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?

1 Answer 1


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

  • $\begingroup$ How can I get the cutoff frequency from the pole location? $\endgroup$ Jul 2, 2019 at 0:03
  • 1
    $\begingroup$ 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, 2019 at 11:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.