Overview

High order allpass structure with integer time delays

Discussion

The allpass_delay_module.m and allpass_delay16_module.m implement the digital system shown below:

The overall transfer response is an allpass filter:

The filter has a constant magnitude response of 1.0 and a non-zero phase response.Â The filter has a DC gain of 1.0.

The arguments to the module instantiation function are:

M=allpass_delay_module(NAME, MAXDELAY, OUTPUTSTATE)

MAXDELAY specifies the size of the delay buffer. At run-time, you can vary the delay in the range 0 to MAXDELAY as well as the coefficient . The third argument OUTPUTSTATE is a Boolean which specifies whether the module has an optional 2^nd output pin.Â The 2^nd output pin gets the output of the delay buffer as shown above.

The module allpass_delay16_module.m has exactly the same functionality except that the data stored in the delay line is quantized to fract16 values in order to save memory. The input data to the delay line must be in the range [-1 +1) or else it will be clipped. The output data from the delay line is converted back to floating-point values for further processing. The allpass_delay16_module.m takes roughly half the memory as compared with the allpass_delay_module.m though with a loss of precision.

Both modules operate on mono data only.

Type Definition

typedef struct _ModuleAllpassDelay
{
   &nbspModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
   &nbspINT32 maxDelay;                               // Maximum delay that can be realized, in samples. The size of the delay buffer (maxDelay+1)*numChannels
   &nbspINT32 currentDelay;                           // Current delay
   &nbspFLOAT32 coef;                                 // Feedback/-feedforward coefficient for the allpass delay.
   &nbspINT32 stateIndex;                             // Index of the oldest state variable in the array of state variables
   &nbspFLOAT32* state;                               // State variable array
} ModuleAllpassDelayClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
maxDelay	int	const	0	100	1:100000	samples
currentDelay	int	parameter	0	0	0:1:100	samples
coef	float	parameter	0	0.5	-1:1
stateIndex	int	state	1	0	Unrestricted
state	float*	state	1	[1 x 101]	Unrestricted

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

Name: state

Description: audio output

Data type: float

MATLAB Usage

File Name: allpass_delay_module.m

 M=allpass_delay_module(NAME, MAXDELAY, OUTPUTSTATE)
 Delay based allpass (1st order), using integer length delay.  Arguments:
    NAME - name of the module.
    MAXDELAY - Maximum delay, in samples.
    OUTPUTSTATE - optional Boolean argument that specifies whether
                  the module has a second output pin which gets the
                  direct output of the state buffer. By default,
                  OUTPUTSTATE = 0.