[Openal] How to Change/Vary the Speed of Music?

[Chris Robinson]

On Thursday 22 May 2008 12:20:26 am 藺心皓 wrote:
> Hi.
>
> I try to change the speed of music to do some effect like the
> conductor can change the speed of whole band.
> By changing Pitch, it may close to my object -- the speed of music was
> change, but pitch also changed.
> Is  there any method to change the speed but remain orignal pitch?

Unfortunately, it's not that easy. The audio data is just a long series of 
pulses, at various strengths. Going through the pulses faster makes it play 
faster, and gets a higher pitch (and going through it slower gives it a lower 
pitch).

The EFX extension has an effect that can change the pitch independantly of 
playback speed (which could be used along with adjusting the playback speed 
to get a normal pitch at faster/slower speeds), but I wouldn't expect to see 
that effect outside of hardware implementations (neither Creative's software 
implementation, nor OpenAL Soft, provide it). Pitch detection and 
modification is not an easy task to do on a real-time stream without some 
dedicated parallelism.

About the only thing I can think of to do to reasonably get the effect you 
want is to use module files (.mod, .it, .s3m..), or MIDI files with a 
wavetable patchset, and using a converter (like BASS/DUMB or Timidity) to 
convert it into PCM that can be streamed through OpenAL. Unlike PCM data, 
MIDI/MOD files are basically like sheet music.. they simply say what notes 
should be played when and how. With "rendering" it in realtime, it can play 
through the notes slower/faster without altering the pitch of the notes 
themselves (assuming it has controlls to speed it up or slow it down, of 
course).

Hope that helps.