Multirate FIR Room Equalization for Low-Frequency Acoustic Peaks

Solution Overview

Problem

Achieving effective low-frequency equalization below 300 Hz with realizable digital equalization filters is challenging due to significant spectral peaks and dips in the Loudspeaker-Room Transfer Function, leading to sound degradation in loudspeaker-room acoustic systems.

Innovation Solution

A combined multirate-based Finite Impulse Response (FIR) filter technique using a low-order FIR filter at a lower rate for low-frequency equalization and a complementary minimum-phase FIR filter at a higher rate for high-frequency equalization, with splicing of outputs and level adjustment to maintain a flat magnitude response, reducing system delay and computational complexity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If a single high-order FIR filter is used for low-frequency equalization below 300 Hz, then equalization performance improves, but computational complexity and filter order increase significantly

Engineering Contradiction:

Improveequalization performanceVSAvoidfilter order

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The frequency range is segmented into low-frequency band (below 300 Hz) and high-frequency band (above 300 Hz). Separate FIR filters are designed for each band, allowing the low-frequency filter to operate at a lower rate with reduced order while maintaining effective equalization performance in the challenging low-frequency region.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The sampling rate of the low-frequency FIR filter is reduced to a lower rate compared to the original audio sampling rate. This parameter change allows the filter to achieve effective low-frequency equalization with reduced computational complexity and lower filter order, as the reduced sampling rate decreases the number of coefficients needed.

Inventive Principle:

Principle #35Parameter changes

2Manufacturing precision

If a single high-order FIR filter is used for low-frequency equalization below 300 Hz, then equalization performance improves, but computational requirements increase

Engineering Contradiction:

Improveequalization performanceVSAvoidcomputational requirements

Core Design Contradiction:

Manufacturing precisionVSUse of energy by moving object

Solution Approach 1:

The frequency range is segmented into low-frequency band (below 300 Hz) and high-frequency band (above 300 Hz). Separate FIR filters are designed for each band, allowing the low-frequency filter to operate at a lower rate with reduced order while maintaining effective equalization performance in the challenging low-frequency region.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The sampling rate of the low-frequency FIR filter is reduced to a lower rate compared to the original audio sampling rate. This parameter change allows the filter to achieve effective low-frequency equalization with reduced computational complexity and lower filter order, as the reduced sampling rate decreases the number of coefficients needed.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If multirate-based filtering is used for low-frequency equalization, then computational complexity reduces, but system delay may increase

Engineering Contradiction:

Improvecomputational complexityVSAvoidsystem delay

Core Design Contradiction:

Device complexityVSLoss of time

Solution Approach 1:

The frequency range is segmented into low-frequency band (below 300 Hz) and high-frequency band (above 300 Hz). Separate FIR filters are designed for each band, allowing the low-frequency filter to operate at a lower rate with reduced order while maintaining effective equalization performance in the challenging low-frequency region.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The input signal is pre-filtered using a low-pass filter before being fed to the low-frequency FIR filter. This preliminary action prepares the signal for efficient processing at the reduced sampling rate, minimizing the impact of multirate conversion on system delay while maintaining computational efficiency.

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentUS8077880B2Combined multirate-based and fir-based filtering technique for room acoustic equalization

Publication Date: 2011.12.13 SOUND UNITED LLC

AI summary

A combined multirate-based Finite Impulse Response (FIR) filter equalization technique combines a low-order FIR equalization filter operating at a lower rate for equalization of a loudspeaker-room response at low frequencies, and a complementary low-order minimum-phase FIR equalization filter operating at a higher rate for equalization of the loudspeaker-room response at higher frequencies. The design of two complementary band filters for separately performing low and high frequency equalization, keeps the system delay at a minimum while maintaining excellent equalization performance. Splicing between the two equalization filters, for maintaining a flat magnitude response in the transition region of the two complementary filters, is done automatically through level adjustment of one equalization filter relative to the other. The present invention achieves excellent equalization at low filter orders and hence reduced computational complexity.