DAFX: Digital Audio Effects, Second Edition

2.6 Delay-based Audio Effects

2.6.1 Vibrato

When a car is passing by, we hear a pitch deviation due to the Doppler effect [Dut91]. This effect will be explained in another chapter, but we can keep in mind that the pitch variation is due to the fact that the distance between the source and our ears is being varied. Varying the distance is, for our application, equivalent to varying the time delay. If we keep on varying periodically the time delay we will produce a periodical pitch variation. This is precisely a vibrato effect. For that purpose we need a delay line and a low-frequency oscillator to drive the delay time parameter. We should only listen to the delayed signal. Typical values of the parameters are 5 to 10 ms as the average delaytime and 5 to 14 Hz rate for the low-frequency oscillator (see Figure 2.32) [And95, Whi93]. M-file 2.8 shows the implementation for vibrato [Dis99].

M-file 2.8 (vibrato.m)

function y=vibrato(y,SAMPLERATE,Modfreq,Width)

% Author: S. Disch

ya_alt=0;

Delay=Width; % basic delay of input sample in sec

DELAY=round(Delay*SAMPLERATE); % basic delay in # samples

WIDTH=round(Width*SAMPLERATE); % modulation width in # samples

if WIDTH>DELAY

  error(’delay greater than basic delay !!!’);

  return;

end

MODFREQ=Modfreq/SAMPLERATE; % modulation frequency in # samples

LEN=length(x);        % # of samples in WAV-file

L=2+DELAY+WIDTH*2;    % length of the entire delay

Delayline=zeros(L,1); % memory allocation for delay

y=zeros(size(x));     % memory allocation for output vector ...