Real-Valued Filterbank Gain Coupling for Aliasing Control

Solution Overview

Problem

Real-valued subband filterbanks used for spectral envelope adjustment introduce severe aliasing, particularly in audio signals with strong tonal structures, leading to intermodulation distortion due to independent gain values in adjacent channels.

Innovation Solution

The solution involves analyzing subband channels to assess strong tonal components using low-order linear prediction and grouping gain values of adjacent channels based on the location of zeros in the LPC polynomial, ensuring coupled gain factors for channels sharing sinusoidal components to maintain aliasing cancellation properties.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If a real-valued cosine modulated filterbank is used for spectral envelope adjustment, then computational complexity is reduced compared to complex exponential modulated filterbank, but severe aliasing is introduced particularly for signals with strong tonal structure

Engineering Contradiction:

Improvecomputational complexityVSAvoidaliasing

Core Design Contradiction:

Device complexityVSObject-generated harmful factors

Solution Approach 1:

The patent applies local quality by making the gain adjustment signal-dependent. Specifically, the gain values are modified based on the detected tonal components in each subband channel. When a strong tonal component is detected at a specific frequency, the gain adjustment is localized to affect only the channels surrounding that frequency, rather than applying uniform gain adjustment across all channels. This localized approach maintains aliasing cancellation properties while enabling spectral envelope adjustment.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent implements dynamics by making the gain values adaptive rather than static. The gain adjustment is dynamically determined based on the instantaneous spectral content of the signal, specifically detecting strong tonal components and adjusting gains accordingly. This dynamic adaptation allows the filterbank to maintain optimal performance for spectral envelope adjustment while preserving aliasing cancellation properties when needed.

Inventive Principle:

Principle #15Dynamics

2Manufacturing precision

If independent gain values are applied to adjacent channels for spectral envelope adjustment, then spectral envelope control is improved, but intermodulation distortion occurs due to aliasing components

Engineering Contradiction:

Improvespectral envelope controlVSAvoidintermodulation distortion

Core Design Contradiction:

Manufacturing precisionVSObject-generated harmful factors

Solution Approach 1:

The patent applies local quality by making the gain adjustment signal-dependent. Specifically, the gain values are modified based on the detected tonal components in each subband channel. When a strong tonal component is detected at a specific frequency, the gain adjustment is localized to affect only the channels surrounding that frequency, rather than applying uniform gain adjustment across all channels. This localized approach maintains aliasing cancellation properties while enabling spectral envelope adjustment.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent converts the harmful aliasing effect into a beneficial property by exploiting the aliasing cancellation phenomenon. By detecting strong tonal components and adjusting gain values specifically for channels surrounding those frequencies, the patent causes the aliasing components from adjacent channels to cancel each other out. This transforms what would normally be harmful intermodulation distortion into a mechanism that preserves signal quality while enabling spectral envelope control.

Inventive Principle:

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

3Object-generated harmful factors

If gain values are restrained in a signal dependent manner to reduce aliasing, then aliasing components are reduced below stop-band level, but spectral envelope adjustment flexibility is limited

Engineering Contradiction:

Improvealiasing componentsVSAvoidspectral envelope adjustment flexibility

Core Design Contradiction:

Object-generated harmful factorsVSAdaptability or versatility

Solution Approach 1:

The patent implements dynamics by making the gain values adaptive rather than static. The gain adjustment is dynamically determined based on the instantaneous spectral content of the signal, specifically detecting strong tonal components and adjusting gains accordingly. This dynamic adaptation allows the filterbank to maintain optimal performance for spectral envelope adjustment while preserving aliasing cancellation properties when needed.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent applies parameter changes by modifying the gain values based on detected tonal components. When a strong tonal component is detected at frequency f, the gain parameters for channels surrounding that frequency are adjusted in a coordinated manner. This parameter adaptation enables the system to maintain aliasing cancellation properties (keeping aliasing below stop-band level) while still providing sufficient flexibility for spectral envelope adjustment in non-tonal regions.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS7577570B2Method for reduction of aliasing introduced by spectral envelope adjustment in real-valued filterbanks

Publication Date: 2009.08.18 DOLBY INTERNATIONAL AB

AI summary

The present invention proposes a new method for improving the performance of a real-valued filterbank based spectral envelope adjuster. By adaptively locking the gain values for adjacent channels dependent on the sign of the channels, as defined in the application, reduced aliasing is achieved. Furthermore, the grouping of the channels during gain-calculation, gives an improved energy estimate of the real valued subband signals in the filterbank.