From BCI2000 Wiki
Revision as of 14:52, 29 March 2019 by Jordanpowell (talk | contribs) (AudioInputDevice)
Jump to: navigation, search


An environment extension which manages multichannel, low latency audio I/O.




Griffin Milsap (

Version History

  • 2012/06/11: Initial public release;

Source Code Revisions

  • Initial development: 4095
  • Tested under: 5813
  • Known to compile under: 5813
  • Broken since: --


  • Fix Known Issues
  • Add per-sample resolution to envelopes

Known Issues

  • Leaving the module in halted state exhibits some sort of bug regarding state logging. When running state is resumed, envelope states may fail to update for 15-30 seconds. The bug seems to be unrelated to how long system was halted -- Not sure if this is an issue with the extension itself, or an issue with the bcievent interface. This bug will never happen on the first run after BCI2000 is started up. If you do see the behavior, either wait for it to go away or restart BCI2000 and perform a new recording.
  • Bandpass filtering in filterbanks doesn't appear to function correctly.
  • AudioExtension processes audio in a separate thread and the internal audio callback is called from yet-another context (Sometimes a system interrupt). In order to prevent deadlock, the Audio callback must not lock or wait for external threads. As such, it will simply copy the last good audio buffer to the output stream if the audio thread has not posted new data to use yet. This can result in slowed "timestretching" effects on audio input files if the audio thread cannot keep up with the audio callbacks. To prevent this behavior, ensure your audio block size is large enough (at least 1024 frames). If you are using a lower latency audio API (such as ASIO) you are probably okay to use audio block sizes around 512 frames. Compiling in debug can also result in a loss of data. Either way, be mindful that audio playback may not necessarily operate in real-time and you will receive NO WARNING WHATSOEVER when it fails to.

Functional Description

Experiments which require audio input or real-time audio synthesis based on system state are now possible with the AudioExtension. This extension is capable of recording multiple channels of audio input, synthesizing tones or noise, and reading encoded audio files. These channels are input to a mixing matrix which mixes those inputs to multiple channels of audio output. Both input and output are run through a simple filterbank, then they have their envelope extracted and logged into states via the bcievent interface. Audio input and output channels can be recorded into audio files losslessly and can be resynchronized offline. The mixing matrix is a matrix of expressions which can be used to dynamically change audio mixing based on the system state.

Integration into BCI2000

Compile the extension into your source module by enabling contributed extensions in your CMake configuration. You can do this by going into your root build folder and deleting CMakeCache.txt and re-running the project batch file, or by running cmake -i and enabling BUILD_AUDIOEXTENSION. Once the extension is built into the source module, enable it by starting the source module with the --EnableAudioExtension=1 command line argument (NB, as explained below, the numeric value here matters, and denotes the audio API to be used: =1 means DirectSound).

Building with ASIO support

ASIO is a driver that allows for recording from devices with up to four input channels. It also can provide lower latency than other audio drivers. To compile with ASIO support, visit and download the ASIO SDK. Extract the downloaded SDK zip file to src/extlib/portaudio and rename it asio. Enable the AudioExtension in CMake and click "Configure". Make sure the "Advanced" option is checked in the CMake GUI and enable PORTAUDIO_ENABLE_ASIO. Click "Generate" and recompile BCI2000. ASIO will now appear as an option under the EnableAudioExtension parameter when BCI2000 is run with the AudioExtension enabled.

Block Diagram



The AudioExtension is configured in the Source tab within the AudioExtension section. The configurable parameters are:


Enables/Disables the AudioExtension. This parameter performs double-duty as an audio host API selector. The following values of this parameter are valid. NOTE: Not all audio APIs are available on all platforms.

    • [0] - Disabled
    • [1] - DirectSound
    • [2] - MME
    • [3] - ASIO
    • [4] - SoundManager
    • [5] - CoreAudio
    • [6] - Disabled
    • [7] - OSS
    • [8] - ALSA
    • [9] - AL
    • [10] - BeOs
    • [11] - WDMKS
    • [12] - JACK
    • [13] - WASAPI
    • [14] - AudioScienceHPI


The Audio Mixer is represented as an N x N Matrix, where N is the number of output channels on the selected device.

If the input device has 2 inputs and 2 outputs, the user must open the AudioMixer and set the matrix size to 2 x 2. To specify which input will be mapped to a specific output you place a 1 at the intersection of the row (input) and column (output).

For the simplest configuration set the number of inputs and outputs and place a 1 in a diagonal line from the top left hand corner to the bottom right hand corner.

   row:1, column:1; row:2, column:2; row:3, column:3; ... , row:(N-1), column:(N-1); row:N, column:N;

By Default the Matrix will have numeric values for all the labels. To specify a different label, double click on the label and type the specified input type.

