A Collection of Useful C++ Classes for Signal Processing

[mystran]

IIR filters with low cutoffs definitely start getting wacky especially as the order increases. I think the typical solution is "oversampling".

Can anyone confirm or deny this?

I'll deny the part about oversampling. The filter "core" only really sees a normalized frequency, and oversampling will make the low normalized frequency even lower. This is make the problem worse!

Anyway, this problem is dependent on the filter structure. Direct forms are probably some of the most sensitive (of the commonly used structures anyway). Like I said in the previous post, I've been meaning to write a tutorial on how to take an arbitrary set of direct form coeffs and map it to arbitrary filter structure (it's not exactly very hard in most cases).

Basic "general" outline is to define the filter in state-space form and take the inverse of (zI-A) and then multiply with input/output vectors, add direct path, and solve for whatever unknowns you have so that the response matches. In some cases (uum, ladders/lattices) there are also published formulas to do it directly (though the state-space approach works too).

[matt42]

IIR filters with low cutoffs definitely start getting wacky especially as the order increases. I think the typical solution is "oversampling".

Can anyone confirm or deny this?

No, oversampling atually lowers the frequency in terms of a ratio of the samplerate. The solution is in my limited experience is to run the filteras as a cascade of lower order filters or to increase the bit depth.

[duncanparsons]

To improve the ratio, one could downsample, filter, then oversample back to original fs again...

[matt42]

To improve the ratio, one could downsample, filter, then oversample back to original fs again...

cool, any info over 10kHz is over commercialized anyway

[DavenH]

To improve the ratio, one could downsample, filter, then oversample back to original fs again...

cool, any info over 10kHz is over commercialized anyway

We're talking about low cutoffs right? Not much info over 10Khz.

[mystran]

To improve the ratio, one could downsample, filter, then oversample back to original fs again...

cool, any info over 10kHz is over commercialized anyway

We're talking about low cutoffs right? Not much info over 10Khz.

The problem does affect high-pass filter equally though (the poles are typically the same or at least close to what you'd have with bandpass/lowpass). When you need a low-cutoff high-pass, you probably don't want to downsample.

[thevinn]

Okay, so to be clear, there is no actual defect in the DspFilters code?

[robrokken]

ok, so i tried to use this. First off, I put the DspFilters folder into my project. I took out the files from the include folder and put them in the main DspFilter folder, and took out the Source stuff and put that in the DspFilter folder. I had to change this, because all the documentation shows is to include "#include "DspFilters/Dsp.h"" which clearly doesn't exist. Its in the include folder.

So then I go to compile and get:

Code: Select all

Dsp::SmoothedFilterDesign<Dsp::RBJ::Design::HighPass, 2> *m_hipassfilter;
m_hipassfilter=new Dsp::SmoothedFilterDesign<Dsp::RBJ::Design::HighPass, 2> (512);
Dsp::Params params;
 params[0] = 44100; // sample rate 
    params[1] = 5; // order 
    params[2] = 4000; // corner frequency 
    params[3] = 6; // shelf gain 
    params[4] = 0.1; // passband ripple 
    f->setParams (params); 


//-------------------------------

1>.\LoJo.cpp(30) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>.\LoJo.cpp(30) : error C2040: 'm_hipassfilter' : 'int' differs in levels of indirection from 'Dsp::SmoothedFilterDesign<DesignClass,Channels> *'
1>        with
1>        [
1>            DesignClass=Dsp::RBJ::Design::HighPass,
1>            Channels=2
1>        ]
1>.\LoJo.cpp(30) : error C2440: 'initializing' : cannot convert from 'Dsp::SmoothedFilterDesign<DesignClass,Channels> *' to 'int'
1>        with
1>        [
1>            DesignClass=Dsp::RBJ::Design::HighPass,
1>            Channels=2
1>        ]
1>        There is no context in which this conversion is possible
1>.\LoJo.cpp(32) : error C2466: cannot allocate an array of constant size 0
1>.\LoJo.cpp(32) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>.\LoJo.cpp(32) : error C2040: 'params' : 'int []' differs in levels of indirection from 'Dsp::Params'
1>.\LoJo.cpp(32) : error C2440: 'initializing' : cannot convert from 'int' to 'int []'
1>        There are no conversions to array types, although there are conversions to references or pointers to arrays
1>.\LoJo.cpp(33) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>.\LoJo.cpp(33) : error C2040: 'params' : 'int [1]' differs in levels of indirection from 'Dsp::Params'

Any ideas? Still not sure why it says "include DspFilters/dsp.h", when that file doesn't even exist.. Shouldn't it say "include DspFilters/include/dsp.h" ???

~Rob.

[thevinn]

Rob:

Not sure what your exact problem is, but did you try compiling the demo project for your platform?

[robrokken]

Yeah, it worked fine. So, I take the downloaded filter code, move the include and source files into the main DspFilter folder, right? Even the example says:

"
// This is the only include you need
#include "DspFilters/Dsp.h"
"

That file doesn't exist....



So i must be doing something wrong.

The file is actually here: #include "DspFilters/include/DspFilters/dsp.h"

Which screws up all the other includes... ughh

EDIT: set additional libraries path to include folder. But still getting the same errors....

Code: Select all

.\LoJo.cpp(12) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>.\LoJo.cpp(12) : error C2040: 'm_hipassfilter' : 'int' differs in levels of indirection from 'Dsp::SmoothedFilterDesign<DesignClass,Channels> *'
1>        with
1>        [
1>            DesignClass=Dsp::RBJ::Design::HighPass,
1>            Channels=2
1>        ]
1>.\LoJo.cpp(12) : error C2440: 'initializing' : cannot convert from 'Dsp::SmoothedFilterDesign<DesignClass,Channels> *' to 'int'
1>        with
1>        [
1>            DesignClass=Dsp::RBJ::Design::HighPass,
1>            Channels=2
1>        ]
1>        There is no context in which this conversion is possible
1>.\LoJo.cpp(14) : error C2466: cannot allocate an array of constant size 0
1>.\LoJo.cpp(14) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>.\LoJo.cpp(14) : error C2040: 'params' : 'int []' differs in levels of indirection from 'Dsp::Params'

Dsp::SmoothedFilterDesign<Dsp::RBJ::Design::HighPass, 2> *m_hipassfilter;

That line throws this warning: warning C4101: 'm_hipassfilter' : unreferenced local variable

[thevinn]

You need to add "DspFilters/include/" into your compile/build systems list of include paths. For example -I"DspFilters/Include" command line option (I think)

[robrokken]

Solved... And getting things in the right place in my plugin class... thanks..

[thevinn]

Try making sure your r-values are of type double when you assign to params

[robrokken]

Are there even peak filters in this?

[thevinn]

Are there even peak filters in this?

Peak filter? You mean a mid-band shelf filter? Yes there is.