FilterExplorer is a standalone program and a VST audio plugin for Microsoft Windows
(9x to XP) that is useful for FIR/IIR filter design, particularly for DSP/audio applications:
It allows you to directly enter the calculation formulas for an FIR/IIR filter (up to 5th
order) in a syntax similar to C or Pascal into the program. You do then immediately get
a plot of the magnitude and phase of the filter transfer function, and with the VST
plugin you can even filter your audio data stream with that filter in realtime and listen
to the results!

You can either enter numerical values directly for a fixed filter, ie "a0=1.34",
"a1=0.2222", ... or you can use the internal expression evaluator for dynamic filter
calculations. You will normally use the more generic dynamic approach since you can
then include for example cutoff frequency or resonance values in the formulas and let
the program re-calculate the coefficients accordingly.

The integrated FEE (fast expression evaluator) allows you to use up to 6 intermediate
terms (t0..t5) besides the regular filter coefficients a0..a5 and b1..b5. Furthermore you
have up to 6 slider variables c0..c5 with the range of 0..1 that allow you to enter
dynamic values in realtime (ie cutoff frequency).

Besides saving your filter in an internal format (simple text file with the terms), you
can also export your filter (with optimized coefficients) as a C++ or Delphi class ready
to use in your own DSP applications with just one mouse-click.
A large number of popular filter algorithms is already included.
 

You can buy FilterExplorer Pro for only $25.00.

Download FilterExplorer Demo

The download above contains a free version of Filter Explorer (standalone version
only) with limited features. It is not save-disabled and you can fully evaluate the
program!
 

Some user comments about FilterExplorer:

"Really useful for designing digital filter fast n' easily. Also thanks for including the
Cookbook filters already - tweaking them is a lot of fun!"

"Great program! Saved me a lot of time in designing some basic filters!"