Low delay decimator and interpolator filters
Systems and methods for low latency adaptive noise cancellation include an audio sensor to sense environmental noise and generate a noise signal, an audio processing path to receive an audio signal, process the audio signal through an interpolation filter, and generate a primary audio signal having a first sample frequency, an adaptive noise cancellation processor to receive the noise signal and generate an anti-noise signal, a direct interpolator to receive the anti-noise signal and generate an anti-noise signal having the first sample frequency, and a limiter to provide clipping to reduce a number of bits in the anti-noise signal, an adder operable to combine the primary audio signal and the anti-noise signal and generate a combined output signal, and a low latency filter to process the combined output signal.
1. A system comprising:
an audio sensor configured to sense environmental noise and generate a noise signal;
an audio processing path configured to receive an audio signal, process the audio signal through an interpolation filter, and generate a primary audio signal having a first sample frequency;
an adaptive noise cancellation processor configured to receive the noise signal and generate a corresponding anti-noise signal;
a direct interpolator configured to receive the anti-noise signal and generate an upsampled anti-noise signal having the first sample frequency without filtering aliased images;
an adder configured to receive and combine the primary audio signal and the upsampled anti-noise signal and generate a combined output signal; and
a low latency filter configured to process the combined output signal to filter out the aliased images.
2. The system of claim 1 , wherein the low latency filter comprises a plurality of filters, each performing filtering at a different sample frequency.
3. The system of claim 2 , wherein the low latency filter comprises a plurality of lattice wave filters disposed in a cascaded arrangement, wherein each of the plurality of lattice wave filters processes a different frequency band.
4. The system of claim 3 , wherein the sample frequency is increased in integer steps in each successive filter.
5. The system of claim 3 , wherein the lattice wave filters include a plurality of delay elements; and wherein direct sampling at a particular output sample frequency is achieved by interlacing multiple filters.
6. The system of claim 5 , wherein N delay elements are provided in reflector sections and one path is delayed by N/2 delay elements, and another path is directly connected to an input signal; and wherein N is a sequence of positive integer exponents of 2.
7. The system of claim 3 , wherein each lattice wave filter comprises two paths, including one path including a plurality of reflector elements with each reflector element delayed by N unit delays, where N is an integer greater than one, and one path delayed by M delay elements, where M is an integer greater than one.
8. The system of claim 1 , wherein the adaptive noise cancellation processor is further operable to derive the anti-noise signal by calculating filter coefficients using by a filtered-X least mean squares process.
9. The system of claim 1 wherein the direct interpolator comprises a sign extension stage operable to extend most significant bits of the anti-noise signal to avoid overflow, and a limiter operable to provide clipping to reduce a number of bits in the upsampled anti-noise signal.
10. A system comprising
an audio processing path configured to receive and process an oversampled primary audio signal having a first sample frequency;
an adaptive noise cancellation path comprising a decimator configured to downsample the primary audio signal to a second sampling frequency, an adaptive noise cancellation processor configured to receive the primary audio signal and a noise signal at the second sample frequency and generate an anti-noise signal having the second sample frequency, and an interpolator configured to upsample the anti-noise signal to the first sample frequency without filtering aliased images;
an adder configured to combine the anti-noise signal and the primary audio signal at the first sample frequency; and
a low latency filter configured to process the combined anti-noise signal and the primary audio signal to filter out the aliased images;
wherein the decimator comprises a first lattice wave filter, and the interpolator comprises a second lattice wave filter having two paths with N delay elements and M delay elements, respectively.
11. The system of claim 10
wherein the first lattice wave filter comprises
a first path including a plurality of reflector elements with each reflector element delayed by N delay elements, where N is an integer greater than two; and
a second path delayed by M delay elements, where M is an integer greater than one.
12. The system of claim 10 , further comprising a microphone operable to sense environmental noise and generate corresponding electrical signals; and a low delay decimator to generate the noise signal at the second sample frequency.
13. The system of claim 10 , further comprising an oversampled interpolation filter that has input and output sample frequencies that match the first sample frequency; and
wherein the oversampled interpolation filter is operable to remove aliased images generated by the interpolator in the adaptive noise cancellation path.
14. The system of claim 10 , wherein the first lattice wave filter and the second lattice wave filter each comprise a multi-stage lattice wave filter structure where each stage changes an operating sample rate by a factor of two.
15. The system of claim 14 , wherein the decimator and interpolator each comprise a sign extension stage operable to extend most significant bits of a received signal to avoid overflow, and a limiter operable to provide clipping to reduce a number of output bits.
16. A method comprising
Sensing environmental noise and generating a noise signal;
Processing an audio signal through an interpolation filter to generate a primary audio signal having a first sample frequency;
generating, from the noise signal, an anti-noise signal having a second sample frequency; directly interpolating the anti-noise signal to generate an upsampled anti-noise signal having the first sample frequency without filtering aliased images; combining the primary audio signal and the upsampled anti-noise signal to produce a combined output signal; and processing the combined output signal through a low latency filter to filter the aliased images.
17. The method of claim 16 , wherein filtering comprises applying a plurality of lattice wave filters disposed in a cascaded arrangement, wherein each of the plurality of lattice wave filters processes a different sample frequency that is successively changed in each successive filter.
18. The method of claim 16 , wherein directly interpolating includes extending most significant bits of the anti-noise signal to avoid overflow, and clipping to reduce a number of output bits in the upsampled anti-noise signal.
19. The method of claim 16 , further comprising decimating the primary audio signal to downsample the primary audio signal to the second sample frequency; and wherein the generating, from the noise signal, the anti-noise signal having the second sample frequency further includes analyzing the downsampled primary audio signal.
20. The method of claim 16 , wherein generating, from the noise signal, the anti-noise signal having the second sample frequency comprises calculating filter coefficient using a filtered-X least mean squares process.