[sdlsound] alt_audio_convert

[Ryan C. Gordon]

> I've assumed it's incomplete, since it's not the default.  I gave it a
> brief try, since a lot of people are complaining about the low quality
> sample rate conversions (esp. 22khz and 48khz; we always play in 44khz),
> but I only got noise.
>
> Should I try to fix it yet?

It's partially functional, but definitely incomplete. I haven't gotten any
patches from the person who wrote it for several months, so if you want to
tackle it, you are most welcome to do so.

The default (non-alt) rate converter is a cut-and-paste from SDL12. The
original intention was to improve it in SDL_sound and merge the changes
back into SDL itself, but this never happened...ideally, we should either
fix altcvt or at least get rid of the cut-and-paste job and call SDL's
converter on the default path to get rid of the duplicate code.

SDL's rate converter only handles powers of 2, so you can go from 22khz to
44, but not 48...this tends to be, well, disastrous, so I'd really like to
see altcvt finished.

--ryan.