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diff --git a/docs/hrtf.txt b/docs/hrtf.txt new file mode 100644 index 00000000..ba8cd8fa --- /dev/null +++ b/docs/hrtf.txt @@ -0,0 +1,84 @@ +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] = "MinPHR02"; +ALuint sampleRate; +ALubyte sampleType; /* Can be 0 (16-bit) or 1 (24-bit). */ +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: ALtype can be ALshort (16-bit) or ALbyte[3] (24-bit) depending on + * sampleType, + * hrirCount is the sum of all azCounts. + * channels can be 1 (mono) or 2 (stereo) depending on channelType. + */ +ALtype coefficients[hrirCount][hrirSize][channels]; +ALubyte delays[hrirCount][channels]; /* Each can be 0 to 63. */ +== + +The data is described as thus: + +The file first starts with the 8-byte marker, "MinPHR02", 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 fields used by the +data set. 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 16-bit or 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, 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. |