From 842a698f2b8f08e29a0eef9a2f255ff358d5c16b Mon Sep 17 00:00:00 2001 From: Chris Robinson Date: Sun, 11 Mar 2012 19:27:40 -0700 Subject: Add a text file explaining OpenAL Soft's HRTF support and data file format --- hrtf.txt | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 82 insertions(+) create mode 100644 hrtf.txt (limited to 'hrtf.txt') diff --git a/hrtf.txt b/hrtf.txt new file mode 100644 index 00000000..5810ea7a --- /dev/null +++ b/hrtf.txt @@ -0,0 +1,82 @@ +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 . 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 are 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 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 set). The +following unsigned 8-bit integer specifies the number of elevations used by the +data set. + +Following this is an array of unsigned 16-bit integers, one for each elevation +which specifies the offset to the start of the HRIR sets for each given +elevation (the number of HRIRs 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" set (that is, one that is applied +equally to the left and right outputs). After this, the HRIR sets follow a +clockwise pattern, with the first half specifying the "occluded" HRIRs (for the +ears on the opposite side of the sound source) for half of the head, and the +remaining specify the un-occluded HRIRs (for the ears on the side of the sound +source) for half of the head. OpenAL Soft will automatically select the HRIR +sets to use based on the angle to the source from the listener, and swap them +depending on which side the sound is on. + +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. -- cgit v1.2.3