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 ALSA

ALSA needs to know how to communicate with all eight channels of the audio card. This normally would be achieved simply by using the device named surround71, but it is not fully compatible with the spatial sound API Mustajuuri. Mustajuuri requires support for input channels. The device surround71 supports eight output channels but no input channels. Therefore, it is necessary to define a new device that has eight output channels and some input channels.

In order to meet this requirement, an asymmetric device is defined. The device is called asymmetric because the number of input and output channels are not necessarily the same. Notice that the number of input channels is not stated explicitly. ALSA determines the number of input channels automatically and assigns the maximum; the Audigy card we used has two.

To configure ALSA, add the following text to the file /etc/asound.conf or create the file if necessary. This file holds information about user-defined devices, so we use the following text to add an asymmetric device called eightout:

ctl.eightout {
   type hw
   card 0
}

pcm.eightout {
   type asym
   playback.pcm {
      type route
      slave.pcm surround71
      ttable.0.0 1
      ttable.1.1 1
      ttable.2.2 1
      ttable.3.3 1
      ttable.4.4 1
      ttable.5.5 1
      ttable.6.6 1
      ttable.7.7 1
   }
   capture.pcm {
      type hw
      card 0
   }
}

Next, an environment variable must be set to allow Mustajuuri to talk to the audio card through ALSA. Set the following environment variable:

export MJ_AUDIO_CONF=
"Input=2=hw:0,0 | Output=8=eightout"

Once this is done, Mustajuuri should be able to output audio through all eight channels of the audio card.

Configuring the Mustajuuri Mixer Panel

Mustajuuri uses a mixer-board-style GUI for sending input audio streams to a speaker array, combining them or just passing them through intact. The input streams can come either from sound files or from live sources, such as a microphone. The GUI lays out several strips of channels that can be assigned different functions applied in a sequential process. Some example functions are input, send (to speaker), amplitude gain, panning and synthesizer. The gain and panning modify how the audio is distributed to individual output audio channels.

The Mixer Panel configuration we use is shown in Figure 4, which uses two mixer strips. The first has two interesting channels: a synthesizer channel, which manages the sound files, and a panning module, which handles the VBAP-based panning across speakers. The second strip is used to manage remote connections from external applications and does not accept an audio stream as input. It sends commands to the synthesizer and the VBAP module.

Figure 4. Screenshot of the Mustajuuri Mixer GUI (Tommi Ilmonen, Mustajuuri-2004)

To create a similar configuration, launch Mustajuuri and create a new mixer from the File menu. This mixer has several strips already, and all of these strips essentially are blank. The number of strips and the number of modules per strip can be changed using the Edit menu, if needed. Modules can be assigned by clicking with the mouse on a particular slot. To adjust the module's properties, simply click on the blue link defining the module's type, such as Synth1 or Mixer Input. The Strip X button at the top of a strip can be used to modify and remove the modules in any slot in that strip. All mixer configuration changes are saved by using the save options from the File menu. The resulting configuration file, for example, SpatialAudio.mj, is specified on the command line when Mustajuuri is called.

Specifying Speaker Placement

In order to use VBAP, it is necessary for Mustajuuri to know the locations of the speakers in the 3-D array. Mustajuuri does this through a configuration file that is specified as part of the VBAP panning module setup; this module was created as part of configuring the Mustajuuri Mixer Panel. This file specifies the azimuth and elevation angles (in degrees) for each speaker relative to the listener. Because our system uses eight speakers arranged in a cube configuration, our configuration file is specified as follows:

3        # dimensionality
# Azimuth, followed by elevation.
# 0 0 would be straight ahead.
-45 45   # Front upper left
45 45    # Front upper right
-135 45  # Back  upper left
135 45   # Back  upper right
-45 -45  # Front lower left
45 -45   # Front lower right
-135 -45 # Back  lower left
135 -45  # Back  lower right

This configuration file is used by the main configuration file for the Mustajuuri Mixer. This file assumes that all speakers are equidistant from the listener. If this is not the case, adjust the gain and delay for each speaker manually by using the Mustajuuri Mixer. In our system, such adjustments were not necessary, and it would involve significant work if it were necessary. The easiest solution is to try to place all speakers equidistant from the ideal listening position.

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