# Moog VCF, variation 1¶

**Author or source:**CSound source code, Stilson/Smith CCRMA paper., Paul Kellett version**Type:**24db resonant lowpass**Created:**2002-01-17 02:03:52

```
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.
```

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 | ```
// 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¶

**Date**: 2005-01-06 00:57:41**By**: ten.xmg@42nitram.leinad

```
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...
```

**Date**: 2005-01-06 01:30:37**By**: ten.xmg@42nitram.leinad

```
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...
```

**Date**: 2005-08-19 00:17:24**By**: ten.epacsten@0002skcuswodniw

```
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!
```

**Date**: 2005-08-20 13:28:54**By**: ed.luosfosruoivas@naitsirhC

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

**Date**: 2007-02-05 18:28:48**By**: moc.erehwon@ydobon

```
>>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)
```

**Date**: 2009-10-23 19:05:04**By**: moc.erehwon@yugsiht

```
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));
```

**Date**: 2012-02-24 13:45:09**By**: ed.redienhcsslin@psdcisum

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