Fast Slope HPF Filter

[Rocko]

Hi,

I'm looking into 'fast slope' filters for an HPF implementation and came across 'Elliptic Filter':
https://en.wikipedia.org/wiki/Elliptic_filter

Is anyone experienced with this type of filter on FlowStone?
Can it be implemented in FlowStone (static parameters)?
Can it be implemented in FlowStone with dynamic parameters (On The Fly changes) ?

Appreciated,
Rocko

[martinvicanek]

You can do elliptic filters in FS, no problem. You can use this excellent online tool to get your filter coefficients: http://cnx.org/contents/8c10bada-99b8-4 ... Filter-Des
I'd recommend to implement the filter as a cascade of biquads. Search this forum and FS Guru for a recipe.

Once you have the coefficients for a certain filter specification you can transform to another cutoff frequency. The math is a bit tedious and it will be heavy on CPU if done at sample rate.

However, note that steep filters tend to ring if you feed them with percussive content.

[martinvicanek]

Here is a tunable 12th order elliptic lowpass as an example:

EllipticFilter.png (31.93 KiB) Viewed 41430 times

[Rocko]

Hi,

Thanks for sharing. It is interesting.

I had reached the page of the tool, it says 1,140 USD for the tool itself
What does the tool do? Does it provide the a and b coefficient set, per filter, which I can then copy into the 'text' boxes of the module you had sent?
I'm interested in HPF, not LPF. Can I calculate the biquads (a,b) for HPF with this LabView tool?

Could I use MATLAB (more available) to reach the same biquads to copy paste into the module you had sent?

Thanks for sharing the info !

Rocko

[tulamide]

Hi,

Thanks for sharing. It is interesting.

I had reached the page of the tool, it says 1,140 USD for the tool itself
What does the tool do? Does it provide the a and b coefficient set, per filter, which I can then copy into the 'text' boxes of the module you had sent?
I'm interested in HPF, not LPF. Can I calculate the biquads (a,b) for HPF with this LabView tool?

Could I use MATLAB (more available) to reach the same biquads to copy paste into the module you had sent?

Thanks for sharing the info !

Rocko

I might be wrong, but from my reading of the page it seems the software used to create the online tool is sold, not the online tool itself. The online tool is a downloadable plugin for your browser, that you need to install to have access to it. At least that's how I understood it.

[martinvicanek]

You are right, tulamide, the link takes you to a free online tool (give it some time to load). Other free filter design tools that include elliptic filters are:
1. IIR Filter Designer from Iowa Hills Software
2. Octave (the free Matlab, so to say), available also via browser interface here

Enter the following in the Octave console:

Code: Select all

format long;   // for double precision
[B,A] = ellip(8,0.15,60,0.5,"high");   // calculates transfer funcion filter coefficients
tf2sos(B,A);   // breaks it down into biquad sections


You can copy and paste the coefficients from the cnx site into the textboxes in my schematic. The other tools have a different output format, you have to unscramble the coefficients before pasting.
I am attaching a highpass filter example.

[Rocko]

Hi,

I must have missed something, thanks for the help and assistance.

Appreciated,
Rocko

[Rocko]

BTW -

How would you compare Octave to Matlab as an audio/filters playground?
It is a hobby, not profession, so existing libraries and examples are a big advantage since I can see what is there and play with it some.

Any comment is appreciated.

Rocko

[stw]

Enter the following in the Octave console:

Code: Select all

format long;   // for double precision
[B,A] = ellip(8,0.15,60,0.5,"high");   // calculates transfer funcion filter coefficients
tf2sos(B,A);   // breaks it down into biquad sections


Hi Martin,
...as usual...thanks for sharing. One remark:
If someone actually wants to do it in Octave one seems to need to install
The control package and The signal package
and just for completeness: How To Install Packages

[martinvicanek]

If someone actually wants to do it in Octave one seems to need to install
The control package and The signal package
and just for completeness: How To Install Packages

If you don't want to install stuff on your PC then you can access Octave online via browser interface here. Apparently the relevant packages are already loaded.