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?
here's a descriptive implementation:
short int mix_sample(short int sample1, short int sample2) {
const int32_t result(static_cast<int32_t>(sample1) + static_cast<int32_t>(sample2));
typedef std::numeric_limits<short int> Range;
if (Range::max() < result)
return Range::max();
else if (Range::min() > result)
return Range::min();
else
return result;
}
to mix, it's just add and clip!
to avoid clipping artifacts, you will want to use saturation or a limiter. ideally, you will have a small int32_t
buffer with a small amount of lookahead. this will introduce latency.
more common than limiting everywhere, is to leave a few bits' worth of 'headroom' in your signal.