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- 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 default data set is based on the KEMAR HRTF data provided by MIT, which can
- be found at <http://sound.media.mit.edu/resources/KEMAR.html>.
-
-
- Custom HRTF Data Sets
- =====================
-
- OpenAL Soft also provides an option to use user-specified data sets, in
- addition to or in place of the default set. This allows users to provide data
- sets that could be better suited for their heads, or to work with stereo
- speakers instead of headphones, for example.
-
- The file format is specified below. It uses little-endian byte order.
-
- ==
- ALchar magic[8] = "MinPHR03";
- ALuint sampleRate;
- ALubyte channelType; /* Can be 0 (mono) or 1 (stereo). */
- ALubyte hrirSize; /* Can be 8 to 128 in steps of 8. */
- ALubyte fdCount; /* Can be 1 to 16. */
-
- struct {
- ALushort distance; /* Can be 50mm to 2500mm. */
- ALubyte evCount; /* Can be 5 to 128. */
- ALubyte azCount[evCount]; /* Each can be 1 to 128. */
- } fields[fdCount];
-
- /* NOTE: ALbyte3 is a packed 24-bit sample type,
- * hrirCount is the sum of all azCounts.
- * channels can be 1 (mono) or 2 (stereo) depending on channelType.
- */
- ALbyte3 coefficients[hrirCount][hrirSize][channels];
- ALubyte delays[hrirCount][channels]; /* Each can be 0 to 63. */
- ==
-
- The data layout is as follows:
-
- The file first starts with the 8-byte marker, "MinPHR03", 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 resample the HRIRs
- if the output device's playback rate doesn't match).
-
- Afterward, an unsigned 8-bit integer specifies the channel type, which can be 0
- (mono, single-channel) or 1 (stereo, dual-channel). After this is another 8-bit
- integer which specifies how many sample points (or finite impulse response
- filter coefficients) make up each HRIR.
-
- The following unsigned 8-bit integer specifies the number of fields used by the
- data set, which must be in descending order (farthest first, closest last).
- Then for each field an unsigned 16-bit short specifies the distance for that
- field in millimeters, followed by an 8-bit integer for the number of
- elevations. These elevations start at the bottom (-90 degrees), and increment
- upwards. Following this is an array of unsigned 8-bit integers, one for each
- elevation which specifies the number of azimuths (and thus HRIRs) that make up
- each elevation. Azimuths start clockwise from the front, constructing a full
- circle. Mono HRTFs use the same HRIRs for both ears by reversing the azimuth
- calculation (ie. left = angle, right = 360-angle).
-
- The actual coefficients follow. Each coefficient is a signed 24-bit sample.
- Stereo HRTFs interleave left/right ear coefficients. The HRIRs must be
- minimum-phase. This allows the use of a smaller filter length, reducing
- computation. For reference, the default data set uses a 32-point filter while
- even the smallest data set provided by MIT used a 128-sample filter (a 4x
- reduction by applying minimum-phase reconstruction).
-
- After the coefficients is an array of unsigned 8-bit delay values as 6.2 fixed-
- point integers, one for each HRIR (with stereo HRTFs interleaving left/right
- ear delays). This is the propagation delay in samples a signal must wait before
- being convolved with the corresponding minimum-phase HRIR filter.
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