Dirt-Cheap 3-D Spatial Audio

With the addition of free audio software, an ordinary inexpensive surround sound card becomes the basis for a 3-D cube for simulation, visualization or gaming.
Configuring the Sound File Loader

In order to use a sound file from Mustajuuri, that sound file must be known to Mustajuuri at load time. The mechanism used to do that is a configuration file that specifies all of the sound files that possibly might be used by Mustajuuri. This configuration file is used by the synthesizer mixer module, which was created when we configured the Mustajuuri Mixer Panel above. A sample configuration file that loads three sound files follows. Once a file is created, any name can be assigned; make sure the synthesizer module points to that file.

unusevoices *-stk
polyphony 48

sample audioeffect1.wav
sample audioeffect2.wav
sample sudioeffect3.wav

The polyphony line specifies the maximum number of audio files that should be loaded by Mustajuuri, so it should be at least as large as the number of audio files listed in this file. The last three lines specify three sample audio WAV files. Any audio files specified here must be placed in a directory specified separately as part of the synthesizer module configuration. The unusevoices line is a somewhat more-advanced setting, but one that should help improve efficiency somewhat.

Configuring Mustajuuri for Remote Control

Mustajuuri is designed to act as a standalone program to manipulate audio, not as a library to be linked against by another application. To control Mustajuuri from another application, as is the case with our project, two steps are required. The first involves setting up Mustajuuri to listen for control commands over the network. The second step consists of writing a simple API in the main application to talk to Mustajuuri.

To get Mustajuuri to accept commands from the network, a network module must be loaded. This network module is the only way for an external application to control Mustajuuri, even if both the application and Mustajuuri are running on the same machine. Adding this module is a simple task and simply requires configuring which port Mustajuuri will listen to; the default port is 10030. This module automatically communicates with the synthesizer module and the VBAP panning modules, if they are in the mixer.

In order to control Mustajuuri from an application, it is necessary to add code included with the Mustajuuri API into the application. We present example code segments here that show how to connect to a remote audio server, play audio specified coming from a given 3-D position and change the position of a sound source and listener position.

The first code segment shows the initial commands to connect the application to the remote Mustajuuri audio server and initialize it. If Mustajuuri is running on another machine, change the address to reflect this. To change the default port, 10030, that Mustajuuri listens to, specify the new port in the address string, for example, mjserver.mydomain.com:12345. The two objects that we created, one instance each of AC_Control and AC_VrControl, are used later to send commands to Mustajuuri:


// connecting to a remote server
  #include <ac_vr_control.h>

  AC_Control acControl =
    new AC_Control();
  char* mjServerAddress = "127.0.0.1";
  if(!acControl->init(mjServerAddress))
  {
    // error handling code here
  }

  AC_VrControl acVrControl =
    new AC_VrControl( acControl );

The next code segment shows how to specify the position of the source of the audio and play it. One interesting thing to note here is that the variable outputChannel identifies the intended sound source to use. The number of supported sound sources was specified in the synthesizer module from the Configuring the Mustajuuri Mixer Panel section, and outputChannel should be between 0 and the number of sources, minus one. The variable soundFilename should not have a path as part of the filename. The filename should be one of the files listed in the configuration file created as part of configuring the Sound File Loader file. Lastly, the soundLevel is essentially the initial gain level for the new sound. This needs to be experimented with to find an appropriate setting:

  AC_Vector3 location(
    positionX,
    positionY,
    positionZ);

  int soundId =
    acVrControl->playSample(
      outputChannel,
      soundFilename,
      soundLevel,
      location,
      true,
      0, 0);

The last code segment shows how to reposition the sound source location and the orientation and position of the listener. The outputChannel variable refers to the sound source that is desired to be moved and should be the same value used to call playSample from the previous example. The listenerRotation matrix specifies the orientation of the listener relative to the world, and the worldRotation matrix specifies the orientation of the world relative to the speakers:

// reposition a source of sound
  AC_Vector3 location(
    positionX,
    positionY,
    positionZ);

  acVrControl->moveSource(outputChannel,
    0.05, location);

// reposition the listener orientation
  AC_Matrix3 listenerRotation(
    ... listener rotation matrix ... );
  AC_Matrix3 worldRotation(
    ... world rotation matrix ... );

  acVrControl->setTransformations(
    location, listenerRotation,
    worldRotation, 0.05);

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