diff options
Diffstat (limited to 'hrtf.txt')
| -rw-r--r-- | hrtf.txt | 76 |
1 files changed, 28 insertions, 48 deletions
@@ -12,9 +12,9 @@ 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 <http://sound.media.mit.edu/resources/KEMAR.html>. It's -only available when using 44100hz playback. +The built-in 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 playback. External HRTF Data Sets @@ -26,68 +26,48 @@ 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). +The file format is specified below. It uses little-endian byte order. == -ALchar magic[8] = "MinPHR00"; +ALchar magic[8] = "MinPHR01"; ALuint sampleRate; -ALushort hrirCount; /* Required value: 828 */ -ALushort hrirSize; /* Required value: 32 */ -ALubyte evCount; /* Required value: 19 */ +ALushort hrirSize; /* Can be 8 to 128. */ +ALubyte evCount; /* Can be 5 to 128. */ -ALushort evOffset[evCount]; /* Required values: - { 0, 1, 13, 37, 73, 118, 174, 234, 306, 378, 450, 522, 594, 654, 710, 755, - 791, 815, 827 } */ +ALushort azCount[evCount]; /* Each can be 1 to 128. */ ALshort coefficients[hrirCount][hrirSize]; -ALubyte delays[hrirCount]; /* Element values must not exceed 127 */ +ALubyte delays[hrirCount]; /* Each can be 0 to 63. */ == The data is described as thus: -The file first starts with the 8-byte marker, "MinPHR00", to identify it as an +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 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 filter). The -following unsigned 8-bit integer specifies the number of elevations used by the -data set. The elevations start at the bottom, and increment upwards. +Afterward, an unsigned 16-bit integer specifies how many sample points (or +finite impulse response filter coefficients) make up each HRIR. -Following this is an array of unsigned 16-bit integers, one for each elevation -which specifies the index offset to the start of the HRIR sets for each given -elevation (the number of HRIR sets at each elevation is infered by the offset -to the next elevation, or by the total count for the last elevation). +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 16-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 set being a consecutive number of samples. For each elevation, -the HRIR sets first start with a neutral "in-front" set (that is, one that is -applied equally to the left and right outputs). After this, the sets follow a -clockwise pattern, constructing a full circle for the left ear only. The right -ear uses the same sets but in reverse (ie, left = angle, right = 360-angle). - -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 and gets heard. - - -Note that the HRTF data is expected to be minimum-phase reconstructed. The -time delays are handled by OpenAL Soft according to the specified delay[] -values, and afterward the samples are fed into the convolution filter using the -corresponding coefficients. This allows for less processing by using a shorter -convolution filter, as it skips the first coefficients that do little more than -cause a timed delay, as well as the tailing coefficients that are used to -equalize the length of all the sets and contribute nothing. - -For reference, the built-in data set uses a 32-sample convolution filter while +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-sample convolution filter -with little quality loss. +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. |