[sdlsound] alt_audio_convert

Frank Ranostaj ranostaj at stud.uni-frankfurt.de
Sun Apr 6 14:25:44 EDT 2003

----- Original Message -----
From: "Glenn Maynard" <g_sdl at zewt.org>
To: <sdlsound at icculus.org>
Sent: Wednesday, March 12, 2003 2:20 AM
Subject: Re: [sdlsound] alt_audio_convert

> On Wed, Mar 12, 2003 at 01:10:36AM +0100, Max Horn wrote:
> > Actually even the rate 2 conversion is pretty bad. One might think that
> > just duplicating every sample is sufficient for a rate conversion from
> > 22khz to 44khz (or vice versa, leaving out every second sample) but
> > that's actually not true - you are introducing undesired new bands into
> I don't think anyone thinks this is really a good way to do it.  :)
> It's just the easiest, since you don't need any lookahead.  Even a
> simple linear filter (one frame of lookahead) would be a lot better (but
> still pretty bad).
> > the output data. The "proper" way is to add some filters afterwards....
> > admittedly it's been far too long since i did anything on this topic so
> > please excuse the sloppy wording, but I hope the idea gets across.
> > There are some great "full" resamplers out there, but I am not sure how
> > etc
> I believe the alt-convert stuff does all of this.  It's just not yet
> fully functional.

Yes, it is intended to do so. I though, it was  more or less working.
The allocation interface in playsound should take a additional buffer
length into account for the settling time of the filter.
The conversion e.g. 44.1 <-> 48.0 is approximated by 44.0 <-> 48.0.
The use of Splines can be superior to the windowed sin(x)/x filter
used in the alt-converter.

I presumed there was not much interest in the rate converter and my
lousy english has made it diffcult to explain functions of the converter,
so I am uncertain on how to proceed.

    Frank Ranostaj

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