Phase Inversion Filter for Correcting Low Frequency Phase Distortion in a Loudspeaker System

Abstract

A filter for correcting phase distortion produced at low frequencies in a loudspeaker system is created by inverting the phase response of the determined complex-valued frequency response of a loudspeaker system. The inverted phase response is obtained by taking the complex conjugate of the phase response. The impulse response for the inverted phase response is obtained by means of an inverse Fourier transform of the inverted phase response. The impulse response provides a linear phase FIR filter having a long filter length. The linear phase FIR filter is applied to the audio signal input to the loudspeaker system. Prior to inverting the phase response, the determined complex-valued frequency response of a loudspeaker system can be subjected to high frequency blanking and polynomial smoothing. Also, the linear phase FIR filter can be subjected to a window function prior to applying the filter to the audio signal.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation of prior U.S. application Ser. No. 14 / 525,898 filed Oct. 28, 2014, which claims the benefit of U.S. Provisional Patent Application No. 61 / 896,899 filed Oct. 29, 2013.BACKGROUND OF INVENTION[0002]The present invention generally relates to systems and methods for correcting the frequency response of loudspeaker systems and more particularly relates to correcting phase distortion produced at low frequencies in a loudspeaker system.[0003]In many cases loudspeakers are perceived as having a particular tonal quality or sound, which is the result of the non-linear distortion in the loudspeaker's frequency response. One such example is a loudspeaker designed to impart a distinct sound to a guitar amplifier. In this case, non-linear distortion adds harmonic content or warmth, and diminished response at high frequencies prevents the guitar from sounding harsh. However, a loudspeaker meant to reproduce different so...

AI Technical Summary

Benefits of technology

The patent text describes a method to reduce noise in a system by smoothing the phase response. This is done by interpolating the phase response on a logarithmic frequency scale to ensure constant information per octave. A smoothing function is then fitted to the phase to create a polynomial approximation of the phase response. The technical effect is improved signal quality with reduced noise.

Problems solved by technology

Any non-linearity in a studio monitor will distort the audio output, a particularly undesirable result in a studio listening environment where decisions are made about the audio mix, microphone placement, et cetera, based on the audio output produced by the monitor.

A limitation in achieving accurate sound reproduction from studio monitors, and other full range loudspeakers, is phase distortion introduced at low frequencies.

This low-frequency roll-off in the magnitude response also results in a phase shift or phase lag.

With such filters, the cost for a flat low frequency magnitude response is low frequency phase distortion.

However, headphones do not provide an accurate stereo image and prevent easy interaction among studio professionals.

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