title: Delay-based audio effects processor
body: <p>I'm doing a script to implement flanger sound effect in octave.
  I'm having the following error at line 33:
  error: 'ampx' undefined near line 33, column 33
  error: called from
  flanger at line 33 column 6</p>
  <p>Can someone help me? I'll post script next.
  Thanks in advance</p>
  <pre><code>close all 
  clear all
  clc
  
  %flanger.m
  % Creates a single FIR delay with the delay time oscillating from
  % Either 0-3 ms or 0-15 ms at 0.1 - 5 Hz
  infile=&quot;acoustic.wav&quot;;
  [som, fs]=audioread(infile);   %som-som, fs sampling rate
  #outfile=audiowrite('out_flanger.wav');
  % read the sample waveform
  [som, fs] = audioread(infile);
  % parameters to vary the effect %
  max_time_delay=0.003; % 3ms max delay in seconds
  rate=1; %rate of flange in Hz
  index=1:length(som);
  % sin reference to create oscillating delay
  sin_ref = (sin(2*pi*index*(rate/fs)))';
  %convert delay in ms to max delay in samples
  max_samp_delay=round(max_time_delay*fs);
  % create empty out vector
  y = zeros(length(som),1);
  % to avoid referencing of negative samples
  y(1:max_samp_delay)=som(1:max_samp_delay);
  % set amp suggested coefficient from page 71 DAFX
  amp=0.7;
  % for each sample
  for i = (max_samp_delay+1):length(som),
  cur_sin=abs(sin_ref(i)); %abs of current sin val 0-1
  % generate delay from 1-max_samp_delay and ensure whole number
  cur_delay=ceil(cur_sin*max_samp_delay);
  % add delayed sample
  y(i) = (ampx(i)) + amp*(som(i-cur_delay));           %%LINE 33!!!!!!!
  end
  % write output
  audiowrite(outfile,y,Fs);
  </code></pre>