E85.2607 - Practical: Schroeder Allpass Reverberator

Write a MATLAB function that uses Schroeder allpass sections in series to produce an artificial diffuse reverb. The function declaration should be:

function output = schroederReverb(x, k, fs)
% y = schroederReverb(x, k, fs)
% where x is the input signal, k is the number of allpass
% sections, and fs is the sampling rate.

To do this you should write a function corresponding to a single allpass section, with the following signature:

function output = schroederAllpass(x, M, g)
% y = schroederAllpass(x, M, g)
% where M is the length of the delay line and g is the gain.
which you should call from schroederReverb. Each allpass filter should have a transfer function of the form:
H(z) = (-g + z^(-M)) / (1 - g * z^(-M))
where g = 0.7, and the delay length M for allpass section i=0,1,...,k-1 is given by:
M = ceil(0.1 * fs / (3^i))

That is, 100ms worth of samples divided by 3^i (as recommended by Schroeder), rounded up. So, for the first allpass filter (i=0), M = ceil(0.1*fs), for the second (i=1), M = ceil(0.1*fs/3), etc., up to i=k-1.

Make some cool sounds (you can find a good input here), and show what's happening in some spectrograms. Describe what happens for different values of k and g.

(borrowed again from Matt Hoffman)

Last updated on 2010-02-18.