If you need 3D spatialized sound output, read on!
Psychtoolbox allows you to directly call low-level OpenAL commands from
the Matlab environment in nearly the same way as you could do from native
C or C++ code. This way you can code and use sound recording
algorithms and 3D sound output algorithms in Matlab, utilizing the power of
modern sound hardware by calling OpenAL functions.
OpenAL is a free (free software), cross-platform (Windows, Macintosh, Linux),
highly efficient, easy to learn, flexible audio library which incorporates
advanced features like 3D spatialized sound, multi-channel support, streaming,
audio capture, special effects like echo, reverb, attenuation, doppler effects
and other stuff. Its programming model and API closely resembles OpenGL, so
OpenGL programmers will find themselves immediatels at home with this library,
and its very easy to extend 3D graphics code with corresponding 3D sound.
MOAL provides one Matlab wrapper M-File for each corresponding OpenAL
function. The wrapper file calls into a special MEX file (moalcore) which
implements the C-language interface to OpenAL. The syntax of a Matlab
OpenAL command is mostly identical to its C counterpart with a few
small exceptions that are imposed to us by the design of Matlab:
- Return values are returned in Matlab-style, as left-hand side
arguments of the calls, instead of being right-hand side arguments as in
E.g., the C language call alGetIntegerv(GLenum;
becomes params = alGetIntegerv(pname); in Matlab, because ‘params’ is a
return argument of alGetIntegerv.
- Commands that don’t take arguments don’t have empty braces, because
Matlab doesn’t allow this:
E.g., the C language call alEnd(); becomes alEnd; in Matlab.
- All AL, ALU and ALC constants start with prefix AL. instead of AL_
E.g., AL_TRUE becomes AL.TRUE …
Each subroutine that intends to use AL constants needs to define the
variable AL as global: Example
global AL; % Define AL variable as global.
…rest of function implementation…
If you want to use ALC constants, then ‘global ALC’ is also needed.
- In your main Matlab script or M-File you need to call the function
InitializeMatlabOpenAL; This command initializes the OpenAL for Matlab toolbox and
sets up Psychtoolbox to play nicely with Matlab-OpenAL and other OpenAL
Support for 3rd party OpenAL MEX-Files:
You can also code up OpenAL algorithms in the C programming language and
compile them into Matlab-MEX files if you have “need for speed”. Your Mex
files will just contain the mixture of ANSI C code and OpenAL calls, but
no code to setup the OpenAL rendering context. You just need to call the
InitializeMatlabOpenAL; function. After that call, an audio device will
be set up and the associated OpenAL rendering context will be active. All
commands in your MEX-File will apply to that rendering context.
If you want to write OpenAL mex-files that are portable across different
operating systems (OS-X, Windows, Linux) then have a look at:
‘Psychtoolbox/PsychSound/MOAL/source’ for how to do this. This folder
contains the source code and Makefiles for our own moalcore mex-file…
Depending on your specific code, rendering speed in Matlab may be
slightly lower than when executing the same code from C or C++. This
is the price you’ll have to pay for using Matlab, but our abstraction
layer is very thin, so most applications won’t be really affected.
Some OpenAL extension functions are not yet implemented in the toolbox, because
these functions can’t get automatically generated, so their wrappers need
to be coded manually. Our goal is to provide sufficient support for the
OpenAL-API, but finalizing all functions may take some time.
Apart from these limitations that will get removed in the future, there
are limitations imposed by your operating system and sound hardware.
Support for OpenAL functions varies between different sound hardware,
so if you want to use the latest and greatest OpenAL functions, you’ll
need to buy and install the latest and greatest sound hardware.
Typical limitations of low-end versus high-end hardware:
- Number of sound sources / channels that can be played back simultaneously.
- Complexity of audio processing that can be performed.
- Latency in sound processing.
- Quality of 3D spatial audio rendering.
* High-level helper functions can be found in ‘Psychtoolbox/PsychSound/’
and its subfolders.
* Demos can be found in ‘Psychtoolbox/PsychDemos/SoundDemos’
Lot’s of documentation, tutorials, code samples and news about OpenAL can
be found by following links at:
A FOSS open-source software implementation of OpenAL, which is typically used
on GNU/Linux, and which has many advanced features implemented as extensions,
e.g., pretty flexible HRTF based sound rendering, can be found at:
Googling for OpenAL is also a option.