Signal Input Range and Raw Data Files in Cheetah Software
The Neuralynx Raw Data (.NRD) files available in our Cheetah software offer several benefits over our “legacy” Neuralynx Continuously Sampled (.NCS) channel and Spike file (.nse, .nst, .ntt) formats because the entire input signal from all channels is saved to a single file. Legacy files may show “clipped” data while the .NRD files contain raw A/D values void of clipping.
Cheetah has legacy file formats as the default file types for backward
compatibility with existing data analysis software. The .NRD file may be written simultaneously with the standard legacy files during an experiment.
- a single data file for all channels
- storage of all channels at the same base sampling rate
- storage of all analog data as full 24-bit, full bandwidth,
unfiltered signal (pre-DSP processed by Cheetah) as acquired
by the Input Board
- A/D values stored as sign-extended 32-bit integers
for easy access and processing
- 64-bit microsecond timestamp and digital I/O port values for the sample period contained in each “A/D scan record”
The .NRD record format is very close to the packets transferred on the Fiber Optic cable from the Digital Lynx interface. Also, .NRD files serve
as a “backup” of the acquired signals since they may be re-processed with Cheetah (and other utilities in the future) if problems are found in the collected legacy files.
24-Bit Data Acquisition
The Digital Lynx Input Boards have a very large +/-132 millivolt fixed signal input range and bandwidth of DC to 7 kHz. This allows accurate signal recording in the presence of large artifacts and electrode offset voltages to ensure that inputs are rarely “clipped” (exceeding this large
linear input range).
The Hybrid Input Boards are DC coupled with a gain reduced “pre-emphasis” applied to the very low frequency range of DC to 0.1 Hz to
prevent large electrode offsets (up to +/- 1v) from saturating the inputs.
The older AC Input boards are capacitively coupled at 0.1Hz; therefore, DC offset voltages are not a problem. Both Input Boards’ 24-bit A/D converters have a “perfect” linear range of 19-bits, providing sub-microvolt resolution over the entire input range
Cheetah Legacy File Processing
The 24-bit A/D values will not fit into the legacy 16-bit data sizes. To support these legacy formats, Cheetah software scales the digitized signals to the user-specified Input Range (usually in the millivolt range) - a concept similar to the retired Lynx-8 programmable gain analog amplifier. Any signal exceeding the specified Input Range will be clipped in data files, even though the 24-bit A/D values are not clipped. This is usually not a problem since Low Cut DSP Filters in Cheetah will remove any DC offsets, but large artifacts will still cause clipping in the 16-bit legacy files.
Further .NRD Benefits
These .NRD files offer several key benefits over the Continuously Sampled files (.NCS) because all of the A/D sampled data is saved - at the full
24-bit resolution, full bandwidth (pre-filter signal processing) and full sampling rate. This allows further processing of frequency bands originally filtered out by Cheetah and written to the legacy files. For example, if an EEG signal is filtered from 1 to 50 Hz, the .NRD file can be accessed later to view the 40 to 200 Hz band for HFOs. The same is true for Spike channels which are filtered from 600 to 6kHz and thresholded.
.NRD files may be viewed in NeuraView 2.0; replayed through Cheetah (with adjusted filter and/or input ranges and spike thresholds, new .NCS and spike files written); and read with the MATLAB® import/export libraries.
The .NRD file format specifications are posted on our website:
We encourage you to try the Neuralynx Raw Data (.NRD) file format and evaluate the .NRD files for your use. If the .NRD file is not needed after a recording session, it may be deleted. Recording both .NRD and legacy files during an experiment usually won’t burden current computers and disk drives.
MATLAB® is a registered trademark of The MathWorks, Inc.