Below are a list of valid input labels:

  • X - This is automatically interpreted as INPUT[X], where x is the input channel on the device.
  • INPUT[X] - This input will come from channel X on the sound capturing device.
  • FILE[X] - This input will come from channel X in the specified AudioInputFile listed in the Source Tab of BCI2000 Config.
  • TONE[X] - This input will be a synthesized sine wave with the frequency of X Hz.
  • NOISE[X] - This input will be generated white noise at X Hz. NOTE: NOISE[] is white noise at the audio sampling rate (which defaults to 44100)


Requires a number, which corresponds to an input device ID. Each Audio Recording Device connected to the computer has an associated number. To select a specific device, enter the number into the corresponding box. To view a list of detected Audio Input Devices in BCI2000 click on Set Config and the devices will be listed below 'Audio Extension Enabled' in the operator log.


             Audio Input Device ID [ i ] : [Name of Audio Device] supports  N Input Channels

Where i is a number that corresponds to the Name of the Audio Device. A value of -1 selects the default input device on this host API.

Where N is the number of input channels that can capture audio. This is also used as the number to set up the AudioMixer during configuration.


Requires a number, which corresponds to an output device ID. Each Audio Playback Device connected to the computer has an associated number. To select a specific device, enter the number into the corresponding box. To view a list of detected Audio Input Devices in BCI2000 click on Set Config and the devices will be listed below 'Audio Extension Enabled' in the operator log.


             Audio Output Device ID [ X ] : [Name of Audio Device]

Where X is a number that corresponds to the Name of the Audio Device. A value of -1 selects the default output device on this host API.


Audio file to use as audio input to AudioMixer. The selected file can have any non-zero number of channels and be encoded in almost any format (except MP3), but MUST be encoded at 44100 Hz.


Enables/Disables recording of audio data to a file in the DataDirectory.


Enables/Disables recording of audio data to a file in the DataDirectory.


Changes the file format and encoding options of the recorded output files. This parameter has the following three options:

  • Raw - Records to 16 bit Microsoft formatted WAV files with no compression. These files open directly in MATLAB if that's interesting to you.
  • Lossless - Records to FLAC formatted files. These files are slightly smaller than RAW files, but have no quality loss.
  • Lossy - Records to Ogg Vorbis files. These files are similar to MP3 but do not have the associated licensing issues. They are compressed using a lossy algorithm, so the resulting files are very small but sound slightly worse than lossless encoding. This format is good for long recordings where perfect quality is not necessary.

AudioInputFilterbank, AudioOutputFilterbank

A filterbank which filters audio input and output before rectification/smoothing for envelope extraction. These butterworth filters will not be applied to the audible signal. The format of the filter bank is as follows:

  • Type - The characteristic of the filter. The following values are valid.
    • Lowpass - Creates a low pass filter
    • Highpass - Creates a high pass filter
    • Bandpass - Creates a band pass filter *See Known Issues*
    • Bandstop - Creates a band stop, or notch filter
  • Order - The order of the filter model. Higher order filters are more accurate but more expensive computationally.
  • Cutoff1 - The cutoff frequency for Lowpass and Highpass filters, and the cut-on frequency for Bandpass and Bandstop filters.
  • Cutoff2 - The cut-off frequency for Bandpass and Bandstop filters.

The matrix can have as many rows as necessary to filter the signal. Filters can be applied in any order and their transfer functions are multiplied before filtering occurs.


The cutoff frequency for the low pass filter which is applied to the filtered and full-wave rectified audio data. This should be set to the highest frequency you want to see in the resulting audio envelope.

State Variables

The AudioExtension outputs the following state variables:


These are the envelope values of each channel (up to channel 4) of the audio inputs and outputs (in the AudioMixer matrix). These 16 bit unsigned values correspond to the resulting envelope after the audio envelope extraction. For architectural reasons, it is not possible to publish states after system startup, so you are limited to four channels of input and output. The AudioExtension can be easily modified to change the number of channels by editing the #define NUM_INPUT_ENVELOPES 4 and #define NUM_OUTPUT_ENVELOPES lines in AudioExtension.cpp, and recompiling your source module.


This 32 bit unsigned number corresponds to the current frame of audio data in the recorded output files. This can be used to resynchronize the lossless audio to the resulting .dat file offline. Audio is sampled internally at 44100 Hz, so this number will roll over once every 27 hours or so.

See also

User Reference:Logging Input, Contributions:Extensions