diff options
Diffstat (limited to 'docs/hrtf.txt')
| -rw-r--r-- | docs/hrtf.txt | 66 |
1 files changed, 38 insertions, 28 deletions
diff --git a/docs/hrtf.txt b/docs/hrtf.txt index 37a329d2..ba8cd8fa 100644 --- a/docs/hrtf.txt +++ b/docs/hrtf.txt @@ -29,22 +29,31 @@ rates, for example. The file format is specified below. It uses little-endian byte order. == -ALchar magic[8] = "MinPHR01"; +ALchar magic[8] = "MinPHR02"; 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. */ +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, "MinPHR01", to identify it as an +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). @@ -52,23 +61,24 @@ 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 +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). Theoretically, one could -further reduce the minimum-phase version down to a 16-point filter with only a -small reduction in quality. +reduction by applying minimum-phase reconstruction). 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. +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. |