Stream binaural signals from BASS to Matlab

This tutorial shows an example of how to control the BASS component and retrieve audio streams in Matlab, using the matlab-genomix bridge (see Installation of the robotic tools).

Preliminary steps

In order to follow this tutorial, you will need:

  • A Linux system with the robotic tools and BASS installed (c.f. Installation of the robotic tools). We will call this system the BASS host.

  • An ALSA-compliant sound acquisition interface with at least two input channels, and two microphones plugged into it. The interface must be connected to the BASS host.


    Alternatively, if you do not possess an external sound interface but the BASS host has an integrated sound card and microphone, you still might be able to follow the tutorial. Keep in mind though that if there is only one microphone, you will not have a genuine stereo signal, but a simulated one from your mono input.

  • A computer with Matlab and the matlab-genomix bridge installed. We will call it the remote client. The BASS host and the remote client could possibly, but not necessarily, be the same computer.

On the BASS host, open 3 new terminals. In the first terminal, run the command:

$ roscore

This launches the ROS middleware. ROS nodes can now connect to this node called the ROS master. In the second terminal, run the command:

$ genomixd

This launches a genomix server, now waiting for incoming connections from clients on port 8080 by default. In the third terminal, run the command:

$ bass-ros

This is the BASS component, now running on the system. The name bass-ros specifies that this GenoM3 component uses the ROS middleware. So it is actually a ROS node, connected to the ROS master running in the first terminal.

For the moment, the BASS component is not doing anything. It is waiting for requests from a client (which will be Matlab here) to start services. This is the followed process:

  1. The client emits a HTTP message destined for the genomix server, requesting to call a service of the BASS component.
  2. genomix executes the call directed at the BASS component.
  3. When the service is completed, BASS returns its output to genomix, and genomix relays it back to the client.

Keep the third terminal running BASS visible on the screen. When we will call some services, we will notice their effect on the component’s standard output stream (stdout).

Control BASS to start an acquisition

On the remote client, start a Matlab session and make sure that matlab-genomix is in the Matlab path (c.f. Installation of the robotic tools).

Connect to genomix and load BASS

If you have Matlab on the same computer where the genomix server is running, you can simply connect to genomix with:

>> client = genomix.client
client =

  client with no properties.

This will attempt a connection on localhost:8080 by default. Otherwise if your BASS host and your remote client are two different computers, get the IP address of the BASS host and override the default value with:

>> client = genomix.client('') % write the IP address of BASS host

Then, load BASS:

>> bass = client.load('bass')

bass =

  component with properties:

        genom_state: [function_handle]
               kill: [function_handle]
       connect_port: [function_handle]
    connect_service: [function_handle]
               Stop: [function_handle]
        ListDevices: [function_handle]
    DedicatedSocket: [function_handle]
              Audio: [function_handle]
     abort_activity: [function_handle]
            Acquire: [function_handle]
        CloseSocket: [function_handle]

The returned handle bass has a list of properties either corresponding to services (e.g. Acquire) or ports (e.g. Audio) of the component.

Get the name of your sound interface

Invoke the ListDevices service to get the name of your ALSA device:

>> bass.ListDevices();

The detected sound devices are listed on the components’s standard output stream (stdout). On the BASS host, look in the terminal where the component is running, and find a line that matches your interface, something like:

hw:1,0 [Babyface2361116] [USB Audio]

The leading string, hw:1,0 in the example, is the name of your ALSA device.

Start an acquisition

We will now use the Acquire service to start an acquisition.


By default, services are invoked synchronously, i.e. the command to invoke them only returns after completion of the service. As the acquisition runs indefinitely, the Acquire service never completes unless you explicitly stop it. So you must invoke this service asynchronously, i.e. the command invoking the service returns immediately and the service output can be retrieved later on. Otherwise you will be blocked in the Matlab command window without control, including stopping the service. If this happens, a solution is to kill the Matlab process and start again.

The service can be invoked asynchronously by providing the '-a' option:

>> r = bass.Acquire('-a')
 string device: ALSA name of the sound device (hw:1,0) >

The Acquire service expects input arguments. As we did not passed them to the function directly, they are prompted interactively. Enter values according to your sound interface (see the example below):

  • For the device parameter, take the value you obtained at the previous step.

  • For the sampleRate parameter, choose a sampling rate that your device supports. The default value (44100 Hz) is most likely to work.

  • For the nFramesPerChunk parameter, choose a chunk size that your device supports. Some devices only support powers of 2 (e.g. 512, 1024, 2048…), refer to your device manual.

  • For the nChunksOnPort parameter, choose a value that is big enough so that the output port of BASS streams a few seconds of audio data. For instance, with the default values (44100 Hz for the sampling rate and 2205 frames for the chunk size), keep 80 chunks on the port to have 4 seconds:

    \[\begin{split}duration &= nChunksOnPort * nFramesPerChunk / sampleRate \\ &= 80 * 2205 / 44100 \\ &= 4 s\end{split}\]
>> r = bass.Acquire('-a')
 string device: ALSA name of the sound device (hw:1,0) > 'hw:1,0'
 unsigned long sampleRate: Sample rate in Hz (44100) > 44100
 unsigned long nFramesPerChunk: Chunk size in frames (2205) > 2205
 unsigned long nChunksOnPort: Port size in chunks (20) > 80

r =

  request with properties:

       status: 'sent'
       result: []
    exception: []

If starting the acquisition succeeded, you should see the status 'sent' in the returned handle. Otherwise, the status would be 'error', check then the error message printed in the terminal on the BASS host. It could be an invalid input parameter.


The parameter prompts like string device: ALSA name of the sound device (hw:1,0) > contains valuable information, i.e. the data type of the parameter, its name, a short description and a default value between parenthesis that will be used if you press enter without specifying another value. All this information comes from the dotgen file of the component, and is part of its definition.

Get audio data in Matlab

You can read the output port of BASS, named Audio, in Matlab:

>> p = bass.Audio()
p =

    Audio: [1x1 struct]

>> p.Audio
ans =

         sampleRate: 44100
      nChunksOnPort: 80
    nFramesPerChunk: 2205
     lastFrameIndex: 251370
               left: {176400x1 cell}
              right: {176400x1 cell}

The data structure shown here is retrieved when reading the port with function bass.Audio(). The audio signals are stored in the left and right fields. Note the presence of the index lastFrameIndex for keeping track of the data.

If your remote client computer has speakers, you can listen to the retrieved signals:

% Speak in the microphones for a few seconds

% Read the last few recorded seconds
>> p = bass.Audio();

% Play the recorded sound, on left channel for instance
>> soundsc(cell2mat(p.Audio.left), p.Audio.sampleRate);

Notice how the duration of the sound matches the one you selected with parameter nChunkOnPort when starting the acquisition.

End the session

When you are done, you can clear the used objects in Matlab:

>> delete(bass);
>> delete(client); % This closes the connection to genomix

On the BASS host, you can kill processes roscore, genomixd and bass-ros by typing Control-c in each terminal.