mirror of
https://github.com/DrBeef/JKXR.git
synced 2024-11-23 04:22:27 +00:00
4597b03873
Opens in Android Studio but haven't even tried to build it yet (it won't.. I know that much!)
93 lines
4.5 KiB
Text
93 lines
4.5 KiB
Text
HRTF Support
|
|
============
|
|
|
|
Starting with OpenAL Soft 1.14, HRTFs can be used to enable enhanced
|
|
spatialization for both 3D (mono) and multi-channel sources, when used with
|
|
headphones/stereo output. This can be enabled using the 'hrtf' config option.
|
|
|
|
For multi-channel sources this creates a virtual speaker effect, making it
|
|
sound as if speakers provide a discrete position for each channel around the
|
|
listener. For mono sources this provides much more versatility in the perceived
|
|
placement of sounds, making it seem as though they are coming from all around,
|
|
including above and below the listener, instead of just to the front, back, and
|
|
sides.
|
|
|
|
The built-in data set is based on the KEMAR HRTF diffuse data provided by MIT,
|
|
which can be found at <http://sound.media.mit.edu/resources/KEMAR.html>. It's
|
|
only available when using 44100hz playback.
|
|
|
|
|
|
External HRTF Data Sets
|
|
=======================
|
|
|
|
OpenAL Soft also provides an option to use user-specified data sets, in
|
|
addition to or in place of the built-in set. This allows users to provide their
|
|
own data sets, which could be better suited for their heads, or to work with
|
|
stereo speakers instead of headphones, or to support more playback sample
|
|
rates, for example.
|
|
|
|
The file format for the data sets is specified below. It uses little-endian
|
|
byte order. Certain data fields are restricted to specific values (these
|
|
restriction may be lifted in future versions of the lib).
|
|
|
|
==
|
|
ALchar magic[8] = "MinPHR00";
|
|
ALuint sampleRate;
|
|
|
|
ALushort hrirCount; /* Required value: 828 */
|
|
ALushort hrirSize; /* Required value: 32 */
|
|
ALubyte evCount; /* Required value: 19 */
|
|
|
|
ALushort evOffset[evCount]; /* Required values:
|
|
{ 0, 1, 13, 37, 73, 118, 174, 234, 306, 378, 450, 522, 594, 654, 710, 755,
|
|
791, 815, 827 } */
|
|
|
|
ALshort coefficients[hrirCount][hrirSize];
|
|
ALubyte delays[hrirCount]; /* Element values must not exceed 127 */
|
|
==
|
|
|
|
The data is described as thus:
|
|
|
|
The file first starts with the 8-byte marker, "MinPHR00", to identify it as an
|
|
HRTF data set. This is followed by an unsigned 32-bit integer, specifying the
|
|
sample rate the data set is designed for (OpenAL Soft will not use it if the
|
|
output device's playback rate doesn't match).
|
|
|
|
Afterward, an unsigned 16-bit integer specifies the total number of HRIR sets
|
|
(each HRIR set is a collection of impulse responses forming the coefficients
|
|
for a convolution filter). The next unsigned 16-bit integer specifies how many
|
|
samples are in each HRIR set (the number of coefficients in the filter). The
|
|
following unsigned 8-bit integer specifies the number of elevations used by the
|
|
data set. The elevations start at the bottom, and increment upwards.
|
|
|
|
Following this is an array of unsigned 16-bit integers, one for each elevation
|
|
which specifies the index offset to the start of the HRIR sets for each given
|
|
elevation (the number of HRIR sets at each elevation is infered by the offset
|
|
to the next elevation, or by the total count for the last elevation).
|
|
|
|
The actual coefficients follow. Each coefficient is a signed 16-bit sample,
|
|
with each HRIR set being a consecutive number of samples. For each elevation,
|
|
the HRIR sets first start with a neutral "in-front" set (that is, one that is
|
|
applied equally to the left and right outputs). After this, the sets follow a
|
|
clockwise pattern, constructing a full circle for the left ear only. The right
|
|
ear uses the same sets but in reverse (ie, left = angle, right = 360-angle).
|
|
|
|
After the coefficients is an array of unsigned 8-bit delay values, one for each
|
|
HRIR set. This is the delay, in samples, after recieving an input sample before
|
|
before it's added in to the convolution filter that the corresponding HRIR set
|
|
operates on and gets heard.
|
|
|
|
|
|
Note that the HRTF data is expected to be minimum-phase reconstructed. The
|
|
time delays are handled by OpenAL Soft according to the specified delay[]
|
|
values, and afterward the samples are fed into the convolution filter using the
|
|
corresponding coefficients. This allows for less processing by using a shorter
|
|
convolution filter, as it skips the first coefficients that do little more than
|
|
cause a timed delay, as well as the tailing coefficients that are used to
|
|
equalize the length of all the sets and contribute nothing.
|
|
|
|
For reference, the built-in data set uses a 32-sample convolution filter while
|
|
even the smallest data set provided by MIT used a 128-sample filter (a 4x
|
|
reduction by applying minimum-phase reconstruction). Theoretically, one could
|
|
further reduce the minimum-phase version down to a 16-sample convolution filter
|
|
with little quality loss.
|