Chris Robinson
4d36ef65b2
For mono sources, third-order ambisonics is utilized to generate panning gains.
The general idea is that a panned mono sound can be encoded into b-format
ambisonics as:
w[i] = sample[i] * 0.7071;
x[i] = sample[i] * dir[0];
y[i] = sample[i] * dir[1];
...
and subsequently rendered using:
output[chan][i] = w[i] * w_coeffs[chan] +
x[i] * x_coeffs[chan] +
y[i] * y_coeffs[chan] +
...;
By reordering the math, channel gains can be generated by doing:
gain[chan] = 0.7071 * w_coeffs[chan] +
dir[0] * x_coeffs[chan] +
dir[1] * y_coeffs[chan] +
...;
which then get applied as normal:
output[chan][i] = sample[i] * gain[chan];
One of the reasons to use ambisonics for panning is that it provides arguably
better reproduction for sounds emanating from between two speakers. As well,
this makes it easier to pan in all 3 dimensions, with for instance a "3D7.1" or
8-channel cube speaker configuration by simply providing the necessary
coefficients (this will need some work since some methods still use angle-based
panpot, particularly multi-channel sources).
Unfortunately, the math to reliably generate the coefficients for a given
speaker configuration is too costly to do at run-time. They have to be pre-
generated based on a pre-specified speaker arangement, which means the config
options for tweaking speaker angles are no longer supportable. Eventually I
hope to provide config options for custom coefficients, which can either be
generated and written in manually, or via alsoft-config from user-specified
speaker positions.
The current default set of coefficients were generated using the MATLAB scripts
(compatible with GNU Octave) from the excellent Ambisonic Decoder Toolbox, at
https://bitbucket.org/ambidecodertoolbox/adt/
Source Install ============== To install OpenAL Soft, use your favorite shell to go into the build/ directory, and run: cmake .. Assuming configuration went well, you can then build it, typically using GNU Make (KDevelop, MSVC, and others are possible depending on your system setup and CMake configuration). Please Note: Double check that the appropriate backends were detected. Often, complaints of no sound, crashing, and missing devices can be solved by making sure the correct backends are being used. CMake's output will identify which backends were enabled. For most systems, you will likely want to make sure ALSA, OSS, and PulseAudio were detected (if your target system uses them). For Windows, make sure DirectSound was detected. Utilities ========= The source package comes with an informational utility, openal-info, and is built by default. It prints out information provided by the ALC and AL sub- systems, including discovered devices, version information, and extensions. Configuration ============= OpenAL Soft can be configured on a per-user and per-system basis. This allows users and sysadmins to control information provided to applications, as well as application-agnostic behavior of the library. See alsoftrc.sample for available settings. Acknowledgements ================ Special thanks go to: Creative Labs for the original source code this is based off of. Christopher Fitzgerald for the current reverb effect implementation, and helping with the low-pass filter. Christian Borss for the 3D panning code the current implementation is heavilly based on. Ben Davis for the idea behind the current click-removal code.