[SDL] SDL_Mixer and speed/pitch shift

Mon Aug 6 13:01:54 PDT 2007

Errata: after a little research it appears I aim at "pitch
transposition" / "sample rate conversion" (which changes the length of
the buffer) rather than actual "pitch shifting" (which does not change
the length of the buffer).

Maybe that better explains my worries about implementing it via a
SDL_Mixer callback :)

-- 
Sylvain

On Mon, Aug 06, 2007 at 07:59:02AM +0200, Sylvain Beucler wrote:
> 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>
> > >Reply-To: "A list for developers using the SDL library. (includes  
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> > >To: sdl at lists.libsdl.org
> > >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!