Mixing 16 bit linear PCM streams and avoiding clipping/overflow

Question

I've trying to mix together 2 16bit linear PCM audio streams and I can't seem to overcome the noise issues. I think they are coming from overflow when mixing samples together.

I have following function ...

short int mix_sample(short int sample1, short int sample2)
{
    return #mixing_algorithm#;
}

... and here's what I have tried as #mixing_algorithm#

sample1/2 + sample2/2
2*(sample1 + sample2) - 2*(sample1*sample2) - 65535
(sample1 + sample2) - sample1*sample2
(sample1 + sample2) - sample1*sample2 - 65535
(sample1 + sample2) - ((sample1*sample2) >> 0x10) // same as divide by 65535

Some of them have produced better results than others but even the best result contained quite a lot of noise.

Any ideas how to solve it?

Answer 1

The best solution I have found is given by Viktor Toth. He provides a solution for 8-bit unsigned PCM, and changing that for 16-bit signed PCM, produces this:

int a = 111; // first sample (-32768..32767)
int b = 222; // second sample
int m; // mixed result will go here

// Make both samples unsigned (0..65535)
a += 32768;
b += 32768;

// Pick the equation
if ((a < 32768) || (b < 32768)) {
    // Viktor's first equation when both sources are "quiet"
    // (i.e. less than middle of the dynamic range)
    m = a * b / 32768;
} else {
    // Viktor's second equation when one or both sources are loud
    m = 2 * (a + b) - (a * b) / 32768 - 65536;
}

// Output is unsigned (0..65536) so convert back to signed (-32768..32767)
if (m == 65536) m = 65535;
m -= 32768;

Using this algorithm means there is almost no need to clip the output as it is only one value short of being within range. Unlike straight averaging, the volume of one source is not reduced even when the other source is silent.