Question

I’m trying to write an .m file to extract energy features from an audio track but I seem to be having trouble in its implementation:

% Formula for calculating RMS

[f, fs, nb] = wavread('Three.wav');

frameWidth=441; %10ms
numSamples=length(x);
numFrames=(numSamples/1);
energy(frame)=0;

for frame=1:numFrames,
    startSample=(frame-1)*frameWidth+1;
    endSample=startSample+frameWidth-1;
% Calculate frame energy
    for i=startSample:endSample
        energy(frame)=energy(frame)+x(i)^2;
    end
end

I run that file in MATLAB and get the following error:

??? Attempted to access x(2); index out of bounds because numel(x)=1.
Error in ==> myrms at 12
energy(frame)=energy(frame)+x(i)^2;

Any help would be much appreciated.

Answer 1

You should be using f instead of x, since f is the actual signal loaded from your .wav file. The variable x was probably just some other scalar in your workspace, which is why you were getting the error you saw.

There are a few other corrections/improvements that should be made to your code. First, as Paul R pointed out, you need to correct how you compute numFrames. Second, energy should be initialized as a vector of zeroes. Third, you can reduce the inner for loop to a one-line vectorized operation.

Here’s how I would rewrite your code (EDIT: Based on comments, I have updated the code to save a few extra variables computed in the loop):

[y, fs, nb] = wavread('Three.wav');  %# Load the signal into variable y

frameWidth = 441;                          %# 10 msec
numSamples = length(y);                    %# Number of samples in y
numFrames = floor(numSamples/frameWidth);  %# Number of full frames in y
energy = zeros(1,numFrames);               %# Initialize energy
startSample = zeros(1,numFrames);          %# Initialize start indices of frame
endSample = zeros(1,numFrames);            %# Initialize end indices of frame

for frame = 1:numFrames                              %# Loop over frames
  startSample(frame) = (frame-1)*frameWidth+1;       %# Starting index of frame
  endSample(frame) = frame*frameWidth;               %# Ending index of frame
  frameIndex = startSample(frame):endSample(frame);  %# Indices of frame samples
  energy(frame) = sum(y(frameIndex).^2);             %# Calculate frame energy
end