Index exceeds the number of array elements When Creating a Ping Pong Delay Algorithm

Hello, I'm following an example from a Will Pirkle book to create a Ping Pong Delay, when indexing from the first part of the if statement there are no errors, however when indexing from the else if statement I get the error from the title

Any help would be greatly appreciated, cheers

% Initialise the Script

clear all 
clc
close all

% Declare Option Variable

algorithm = 2;  % 1 = straight 
                % 2 = Ping Pong
                % Issue does not Occur on 'algorithm 1'

% Read in Audio for Simulation

Fs = 48000;                         % Sampling Rate
L = 10;                             % Length of Simulation (Seconds)
N = L*Fs;                           % Length of Simulation (Samples)
x = audioread('Drum_Groove.wav');   % Read in Audio File

% Declare Time Delay Variables

tDelayL = 0.208;                % Time Delay for the Left Channel (Seconds)
tDelayR = 0.3;                  % Time Delay for the Right Channel (Seconds)
sDelayL = round(tDelayL*Fs);    % Time Delay for the Left Channel (Samples)
sDelayR = round(tDelayR*Fs);    % Time Delay for the Right Channel (Samples)
delayLineL = zeros(sDelayL,1);  % Storage Array for the Left Channel Delay Line
delayLineR = zeros(sDelayR,1);  % Storage Array for the Right Channel  Delay Line
idxL = 1;                       % Vector to Increment DDL(Left)
idxR = 1;                       % Vector to Increment DDL(Right)

% Declare User Control Parameters

DryL = 1;               % Amplitude Value of the Left Channel Dry Signal
DryR = 1;               % Amplitude Value of the Right Channel Dry Signal

WetL = 0.5;             % Amplitude Value of the Left Channel Wet Signal
WetR = 0.5;             % Amplitude Value of the Right Channel Wet Signal

Fbl = 0.3;              % Feedback Value of the Left Channel Wet Signal
Fbr = 0.4;              % Feedback Value of the Right Channel Wet Signal

GainL = 1;              % Linear Output Gain For Both Left Channel Signals
GainR = 1;              % Linear Output Gain For Both Right Channel Signals

GainDBL = 10^GainL/20;  % Output Gain in Decibels for the Left Channel 
GainDBR = 10^GainR/20;  % Output Gain in Decibels for the Right Channel 

% Create Output Storage Array

yL = zeros(N,1);
yR = zeros(N,1);

% Create Main Time Loop to Iterate Through 

for n = 1:N
    
    % Define the Output
    
    yL(n,1) = GainDBL*(DryL*x(n,1) + WetL*delayLineL(idxL));
    yR(n,2) = GainDBR*(DryR*x(n,2) + WetR*delayLineR(idxR));
    
    % Write Samples
    
    if algorithm == 1
    
    delayLineL(idxL) = x(n,1) + Fbl*delayLineL(idxL);
    delayLineR(idxR) = x(n,2) + Fbr*delayLineR(idxR);
    
    else if algorithm == 2
    
    delayLineL(idxR) = x(n,1) + Fbl*delayLineL(idxR);
    delayLineR(idxL) = x(n,2) + Fbr*delayLineR(idxL);
        
        end
    
    end
    
    % Increment the Index Value
    
    idxL = idxL+1;
    idxR = idxR+1;
    
    % Create a Circular Buffer By Wrapping the Index Value
    
    if idxL >= sDelayL
       
        idxL = 1;
        
    end
    
    if idxR >= sDelayR
       
        idxR = 1;
        
    end
    
end

plot(yL + 0.15)
hold on
plot(yR - 0.15)
hold off

title('Stereo Delay');
legend('Left Channel','Right Channel')
audiowrite('Drum_Groove_Delay.wav',yL+yR,Fs)

Answer 1

[Raag]

Hi Nathanael,

Based on your description and the provided code, the error you are encountering is caused by a mismatch between the index values and the size of the delay line arrays when using the Ping Pong Delay algorithm.

In the ‘algorithm == 2’ block, the code attempts to access ‘delayLineL(idxR)’ and ‘delayLineR(idxL)’. However, the delay lines ‘delayLineL’ and ‘delayLineR’ are initialized with different lengths:

Since ‘idxR’ can exceed the length of ‘delayLineL’, and ‘idxL’ can exceed the length of ‘delayLineR’, this results in an out-of-bounds indexing error.

To resolve the issue, you should avoid cross-indexing the delay lines. Instead, use the correct index for each delay line while still achieving the ping-pong effect by feeding the delayed signal from one channel into the other.

Replace the ‘algorithm == 2’ block with the following:

else if algorithm == 2 

    delayLineL(idxL) = x(n,1) + Fbl * delayLineR(idxR); 

    delayLineR(idxR) = x(n,2) + Fbr * delayLineL(idxL); 

end

After following it the output will look like this:

This ensures that:

• ‘delayLineL’ is indexed using ‘idxL’ (its own index)
• ‘delayLineR’ is indexed using ‘idxR’ (its own index)
• The ping-pong effect is preserved by feeding the delayed signal from one channel into the other.

For more information, you may find the following MATLAB documentation helpful:

1. Array indexing - https://www.mathworks.com/help/matlab/math/array-indexing.html

2. Audio Processing in MATLAB - https://www.mathworks.com/help/audio/audio-processing-algorithm-design.html