Overview

Frequency domain subband adaptive filter

Discussion

Subband based mono echo canceler. This module works in conjunction with the WOLA filterbank modules. The module has two input pins. The first input is a single channel microphone signal and the second is a mono reference signal.

At instantiation time you specify the length of each subband filter. Then at run time, you specify the number of taps currently used. This allows you to easily vary the filter length at run time and verify performance and CPU load.

The module has optional residual noise suppression which is enabled via the enableRNS variable. By default, residual noise suppression is on and the output of the echo canceler (the error signal) is further reduced in level. The residual noise suppression is configured by a 7 element interpolation table. This table operates just like the Table X-Y module with configurable (X, Y) lookup points. The lookup table specifies values in dB for the input and output function

Type Definition

typedef struct _ModuleSbAECV2
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    INT32 maxTaps;                                // Maximum length of the filter
    INT32 numTaps;                                // Current length of the filter
    FLOAT32 V;                                    // Adaptation constant
    FLOAT32 smoothingTime;                        // Time constant of the smoothing process.
    FLOAT32 A;                                    // Smoothing factor
    FLOAT32 p0;                                   // Starting value for covariance estimate
    INT32 numIter;                                // Number of iterations of adaptive loop
    INT32 enableRNS;                              // Enable residual noise suppression
    INT32 freezePin;                              // Availability of the freeze pin
    INT32 stateIndex;                             // Circular buffer index. [0 numTaps-1]
    INT32 RNS_numPoints;                          // Number of values in the lookup table. The total table size is [maxPoints 2].
    INT32 RNS_order;                              // Order of the interpolation. This can be either 2 (for linear) or 4 (for pchip).
    FLOAT32* WW1;                                 // Adaptive filter coefficients.
    FLOAT32* P;                                   // Real estimation error covariance matrix.
    FLOAT32* CC;                                  // Complex state variables (previous inputs).
    FLOAT32* Pplus;                               // Intermediate result (real scratch).
    FLOAT32* Wplus;                               // Intermediate result (complex scratch).
    FLOAT32* energyC;                             // Intermediate result (real scratch).
    FLOAT32* mu;                                  // Mu value (real).
    FLOAT32* W2;                                  // Residual noise suppression weights.
    FLOAT32* RNS_table;                           // Lookup table. The first row is the X values and the second row is the Y values.
    FLOAT32* RNS_polyCoeffs;                      // Interpolation coefficients returned by the grid control.
} ModuleSbAECV2Class;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
maxTaps	int	const	0	16	Unrestricted	samples
numTaps	int	parameter	0	16	1:1:16	samples
V	float	const	0	0.5	Unrestricted
smoothingTime	float	parameter	0	10	0.1:1000	sec
A	float	derived	0	1	0:1
p0	float	parameter	0	1	Unrestricted
numIter	int	parameter	0	3	1:20
enableRNS	int	parameter	0	1	0:1:1
freezePin	int	const	1	0	Unrestricted
stateIndex	int	state	1	0	Unrestricted
RNS_numPoints	int	const	1	7	4:1:1000
RNS_order	int	const	1	2	2:2:4
WW1	float*	state	0	[16 x 32]	Unrestricted
P	float*	state	0	[16 x 32]	Unrestricted
CC	float*	state	0	[16 x 32]	Unrestricted
Pplus	float*	state	0	[1 x 16]	Unrestricted
Wplus	float*	state	0	[1 x 16]	Unrestricted
energyC	float*	state	0	[16 x 32]	Unrestricted
mu	float*	state	0	[16 x 32]	Unrestricted
W2	float*	state	0	[32 x 1]	Unrestricted
RNS_table	float*	parameter	1	[2 x 7]	Unrestricted
RNS_polyCoeffs	float*	state	1	[4 x 6]	Unrestricted

Pins

Input Pins

Name: dataIn

Description: Micro input

Data type: float

Channel range: 1

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Complex

Name: reference

Description: Reference input

Data type: float

Channel range: 1

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Complex

Output Pins

Name: errorSignal

Description: Error signal = difference between filter output and dataIn

Data type: float

MATLAB Usage

File Name: sb_aec_v2_module.m

 M = sb_aec_v2_module(NAME, MAXTAPS, FREEZEPIN)
 Frequency domain subband echo canceler using a Kalman filter based
 algorithm.  Arguments:
    NAME - name of the module.
    MAXTAPS - maximum length of each frequency domain complex adaptive filter
    FREEZEPIN - If 1, a freeze pin is exposed. The freeze pin, if 1, will
    stop the adaptation of the adaptive filter. (Post filter's adaptation
    is not frozen).