**Type :** 24db resonant lowpass

**References :** CSound source code, Stilson/Smith CCRMA paper., Paul Kellett version

**Notes :**

The second "q =" line previously used exp() - I'm not sure if what I've done is any faster, but this line needs playing with anyway as it controls which frequencies will self-oscillate. I

think it could be tweaked to sound better than it currently does.

Highpass / Bandpass :

They are only 6dB/oct, but still seem musically useful - the 'fruity' sound of the 24dB/oct lowpass is retained.

**Code :**

// Moog 24 dB/oct resonant lowpass VCF

// References: CSound source code, Stilson/Smith CCRMA paper.

// Modified by paul.kellett@maxim.abel.co.uk July 2000

float f, p, q; //filter coefficients

float b0, b1, b2, b3, b4; //filter buffers (beware denormals!)

float t1, t2; //temporary buffers

// Set coefficients given frequency & resonance [0.0...1.0]

q = 1.0f - frequency;

p = frequency + 0.8f * frequency * q;

f = p + p - 1.0f;

q = resonance * (1.0f + 0.5f * q * (1.0f - q + 5.6f * q * q));

// Filter (in [-1.0...+1.0])

in -= q * b4; //feedback

t1 = b1; b1 = (in + b0) * p - b1 * f;

t2 = b2; b2 = (b1 + t1) * p - b2 * f;

t1 = b3; b3 = (b2 + t2) * p - b3 * f;

b4 = (b3 + t1) * p - b4 * f;

b4 = b4 - b4 * b4 * b4 * 0.166667f; //clipping

b0 = in;

// Lowpass output: b4

// Highpass output: in - b4;

// Bandpass output: 3.0f * (b3 - b4);

**Comments**

__from__ : daniel[DOT]martin24[AT]gmx[DOT]net

__comment__ : I just tried the filter code and it seems like the highpass output is the same as the lowpass output, or at least another lowpass...
But i´m still testing the filter code...

__from__ : daniel[DOT]martin24[AT]gmx[DOT]net

__comment__ : Sorry for the Confusion, it works....
I just had a typo in my code.
One thing i did to get the HP sound nicer was
HP output: (in - 3.0f * (b3 - b4))-b4
But I´m a newbie to DSP Filters...

__from__ : windowsucks2000[AT]netscape[DOT]net

__comment__ : Hey, thanks for this code. I'm a bit confused as to the range to the frequency and resonance. Is it really 0.0-1.0? If so, how so I specify a certain frequency, such as... 400Hz? THANKS!

__from__ : Christian[AT]savioursofsoul[DOT]de

__comment__ : frequency * nyquist
or
frequency * samplerate
don't know the exact implementation.

__from__ : nobody[AT]nowhere[DOT]com

__comment__ : >>Hey, thanks for this code. I'm a bit confused as to the range to the frequency and resonance. Is it really 0.0-1.0? If so, how so I specify a certain frequency, such as... 400Hz? THANKS!
I'd guess it would be:
frequency/(samplerate/2.f)

__from__ : thisguy[AT]nowhere[DOT]com

__comment__ : The domain seems to be 0-nyquest (samplerate/2.0), but the range is 0-1
A better way to get smoother non-linear mapping of frequency would be this:
(give you a range of 20Hz to 20kHz)
freqinhz = 20.f * 1000.f ^ range;
then
frequency = freqinhz * (1.f/(samplerate/2.0f));

__from__ : musicdsp[AT]nilsschneider[DOT]de

__comment__ : I like the sound of this one, unfortunately, it breaks quite fast, causing the internal values b1-b4 to be "infinity". Any hints?