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BioSemi ActiveTwo MkII (32-channel EEG System)

The BioSemi ActiveTwo MkII provides high-resolution, low-noise recordings using gel-based active electrodes, designed for stationary use in controlled laboratory environments. This configuration is not portable: it requires careful setup with conductive gel, dedicated workspace, and proper electrode preparation before each session.

BioSemi ActiveTwo MkII components

Images © NEUROSPEC AG. Used for documentation and identification purposes.

The system converts analog EEG signals into 24-bit digital data through a shielded A/D (analog-to-digital) amplifier box. The data are transmitted optically to the recording computer via a fiber-optic USB interface.

The 32-channel setup follows the 10–20 system layout.

For complete specifications, see the ActiveTwo Operating Guidelines (PDF) — pp. 9–23.

System Components

Head Cap and Active Electrodes

The system uses pin-type, gel-based active electrodes arranged in a 32-channel head cap following the international 10–20 layout. Each electrode contains an integrated amplifier to minimize cable and motion artifacts.

Key details:

  • Requires electrolyte gel (e.g., SignaGel or Lectron II) applied directly into each electrode holder.
  • A snug head cap fit ensures stable contact and low noise.
  • Electrodes connect through a 68-pin D-sub ribbon cable labeled A1–A32.
  • The CMS (Common Mode Sense) and DRL (Driven Right Leg) electrodes form the system’s reference and must always be applied to the participant.

This setup provides very high signal fidelity but requires preparation and cleanup after each session.
See application steps in the manual.

A/D Box (Amplifier Unit)

The Mark II A/D box digitizes and transmits all EEG signals.

Features specific to the 32-channel setup: - 32 active EEG inputs via the A1–A32 connector.
- 24-bit resolution per channel with high input impedance.
- LED indicators: - Power (green): system on
- CM in Range (blue): CMS/DRL connected and stable
- Low Battery (red): recharge required
- Speed-mode dial sets the sampling rate (typically 512–2048 Hz for EEG).
- Auxiliary inputs allow up to two additional bipolar sensors.

See hardware layout in the manual.

Power Supply and Battery

This system is powered by rechargeable lead-acid batteries.
While it can technically operate on battery power, the setup is intended for stationary use in the lab.

  • Typical battery life (32 channels): ≈ 15 hours per charge
  • Recharge only with the supplied BioSemi charger
  • Replace or recharge when the Low Battery LED appears or as indicated in ActiView

For maintenance procedures, see pp. 69–70.

Signal transmission between the amplifier and the optical receiver uses a multimode (62.5/125 µm) ST–ST fiber-optic cable.
- Provides complete electrical isolation between participant and computer
- Immune to electromagnetic interference
- Standard cable length: 3 m (up to 50 m optional)
- The cable is delicate — avoid bending or crushing it

See p. 18 of the manual.

Optical Receiver / USB 2.0 Interface

The optical receiver connects the amplifier to the recording computer via USB 2.0.

  • Powered directly by the USB port (no AC supply required)
  • Galvanically isolated trigger input/output
  • 37-pin digital I/O port for event triggers and synchronization
  • Front LEDs indicate data input (from amplifier) and data output (to PC)

More information: pp. 19–20.

ActiView Software

ActiView is the open-source acquisition software for ActiveTwo, written in LabVIEW.
It allows real-time signal visualization, electrode offset checks, and recording control.

Main functions: - Real-time EEG and auxiliary channel display
- Channel configuration through .CFG files
- Data saved in BioSemi Data Format (BDF) — compatible with EEGLAB, BESA, and BIOSIG
- Displays battery level, offsets, and trigger status
- Typical offset range for good contact: < ±25 mV

See installation and use instructions in pp. 23–46.

Data and File Format

Recordings are stored as .BDF files (BioSemi Data Format), a 24-bit EDF extension supporting time-locked triggers and full-resolution EEG data.
Compatible with major EEG analysis toolboxes such as EEGLAB, BESA, and BIOSIG.
See pp. 60–61.

Key Facts for This Configuration

  • 32-channel active EEG with gel-based electrodes
  • Not portable – designed for fixed laboratory installations
  • 24-bit precision, high input impedance, and good noise performance
  • Full galvanic isolation for participant safety
  • Battery-powered amplifier, USB-powered receiver
  • Open-source acquisition software (ActiView)
  • Compatible with most EEG analysis platforms

Video Tutorials

Biosemi ActiveTwo - Setup, Recording EEG, and Cleanup

How to clean the EEG Electrodes and Caps?

Further Reading

For in-depth operation, maintenance, and troubleshooting, see the
BioSemi ActiveTwo Operating Guidelines (PDF):

  • Components and Setup: pp. 9–23
  • Electrode Application: pp. 27–35
  • ActiView Software: pp. 23–46
  • Battery Maintenance: pp. 69–70
  • Troubleshooting: pp. 71–76

(Source: BioSemi ActiveTwo Operating Guidelines, Rev. 1, 2016)

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