75 lines
3.3 KiB
Plaintext
75 lines
3.3 KiB
Plaintext
HRTF Support
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============
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Starting with OpenAL Soft 1.14, HRTFs can be used to enable enhanced
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spatialization for both 3D (mono) and multi-channel sources, when used with
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headphones/stereo output. This can be enabled using the 'hrtf' config option.
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For multi-channel sources this creates a virtual speaker effect, making it
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sound as if speakers provide a discrete position for each channel around the
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listener. For mono sources this provides much more versatility in the perceived
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placement of sounds, making it seem as though they are coming from all around,
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including above and below the listener, instead of just to the front, back, and
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sides.
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The default data set is based on the KEMAR HRTF data provided by MIT, which can
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be found at <http://sound.media.mit.edu/resources/KEMAR.html>. It's only
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available when using 44100hz playback.
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Custom HRTF Data Sets
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=====================
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OpenAL Soft also provides an option to use user-specified data sets, in
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addition to or in place of the default set. This allows users to provide their
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own data sets, which could be better suited for their heads, or to work with
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stereo speakers instead of headphones, or to support more playback sample
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rates, for example.
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The file format is specified below. It uses little-endian byte order.
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==
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ALchar magic[8] = "MinPHR01";
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ALuint sampleRate;
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ALubyte hrirSize; /* Can be 8 to 128 in steps of 8. */
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ALubyte evCount; /* Can be 5 to 128. */
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ALubyte azCount[evCount]; /* Each can be 1 to 128. */
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/* NOTE: hrirCount is the sum of all azCounts */
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ALshort coefficients[hrirCount][hrirSize];
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ALubyte delays[hrirCount]; /* Each can be 0 to 63. */
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==
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The data is described as thus:
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The file first starts with the 8-byte marker, "MinPHR01", to identify it as an
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HRTF data set. This is followed by an unsigned 32-bit integer, specifying the
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sample rate the data set is designed for (OpenAL Soft will not use it if the
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output device's playback rate doesn't match).
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Afterward, an unsigned 8-bit integer specifies how many sample points (or
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finite impulse response filter coefficients) make up each HRIR.
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The following unsigned 8-bit integer specifies the number of elevations used
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by the data set. The elevations start at the bottom (-90 degrees), and
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increment upwards. Following this is an array of unsigned 8-bit integers, one
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for each elevation which specifies the number of azimuths (and thus HRIRs) that
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make up each elevation. Azimuths start clockwise from the front, constructing
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a full circle for the left ear only. The right ear uses the same HRIRs but in
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reverse (ie, left = angle, right = 360-angle).
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The actual coefficients follow. Each coefficient is a signed 16-bit sample,
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with each HRIR being a consecutive number of sample points. The HRIRs must be
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minimum-phase. This allows the use of a smaller filter length, reducing
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computation. For reference, the built-in data set uses a 32-point filter while
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even the smallest data set provided by MIT used a 128-sample filter (a 4x
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reduction by applying minimum-phase reconstruction). Theoretically, one could
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further reduce the minimum-phase version down to a 16-point filter with only a
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small reduction in quality.
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After the coefficients is an array of unsigned 8-bit delay values, one for
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each HRIR. This is the propagation delay (in samples) a signal must wait before
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being convolved with the corresponding minimum-phase HRIR filter.
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