[SDL] SDL_Mixer and speed/pitch shift

[James Haley]

Hi,

You're correct that SDL_mixer itself doesn't have support for pitch-shifting 
sounds. However, since it does give you direct access to the sound stream 
buffer via the mixing callback functions (such as the post-mix callback), 
you can implement it yourself.

My Eternity Engine Doom port has a rudimentary pitch shifting ability. 
Looking at it might give you an idea on what would be involved.
Link to the file:
http://eternity.mancubus.net/svn/trunk/eternity/source/sdl/i_sound.c

EE just does all its own digital sound mixing and then pipes it through 
SDL_mixer, which we use mainly for playing music. I should probably note 
that our sound engine crashes on some x86 Mac machines due apparently to 
some mysterious bug in SDL_mixer or its interaction with some particular 
sound driver (it sends us a bad buffer seemingly at random). As of the 
latest release, this is still a problem for one of my users, so watch out 
for that when accessing the stream buffer directly.

Yours,
James Haley

>From: Sylvain Beucler <beuc at beuc.net>
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>Subject: Re: [SDL] SDL_Mixer and speed/pitch shift
>Date: Mon, 6 Aug 2007 00:21:36 +0200
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>Nobody using pitch shift here? :/
>
>Cheers,
>
>--
>Sylvain
>
>On Wed, Aug 01, 2007 at 05:03:15PM +0200, Sylvain Beucler wrote:
> > Hi,
> >
> > I'd like to use SDL_Mixer to play multiple sounds at once, and with
> > different speeds for each sound.
> >
> > This allow, for example, to play a normal voice and an acute voice
> > from the same sound file, by playing it with a greater frequency for
> > the acute voice.
> >
> > This feature (which is often refered to as "pitch shift") seems to be
> > missing from SDL_Mixer.
> > (I don't think I can implement it with an SDL_Mixer Effect either,
> > because the callback can only modify a fixed-length sample.)
> >
> >
> > Is there a way to implement pitch shift with SDL_Mixer? Or an
> > alternate SDL library that would do that (but I also need midi/BGM
> > support in the mixer)?
> >
> >
> > For example, DirectX provides it through
> > DirectSoundBuffer::SetFrequency(frequency), and Allegro provides it
> > with play_sample(sample, t vol, pan, freq, loop).
> >
> >
> > Thanks!
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