audio - Algorithm to mix sound

Question

I have two raw sound streams that I need to add together. For the purposes of this question, we can assume they are the same bitrate and bit depth (say 16 bit sample, 44.1khz sample rate).

Obviously if I just add them together I will overflow and underflow my 16 bit space. If I add them together and divide by two, then the volume of each is halved, which isn't correct sonically - if two people are speaking in a room, their voices don't become quieter by half, and a microphone can pick them both up without hitting the limiter.

• So what's the correct method to add these sounds together in my software mixer?
• Am I wrong and the correct method is to lower the volume of each by half?
• Do I need to add a compressor/limiter or some other processing stage to get the volume and mixing effect I'm trying for?

-Adam

Answer 1

You should add them together, but clip the result to the allowable range to prevent over/underflow.

In the event of the clipping occuring, you will introduce distortion into the audio, but that's unavoidable. You can use your clipping code to "detect" this condition and report it to the user/operator (equivalent of red 'clip' light on a mixer...)

You could implement a more "proper" compressor/limiter, but without knowing your exact application, it's hard to say if it would be worth it.

If you're doing lots of audio processing, you might want to represent your audio levels as floating-point values, and only go back to the 16-bit space at the end of the process. High-end digital audio systems often work this way.