, the device comprising:
an adaptive time domain FIR 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; and
a filter synthesizer for synthesizing a minimum phase time domain filter characteristic which approaches the desired gains, the synthesizing comprising 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 a unit circle into the unit circle, to obtain a minimum phase FIR filter, and the filter synthesizer for updating the adaptive FIR filter with the synthesized filter characteristic.
9. The device of claim 8 wherein the filter synthesizer is for limiting phase non-linearity of the adaptive FIR filter below a perception threshold.