This way takes into account a new stereo-mode config option, which when set to
"headphones" will default to using HRTF. Eventually the device will also be
able to specify if headphones are being used.
This new method mixes sources normally into a 14-channel buffer with the
channels placed all around the listener. HRTF is then applied to the channels
given their positions and written to a 2-channel buffer, which gets written out
to the device.
This method has the benefit that HRTF processing becomes more scalable. The
costly HRTF filters are applied to the 14-channel buffer after the mix is done,
turning it into a post-process with a fixed overhead. Mixing sources is done
with normal non-HRTF methods, so increasing the number of playing sources only
incurs normal mixing costs.
Another benefit is that it improves B-Format playback since the soundfield gets
mixed into speakers covering all three dimensions, which then get filtered
based on their locations.
The main downside to this is that the spatial resolution of the HRTF dataset
does not play a big role anymore. However, the hope is that with ambisonics-
based panning, the perceptual position of panned sounds will still be good. It
is also an option to increase the number of virtual channels for systems that
can handle it, or maybe even decrease it for weaker systems.
Apparently, 5.1 surround sound is supposed to use the "side" channels, not the
back channels, and we've been wrong this whole time. That means the "5.1 Side"
is actually the correct 5.1 setup, and using the back channels is anomalous.
Additionally, this means the 5.1 buffer format should also use the the side
channels instead of the back channels.
A final note: the 5.1 mixing coefficients are changed so both use the original
5.1 surround sound set (with the surround channels at +/-110 degrees). So the
only difference now between 5.1 "side" and 5.1 "back" is the channel labels.
This behavior better matches Creative's hardware drivers and Rapture3D's OpenAL
driver. A compatibility environment variable is provided to restore the old
no-op behavior for any app that behaves badly from this change (set
__ALSOFT_SUSPEND_CONTEXT to "ignore").
If too many apps have a problem with this, the default behavior may need to be
changed to ignore, with the env var providing an option to defer/batch instead.
There's apparently some issues with it causing noise or killing the output. It
might be due to the per-sample changes being too harsh for the filter to keep
up with, but it's not something I can take care of in time for release.
This commit should be reverted after release when work on fixing it can resume.
