[openal] A couple questions about OpenALSoft

[Chris Robinson]

On 02/22/2014 06:45 PM, Austin Hicks wrote:
> What's the possibility of getting OpenALSoft HRTEF to work at 192000
> HZ?  I know that the difference is probably imperceptible, but my audio
> equipment claims to support it, and it'd be nice if this just works
> without changing settings.  Is there a reason that makehrtf limits to
> 96khz?  I'm assuming I can't just disable this limit and expect it to
> work.  Also, looks like this only hits mmdevapi backend-the older ones
> seem to not go over 96khz.

The biggest issue is processing time. At 44~48Khz it uses a 32-point FIR 
filter for each output sample (per channel) to properly filter it. At 
192Khz, you'd need a 128-point filter to get the same result. So not 
only would you need 4 times as many points, you'd need to do it 4 times 
more often (128 taps 192000 times a second instead of 32 taps 48000 
times a second).

Given the limits of human hearing and the filter source, it's probably 
just as good to mix with HRTF at 48khz and then do a fast, high-quality 
4x resampling to 192khz.

That said, I'm not sure it shouldn't technically be possible. There 
could, however, be issues with integer overflow and floating-point 
accuracy when dealing with large samples rates. It would need testing.

> Secondly, how do I make proper use of buffers that are more than 2
> channels?  I'm not sure how to map samples from their desired channel to
> the expected data format, and no information seems available anywhere on
> this.  Does anyone know which order to put the channels in before
> submitting?

The order is like WFX.
mono - mono
stereo - front left, front right
rear - back left, back right
quad - front left, front right, back left, back right
5.1 surround - front left, front right, front center, lfe, back left,
                 back right
6.1 surround - front left, front right, front center, lfe, back center,
                side left, side right
7.1 surround - front left, front right, front center, lfe, back left,
                back right, side left, side right

> On a sidenote, does anyone know how to extract the order from audio
> files, or if there is a standard order that audio files use?
> Camlorn_audio, my python/C++/probably other language bindings can do
> stereo, but it'd be nice to get them automatically doing other
> multichannel formats without extra steps by the user.

Different audio formats specify their own ordering standards. Ogg 
Vorbis, for instance, specifies this:
https://www.xiph.org/vorbis/doc/Vorbis_I_spec.html#x1-800004.3.9

For wav files it's a bit more complicated since the format is just a 
container; the actual audio can be in any number of formats. 
WAVE_FORMAT_PCM (type 0x0001), for instance, doesn't really specify 
anything, and WAVE_FORMAT_EXTENSIBLE (type 0xfffe) does specify an 
ordering with its channel mask field (basically the channels are ordered 
by the lowest set bit first to the highest last).
http://msdn.microsoft.com/en-us/windows/hardware/gg463006.aspx

For other formats, you'll need to find their specification. Anything 
more than two channels is highly format-dependent.