Overview

Single band of a cascade graphic equalizer with High Precision Biquad fract32 algorithm

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_subsystem.m. This module is typically not used directly. Instead, use the graphic_eq_subsystem.m

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, 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 High Precision second order biquad sections. These filters are smoothly updating based upon the variable .smoothingTime. Note, some low frequency filters may go unstable if the smoothingTime is too large.

Type Definition

typedef struct _ModuleGraphicEQBandHpFract32
{
    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
    INT32 updateActive;                           // Specifies whether the filter coefficients are updating (=1) or fixed (=0).
    INT32 bShift;                                 // Number of bits to shift of Numerator coefficients
    INT32 aShift;                                 // Number of bits to shift of Denominator coefficients
    fract32 smoothingCoeff;                       // Smoothing coefficient
    FLOAT32* coeffs;                              // Matrix of filter coefficients. The size of the matrix is 5 x numStages. Each column contains the variables for a biquad arranged as [b0; b1; b2; a1; a2].
    fract32* coeffsFract32;                       // Matrix of filter coefficients. The size of the matrix is 5 x numStages. Each column contains the variables for a biquad arranged as [b0; b1; b2; a1; a2].
    fract32* currentCoeffsFract32;                // Matrix of filter coefficients. The size of the matrix is 5 x numStages. Each column contains the variables for a biquad arranged as [b0; b1; b2; a1; a2].
    fract32* state;                               // State variables. 6 per channel.
} ModuleGraphicEQBandHpFract32Class;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
gain	float	parameter	0	0	-12:12	dB
lowerEdge	float	parameter	0	100	10:20000	Hz
upperEdge	float	parameter	0	500	10:20000	Hz
smoothingTime	float	parameter	0	50	0:1000	msec
numStages	int	const	0	4	Unrestricted
updateActive	int	state	0	1	0:1
bShift	int	derived	1	4	Unrestricted
aShift	int	derived	1	1	Unrestricted
smoothingCoeff	fract32	derived	1	0.01324	Unrestricted
coeffs	float*	derived	1	[5 x 4]	Unrestricted
coeffsFract32	fract32*	derived	1	[5 x 4]	Unrestricted
currentCoeffsFract32	fract32*	state	1	[5 x 4]	Unrestricted
state	fract32*	state	1	[24 x 1]	Unrestricted

Pins

Input Pins

Name: in

Description: audio input

Data type: fract32

Channel range: Unrestricted

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Real

Output Pins

Name: out

Description: audio output

Data type: fract32

MATLAB Usage

File Name: graphic_eq_band_hp_fract32_module.m

 M=graphic_eq_band_hp_fract32_module(NAME, ORDER)
 Implements a single band of a cascade graphic equalizer.  This modules
 forms part of the overall graphic_eq_hp_fract32_subsystem.  Arguments:
    NAME - name of the module.
    ORDER - filter order of the stage.  This must be either 4, 8, or 12.
            By default, ORDER=8.