[SDL] SDL_Mixer and speed/pitch shift

[Sylvain Beucler]

Hi,

I was thinking that was not possible; for example, if you want to play
a sound slower, it takes a bigger buffer to store it, and the callback
works with a fixed-length buffer. So at first glance I believe
SDL_Mixer will stop the pitch-shifted sound before it ends.

Do you think it is possible to bypass this limitation?


Thanks for the link.

-- 
Sylvain

On Mon, Aug 06, 2007 at 12:27:19AM -0500, James Haley wrote:
> 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!