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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>. It's only
- available when using 44100hz or 48000hz playback.
- 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 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 is specified below. It uses little-endian byte order.
- ==
- ALchar magic[8] = "MinPHR01";
- ALuint sampleRate;
- ALubyte hrirSize; /* Can be 8 to 128 in steps of 8. */
- ALubyte evCount; /* Can be 5 to 128. */
- ALubyte azCount[evCount]; /* Each can be 1 to 128. */
- /* NOTE: hrirCount is the sum of all azCounts */
- ALshort coefficients[hrirCount][hrirSize];
- ALubyte delays[hrirCount]; /* Each can be 0 to 63. */
- ==
- The data is described as thus:
- The file first starts with the 8-byte marker, "MinPHR01", 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 8-bit integer 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 elevations used
- by the data set. The 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 for the left ear only. The right ear uses the same HRIRs but in
- reverse (ie, left = angle, right = 360-angle).
- The actual coefficients follow. Each coefficient is a signed 16-bit sample,
- with each HRIR being a consecutive number of sample points. The HRIRs must be
- minimum-phase. This allows the use of a smaller filter length, reducing
- computation. For reference, the built-in 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). Theoretically, one could
- further reduce the minimum-phase version down to a 16-point filter with only a
- small reduction in quality.
- After the coefficients is an array of unsigned 8-bit delay values, one for
- each HRIR. 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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