Overview
Subband normalized LMS adaptive filter
Discussion
The SbNLMSV3 module implements a subband-based normalized LMS algorithm. This module is identical to v2 except for a clarification of the names of the input and output signals. NLMS, when used as an Acoustic Echo Canceller, should use the "desired" signal for the mic input, and the "input" signal for the AEC reference. The pin locations have been swapped to match those of other sb aec modules. There is a separate complex filter within each subband. The module takes 2 single channel inputs: A desired signal and an input to be filtered so that can match the desired signal. For an AEC application, the reference signal goes to input, and the mic signal goes to desired. For every data sample the filtered output is compared to the desired signal and this information is used to update the filter taps according to the normalized LMS algorithm. The module always has 2 single channel ouputs: adaptive filter output and error signal, (desired-(filter output)). The error signal is the output for an AEC system, while the filtered output is ignored. The array .coeffs holds complex filter coefficients and is of size numTaps x numBins, where numBins is the number of frequency domain bins (or subbands). numBins equals the FFT size / 2 + 1. The coefficients are stored in coeffs in normal order. The state variable array is complex data of size numTaps x numBins, and circular buffer is used within each subband.
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The constructor argument MUINPUT determines where the adaptation parameter mu is taken from. By default, MUINPUT = 0 and the adaptation parameter is taken from the internal parameter .mu; the same factor is used for all subbands. If MUINPUT = 1, then the module has a 3rd input pin. This pin contains real data of size FFT size / 2 + 1 and each point specifies the mu factor for each subband. This allows external logic to be used when computing mu and allows mu to vary between subbands. When MUINPUT = 1 then the internal paramter .mu is ignored.
Type Definition
Code Block |
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typedef struct _ModuleSbNLMSV3 { ModuleInstanceDescriptor instance; // Common Audio Weaver module instance structure INT32 maxTaps; // Maximum length of the filter INT32 numTaps; // Current length of the filter FLOAT32 mu; // Adaptation constant FLOAT32 offset; // Normalization offset constant INT32 stateIndex; // Index of the oldest state variable in the array of state variables FLOAT32* coeffs; // Coefficient array FLOAT32* state; // State variable array FLOAT32* W; // Weighted error from last iteration } ModuleSbNLMSV3Class; |
Variables
Properties
Name | Type | Usage | isHidden | Default value | Range | Units |
maxTaps | int | const | 0 | 32 | 1:1:5000 | samples |
numTaps | int | parameter | 0 | 32 | 1:1:32 | samples |
mu | float | parameter | 0 | 0.5 | 0:1 | linear |
offset | float | parameter | 0 | 0.001 | 0:0.001 | linear |
stateIndex | int | state | 1 | 0 | Unrestricted | |
coeffs | float* | state | 0 | [32 x 32] | Unrestricted | |
state | float* | state | 1 | [32 x 32] | Unrestricted | |
W | float* | state | 1 | [32 x 1] | Unrestricted |
Pins
Input Pins
Name: desired
Description: Desired signal at filtered output
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Sample rate range: Unrestricted
Complex support: Complex
Output Pins
Name: output
Description: Output of adaptive filtered input
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Description: Error signal = difference between filtered output and desired signal (AEC output)
Data type: float
MATLAB Usage
File Name: sb_nlms_v3_module.m
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