Programmatically merging two pieces of audio

Question

I have two arrays of samples of two different audio clips. If I just programmatically add them together will this be the equivalent of layering one track over another in an audio editing suite? Like if I have one audio clip of bass the other of a drum and I want them playing together.

I would probably do something like this:

 for (int i = 0; i < length_of_array; i++){
   final_array[i] = first_array[i] + second_array[i];
 }

If it is not done this way, could I get some indication of what would be the correct way?

Have you tried to see what it sounds like? (actually, to *hear* what it sounds like is more correct) — Lasse V. Karlsen, Feb 26 '11 at 09:49
Are you able to mix the sound ? if yes please share the code — Ahmad Arslan, Sep 03 '18 at 10:38

Daniel Mošmondor · Accepted Answer · 2011-02-26T10:14:36.230

11

This IS a correct way. Merging is called MIXING in audio jargon.

BUT:

If your samples are short (16 bit signed) - you will have to use int (32 bit signed) for addition and then clip the samples manually. If you don't, your values will wrap and you'll have so much fun listening to what you did :)

Here comes the code:

short first_array[1024];
short second_array[1024];
short final_array[1024];
for (int i = 0; i < length_of_array; i++)
{
    int mixed=(int)first_array[i] + (int)second_array[i];
    if (mixed>32767) mixed=32767;
    if (mixed<-32768) mixed=-32768;
    final_array[i] = (short)mixed;
}

In MOST cases you don't need anything else for normal audio samples, as the clipping will occur in extremely rare conditions. I am talking this from practice, not from theory.

edited Feb 26 '11 at 10:14

answered Feb 26 '11 at 09:53

Daniel Mošmondor

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@Daniel How would you accomplish this with two mp3 (for instance) files in php or any other server side language? – trainoasis May 19 '14 at 14:46
1

@trainoasis you won't. In order to mix 2 mp3s you have to decode them to PCM then (if needed) re-encode back to mp3. Direct mp3 mixing is not possible. At least not at byte level. – Daniel Mošmondor May 21 '14 at 08:57
Yes it will, just clip everything. However, if all the samples are very loud and you are not lucky, you will have lots or clipping and will need to compress the sound somehow. – Daniel Mošmondor Feb 21 '18 at 13:21
@DanielMošmondor how would you mix more than 2 pcm files? – Arunraj Shanmugam Feb 22 '18 at 05:40
Same thing as an example above, just keep adding samples from other samples. – Daniel Mošmondor Feb 22 '18 at 07:14

score 5 · Answer 2 · answered Feb 26 '11 at 18:58

Your above merging method will work if the sample rates and desired mix level of the two audio clips are identical. If the desired mix levels are different, then a slightly more general form of your mixer would be something like:

mixed_result[i] = rescale_and_clip_fix( volume1 * input1[i] + volume2 * input2[i] );

Where rescale_and_clip_fix() might be a limiter or compressor, following making sure the scale after multiplication is correct for the result's data type. If the result array is an integer data type, then you may also want to do rounding or noise filtering while scaling.

If the sample rates are different, then you will need to do a sample rate conversion on one of the input channels first and/or the result afterwards.

score 1 · Answer 3 · answered Feb 26 '11 at 09:52

In general, this will get you what you want - however, watch for clipping. That is, be careful not to end up with integer overflow; and don't avoid this by just limiting the value to the max/minimum of the type in question. You may need to apply a compressor to bring the values back into range after adding them.

Programmatically merging two pieces of audio

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