how to vectorize .

for i=1:no_frame
    temp = downsample(fft_frame(:,i),2);               % Down sampling by 2
    y2(:,i) = [temp; zeros(NFFT/2 - length(temp), 1)];
    temp = downsample(fft_frame(:,i),3);               % Down sampling by 4
    y3(:,i) = [temp; zeros(NFFT/2 - length(temp), 1)];
    temp = downsample(fft_frame(:,i),4);               % Down sampling by 8
    y4(:,i) = [temp; zeros(NFFT/2 - length(temp), 1)];
end

Answer 1

[Greg]

You have already preallocated y2, y3, y4 with zeros. Use a row indexing variable to insert temp and stop re-inserting the zeros.

Also, downsample will work along columns. Just remove your loop and uses of the i variable.

For example:

temp = downsample(fft_frame,2); % Down sampling by 2
y2(1:size(temp,1),:) = temp;