SDL_MixAudio
Name
SDL_MixAudio -- Mix audio data
Synopsis
#include "SDL.h" void SDL_MixAudio(Uint8 *dst, Uint8 *src, Uint32 len, int volume);
Description
This function takes two audio buffers of len bytes each of the playing audio format and mixes them, performing addition, volume adjustment, and overflow clipping. The volume ranges from 0 to SDL_MIX_MAXVOLUME and should be set to the maximum value for full audio volume. Note this does not change hardware volume. This is provided for convenience -- you could also mix your own audio data.
Note: Do not use this function for mixing together more than two streams of sample data. The output from repeated application of this function may be distorted by clipping, because there is no accumulator with greater range than the input (not to mention this being an inefficient way of doing it). Use mixing functions from SDL_mixer, OpenAL, or write your own mixer instead.
Note: It is a common misconception that this function is required to write audiodata to an outputstream in the audio-callback like:
void audio_callback(void *udata, Uint8 *stream, int len)
{
SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME);
}Although this works, for a single channel it's just as good as just writing the audiodata directly to stream, that is if you don't need the volume adjustment.
If you need to mix more channels of sample data writing your own mixer is not that difficult, for starters use a signed audio stream and just add the values and clip the result. There is an example of this in Audio_Examples.