Overview

Single band of a cascade graphic equalizer

Discussion

This module implements a single stage of a graphic equalizer. The stage provides boost or cut over a specified frequency range. A multistage graphic EQ subsystem is constructed as a series cascade of these modules. The multistage graphic EQ is contained within graphic_eq_fract32_subsystem.m. This module is typically not used directly; use the graphic_eq_fract32_subsystem.m instead.

The variables .lowerEdge and .upperEdge specify the range of frequencies, in Hz, over which the filter is effective. The variable .gain specifies the boost or cut in dB.

The design of the module is based upon the paper "Graphic Equalizer Design User Higher-Order Recursive Filters" by Holters and Zoelzer. The order of the filter of either 4, 8, or 12 is specified upon instantiation. Higher order filters provide sharper cutoffs at the expense of additional computation. As a rule of thumb, you can use 4th order filters until you have approximately 5 filter bands, 8th order filters until you have 15 filter bands, and 12th order filters for up to 31 bands. The order of the filter can only be set at instantiation time.

Internally, the filter is implemented as a cascade of second order biquad sections.

Type Definition

typedef struct _ModuleGraphicEQBand
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    FLOAT32 gain;                                 // Gain in the center of the band, in dB.
    FLOAT32 lowerEdge;                            // Lower frequency edge of the band, in Hz.
    FLOAT32 upperEdge;                            // Upper frequency edge of the band, in Hz.
    FLOAT32 smoothingTime;                        // Time constant of the smoothing process.
    INT32 numStages;                              // Number of 2nd order stages in the filter.
    FLOAT32 smoothingCoeff;                       // Smoothing coefficient.
    FLOAT32* targetCoeffs;                        // Target coefficient array.
    FLOAT32* currentCoeffs;                       // Current coefficient array after smoothing.
    FLOAT32* state;                               // State variable array with 4 values per stage per channel.
} ModuleGraphicEQBandClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
gain	float	parameter	0	0	-12:0.1:12	dB
lowerEdge	float	parameter	0	100	20:20000	Hz
upperEdge	float	parameter	0	500	20:20000	Hz
smoothingTime	float	parameter	0	20	0:1000	msec
numStages	int	const	0	4	Unrestricted
smoothingCoeff	float	derived	1	0.03278	Unrestricted
targetCoeffs	float*	parameter	1	[5 x 4]	Unrestricted
currentCoeffs	float*	parameter	1	[5 x 4]	Unrestricted
state	float*	state	1	[8 x 1]	Unrestricted

Pins

Input Pins

Name: in

Description: audio input

Data type: float

Channel range: Unrestricted

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Real

Output Pins

Name: out

Description: audio output