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Old 03-21-2011, 02:51 AM   #1
Tale
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Default Bessel filter implementation

Hi,

I have added a low-pass Bessel filter (besselfilter.h) implementation using matched Z-transform to my WDL repository. I have extracted this implementation from from the source code of mkfilter.

I have divided the implementation into three classes: one for calculating coefficients (WDL_BesselFilterCoeffs), one for the actual filter (WDL_BesselFilterStage), and a third class combining both (WDL_BesselFilter).

Here are a couple of (pseudo) code examples of its use:

Example #1:
Code:
// 8th order anti-alias filter
#define WDL_BESSEL_FILTER_ORDER 8
#include "besselfilter.h"

int oversampling = 8;

WDL_BesselFilterCoeffs bessel;
WDL_BesselFilterStage filter;

bessel.Calc(0.5 / (double)oversampling);
filter.Reset();

for (int i = 0; i < nFrames; ++i)
{
	filter.Process(inputs[0][i], bessel.Coeffs());
	outputs[0][i] = filter.Output();
}
Example #2:
Code:
#include "besselfilter.h"

int order = 4;
int oversampling = 8;

// 2 cascaded filters
WDL_BesselFilterStage filter[2];
filter[0].Reset();
filter[1].Reset();

WDL_BesselFilterCoeffs coeffs;
coeffs.Calc(0.5 / (double)oversampling, order);

for (int i = 0; i < nFrames; ++i)
{
	filter[0].Process(inputs[0][i], &coeffs);
	filter[1].Process(filter[0].Output(), &coeffs);
	outputs[0][i] = filter[1].Output();
}
Example #3:
Code:
#include "besselfilter.h"

int order = 8;
int oversampling = 8;

WDL_BesselFilter bessel;
bessel.Calc(0.5 / (double)oversampling, order);
bessel.Reset();

for (int i = 0; i < nFrames; ++i)
{
	bessel.Process(inputs[0][i]);
	outputs[0][i] = bessel.Output();
}
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Old 03-21-2011, 03:01 AM   #2
junioreq
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Tale this is Great! But the first thing that popped out was the oversampling.. Could this be "modded" to work on its own oversampling without the filter? Seems oversampling for distortions etc is just way over my head at the moment. Thanks for the class though, its so nice to have these things built in and ready for use.

~Rob.
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Old 03-21-2011, 04:55 AM   #3
olilarkin
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wow, thanks for this tale!

rob - the filter is part of the oversampling - you need to filter out frequencies above the nyquist when up or down sampling
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Old 03-26-2011, 03:22 PM   #4
Tale
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I tried to include besselfilter.h in an Xcode project today, but my MacBook wouldn't build it because _hypot is supposed to be called hypot (although Microsoft seems to disagree). Anyway, this has now been fixed (see my Git repository).
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Old 09-18-2011, 06:38 AM   #5
Tale
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Default Added denormal fix

I have added the option to zero denormals in the output of the Bessel filter. I have also added an example using this filter for oversampling:

Code:
mAntiAlias.Calc(0.5 / (double)mOversampling);

...

double sample = <input sample>;
double output;

for (int j = 0; j < mOversampling; ++j)
{
	// Upsample
	if (j > 0) sample = 0.;
	mUpsample.Process(sample, mAntiAlias.Coeffs());
	sample = (double)mOversampling * mUpsample.Output();

	...

	// Downsample
	mDownsample.Process(sample, mAntiAlias.Coeffs());
	if (j == 0) output = mDownsample.Output();
}

<output sample> = output;
The complete (working) example can be found in WDL/IPlug/Example/Distortion in my WDL Git repository.
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Old 10-20-2011, 01:42 AM   #6
olilarkin
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Do you get a dc offset/clicks when you change the drive amount in your example?
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Old 11-06-2011, 04:30 AM   #7
Mich
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Quote:
Originally Posted by Tale View Post
I tried to include besselfilter.h in an Xcode project today, but my MacBook wouldn't build it because _hypot is supposed to be called hypot (although Microsoft seems to disagree).
It is supposed to be called hypot() according to the ANSI C99 standard.
But Microsoft is somewhat confused about it.
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Old 07-24-2012, 02:55 AM   #8
onqel
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I tried this on one of my high gain ampsims, it can't compete with my original polyphase approach unfortunately (for 2X/4X oversampling), aliasing all over the place.. I tried order up to 10

Last edited by onqel; 07-24-2012 at 03:06 AM.
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