1
$\begingroup$

I've been looking into the venerable Freeverb and I see that CCRMA state that it uses 4 Shroeder Allpass filters in series, but when I look at the source code I see:

inline float allpass::process(float input)
{
    float output;
    float bufout;
    
    bufout = buffer[bufidx];
    undenormalise(bufout);
    
    output = -input + bufout;
    buffer[bufidx] = input + (bufout*feedback);

    if(++bufidx>=bufsize) bufidx = 0;

    return output;
}

To me, this doesn't equate to the standard Shroeder Allpass, but instead something like this:

Freeverb Implementation

But it should look like this (from CCRMA):

CCRMA Implementation

Am I overlooking something here, or is CCRMA incorrect?

$\endgroup$
1
$\begingroup$

You're right that the freeverb "allpass" implementation actually isn't a perfect allpass filter. The implemented filter has a transfer function

$$H(z)=\frac{(1+\alpha)z^{-N}-1}{1-\alpha z^{-N}}\tag{1}$$

The only case for which $(1)$ actually is a (scaled) allpass filter occurs if $\alpha$ satisfies

$$\alpha(1+\alpha)=1\tag{2}$$

i.e., $\alpha=(\sqrt{5}-1)/2\approx 0.62$.

The fact that the freeverb filter only approximates an allpass filter is also mentioned here.

$\endgroup$
3
  • $\begingroup$ Thank you! I've looked at that Freeverb page all morning and didn't see the page with that info. Much appreciated. I think I have to assume that the design was intentional. I wonder what the rationale for this unusual choice was? $\endgroup$ Jun 21 at 12:25
  • 1
    $\begingroup$ @kippertoffee: You're welcome! As for your last question, I honestly don't have a clue ... $\endgroup$
    – Matt L.
    Jun 21 at 12:43
  • 1
    $\begingroup$ Manfred Schroeder's original paper on reverb is still a good read: aes.org/e-lib/browse.cfm?elib=343 . His original approach used allpass and comb filters and the freeverb implementation seems to by a hybrid of both. (No idea if that's intentional). $\endgroup$
    – Hilmar
    Jun 21 at 13:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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