EmphasisFilter

Overview

Audio pre- and de-emphasis

Discussion

Implements pre-emphasis and de-emphasis filters. These are first order recursive filters which are used in a variety of applications, typically for noise reduction. The pre-emphasis filter boosts high frequencies while the de-emphasis filter reduces high frequencies in a complementary fashion. The filters are designed so that a cascade of a pre-emphasis and de-emphasis filter with the same tau setting cancel each other out perfectly.

The cutoff frequency of the filter is specified via a time constant tau. tau is specified in microseconds and the 3 dB frequency is related to tau by the formula \(f_c = \frac{1}{2*\pi*\tau}.\)

Typical time constants are:

50 usec for US and Korean FM

75 usec for all other FM

The .filterType variable specifies the behavior of the module. If .filterType = 0, then the filter is bypassed (no change). If .filterType = 1, then the filter implements pre-emphasis (boost high frequencies). If .filterType = 2, then the filter implements de-emphasis (reduce high frequencies).

Module Pack

Advanced

ClassID

classID = 1267

Type Definition

typedef struct _ModuleEmphasisFilter
{
ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
INT32 filterType;                             // Specifies whether the filter implements bypass (=0) pre-emphasis (=1) or de-emphasis (=2)
FLOAT32 tau;                                  // Time constant of the filter
awe_modBiquadSmoothedInstance *filt; // 2nd order smoothly updating IIR filter} ModuleEmphasisFilterClass;

Variables

Properties

Name	Type	Usage	isHidden	Default Value	Range	Units
filterType	int	parameter	0	0	0:2
tau	float	parameter	0	75	0:10000	microseconds

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
Data Type	float

Matlab Usage

File Name: emphasis_filter_module.m 
 SYS = emphasis_filter_module(NAME) 
 Creates a second order filter module which implements audio pre-  
 and de-emphasis.  Arguments: 
    NAME - name of the module.