Overview

Time varying 1st order filter controlled using an input pin.

Discussion

This module implements a time-varying 1st order filter. The first input pin specifies the cutoff frequency of the module, in Hz. We only use the first element as the cutoff frequency for the block processing. The second input pin is the audio signal. The audio signal can contain any number of channels and have any blockSize.

Internally, every time the module processes a block of data, it runs a simple design equation to compute the coefficients of the first order filter. Then, filter coeff smoothing is done on the block bases and processing is on a sample-by-sample basis. The .smoothingTime control variable specifies the smoothing time of the coefficient update (in milliseconds). Note, only use this module if you need time varying behavior. The module can be configured as either LPF or HPF.

Type Definition

typedef struct _ModuleFOFControl
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    FLOAT32 smoothingTime;                        // Time constant of the smoothing process
    INT32 filterType;                             // LPF = 0 and HPF = 1
    FLOAT32 smoothingCoeff;                       // Smoothing coefficient
    FLOAT32 currentA1;                            // Instantaneous a1 filter coefficient
    FLOAT32* coeffs;                              // coefficients
    FLOAT32* state;                               // State variables. 4 per channel
} ModuleFOFControlClass;

Variables

Properties

Pins

Input Pins

Name: f

Description: Cutoff frequency (Hz)

...

Sample rate range: Unrestricted

Complex support: Real

Output Pins

Name: out

Description: audio output

Data type: float

MATLAB Usage

File Name: fof_control_module.m

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