Question

Given is an array of 320 elements (int16), which represent an audio signal (16-bit LPCM) of 20 ms duration. I am looking for a most simple and very fast method which should decide whether this array contains active audio (like speech or music), but not noise or silence. I don’t need a very high quality of the decision, but it must be very fast.

It occurred to me first to add all squares or absolute values of the elements and compare their sum with a threshold, but such a method is very slow on my system, even if it is O(n).

Answer 1

You’re not going to get much faster than a sum-of-squares approach.

One optimization that you may not be doing so far is to use a running total. That is, in each time step, instead of summing the squares of the last n samples, keep a running total and update that with the square of the most recent sample. To avoid your running total from growing and growing over time, add an exponential decay. In pseudocode:

decay_constant=0.999;  // Some suitable value smaller than 1
total=0;
for t=1,...
    // Exponential decay
    total=total*decay_constant;

    // Add in latest sample
    total+=current_sample;

    if total>threshold
        // do something
    end
end

Of course, you’ll have to tune the decay constant and threshold to suit your application. If this isn’t fast enough to run in real time, you have a seriously underpowered DSP…