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
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.
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// 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

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...

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...

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!

frequency * nyquist
or
frequency * samplerate

don't know the exact implementation.

>>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)

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));

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