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I am implementing a reverb based on the Freeverb implementation detailed here. There is also a more in-depth description of the LBCF in question here, but I am not able to find the information I want at that link.

In the diagram given, they use a lowpass-feedback comb-filter, denoted by LBCF(f,d,N). N is clearly the number of samples. I suspect d means the feedback gain, though I'm not sure if a value of .2 makes sense for that. I have no idea what f means.

So here are my questions:

  • What does d mean?
  • What does f mean?
  • What cutoff frequency is used for the lowpass filters?
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It's a Lowpass-Feedback Comb Filter, so let's dissect this one at a time:

  1. Comb filter: You have delay line and add the output back to the original signal. That gives you a single echo
  2. Feedback: You take some of the output of the delay line and put it back into the input of the delay line. That gives you a series of echo's that repeat regularly. You need to put a gain < 1 on this, otherwise it will repeat forever. That gain is called the feedback gain and controls how fast (or slow) the echoes decay.
  3. You put a lowpass into the feedback path to model the typical absorption in a room that's higher at high frequencies and lower at low frequencies. You basically make the feedback gain frequency dependent.

$d$ is the pole location of the lowpass filter. That determines the cutoff frequency of your lowpass filter

$f$ is the feedback gain and determines how quickly the repeating echoes decay. That should be a function of the reverb time (at low frequencies) and the delay size

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  • $\begingroup$ How can I get the cutoff frequency from the pole location? $\endgroup$ – popctrl Jul 2 at 0:03
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    $\begingroup$ Depends on how you define it. This is not a "true" lowpass filter since it doesn't have a zero at Nyquist. In general, amplitude goes down with frequency but you won't hit -3dB if $d$ gets too small. A $d$ of 0.2 you'll get a -3dB point of about 15.5kHz @44.1kHz sample rate. For a $d$ of 0.1 there is no -3dB point at all $\endgroup$ – Hilmar Jul 2 at 11:13

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