DSPPROC callback

User defined DSP callback function.

    HDSP handle,
    DWORD channel,
    void *buffer,
    DWORD length,
    void *user


handleThe DSP handle.
channelChannel that the DSP is being applied to.
bufferPointer to the sample data to apply the DSP to. The data is as follows: 8-bit samples are unsigned, 16-bit samples are signed, 32-bit floating-point samples range from -1 to +1 (not clipped, so can actually be outside this range).
lengthThe number of bytes to process.
userThe user instance data given when BASS_ChannelSetDSP was called.


A DSP function should be as quick as possible; playing streams and MOD musics, and other DSP functions cannot be processed until it has finished.

Some functions can cause problems if called from within a DSP (or stream) function. Do not call BASS_Stop or BASS_Free from within a DSP callback, and do not call BASS_ChannelStop, BASS_MusicFree or BASS_StreamFree with the same channel handle as received by the callback.

If the BASS_CONFIG_FLOATDSP config option is enabled, then DSP callback functions will always be passed 32-bit floating-point sample data, regardless of what the channels' actual sample format is.


A simple DSP function to swap the left/right channels of a stereo 16-bit channel.
void CALLBACK SwapDSP(HDSP handle, DWORD channel, void *buffer, DWORD length, void *user)
    short *s=buffer;
    for (; length; length-=4, s+=2) {
        short temp=s[0];

A panning/balance DSP function for a stereo 16-bit channel.

float pan; // panning position, set as you would the BASS_ATTRIB_PAN attribute

void CALLBACK PanDSP(HDSP handle, DWORD channel, void *buffer, DWORD length, void *user)
    short *s=buffer;
    if (!pan) return; // no processing neeeded for centre panning
    for (; length; length-=4, s+=2) {
        if (pan<0) s[1]=s[1]*(1+pan); // pan left = reduce right
        else s[0]=s[0]*(1-pan); // vice versa

See also