Spherical Microphone Array Equalization for Ambisonics

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Solution Overview

Problem

Spherical microphone arrays on a rigid sphere face challenges in accurately capturing and reconstructing three-dimensional sound fields due to distortion caused by spatial aliasing and missing higher-order Ambisonics coefficients, leading to unbalanced spectral power in reconstructed signals, especially in beam forming applications.

Innovation Solution

A method and apparatus that apply an equalization filter to balance the frequency spectrum of reconstructed Ambisonics signals by estimating the signal-to-noise ratio and computing an adapted transfer function to correct for spatial aliasing and noise, using a noise minimizing filter and inverse microphone array transfer function, which can be stored in a look-up table for reduced computational complexity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If spatial aliasing and missing higher-order coefficients are present in spherical microphone array recordings, then the Ambisonics representation can be obtained with limited computational complexity, but the spectral power of the reconstructed signal becomes unbalanced and distorted

Engineering Contradiction:

Improvecomputational complexityVSAvoidspectral power balance

Core Design Contradiction:

Device complexityVSManufacturing precision

Solution Approach 1:

The patent applies parameter changes by modifying the frequency response characteristics through equalization filters. The transfer function is adjusted to compensate for spectral distortions caused by spatial aliasing and missing higher-order coefficients, thereby balancing the spectral power without requiring complex computational methods.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent introduces an equalization filter as an intermediary component between the microphone array and the Ambisonics decoder. This filter mediates the spectral distortions by pre-compensating for the effects of spatial aliasing and missing higher-order coefficients, achieving balanced spectral power with reduced computational complexity.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Productivity

If higher-order Ambisonics coefficients are removed to reduce computational complexity, then processing becomes more efficient, but the spectral power spectrum becomes unbalanced especially for beam forming applications

Engineering Contradiction:

Improveprocessing efficiencyVSAvoidspectral power balance

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

The patent extracts and compensates for the harmful effects of removed higher-order coefficients by applying equalization filters that specifically target and correct the spectral imbalances introduced by their removal, allowing efficient processing while maintaining spectral power balance.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent changes the frequency response parameters through equalization filters to compensate for the spectral distortions caused by removing higher-order coefficients, thereby maintaining processing efficiency while restoring spectral power balance for accurate beam forming.

Inventive Principle:

Principle #35Parameter changes

3Reliability

If noise reduction is applied to spherical microphone arrays, then signal quality improves, but higher order coefficients become missing and unbalance the spectral power spectrum

Engineering Contradiction:

Improvesignal qualityVSAvoidspectral power balance

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The patent converts the harmful effect of noise reduction into a beneficial process by using equalization filters to compensate for the spectral imbalances introduced by noise reduction. The filters transform the distorted spectral power spectrum back into a balanced representation, turning the negative effect of noise reduction into a controlled process.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent applies parameter changes through equalization filters that adjust the frequency response to compensate for spectral distortions caused by noise reduction. This restores spectral power balance while maintaining the benefits of noise reduction for improved signal quality.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS9420372B2Method and apparatus for processing signals of a spherical microphone array on a rigid sphere used for generating an ambisonics representation of the sound field

Publication Date: 2016.08.16 DOLBY LABORATORIES LICENSING CORP

AI summary

Spherical microphone arrays capture a three-dimensional sound field (P(Ωc,t) for generating an Ambisonics representation (Anm(t)), where the pressure distribution on the surface of the sphere is sampled by the capsules of the array. The impact of the microphones on the captured sound field is removed using the inverse microphone transfer function. The equalization of the transfer function of the microphone array is a big problem because the reciprocal of the transfer function causes high gains for small values in the transfer function and these small values are affected by transducer noise. The invention estimates (73) the signal-to-noise ratio between the average sound field power and the noise power from the microphone array capsules, computes (74) the average spatial signal power at the point of origin for a diffuse sound field, and designs in the frequency domain the frequency response of the equalization filter from the square root of the fraction of a given reference power and the simulated power at the point of origin.