Configuring the TRACKPixx3 Analog Output

The TRACKPixx3 can stream up to four analog output channels directly to your neurophysiological recording system via the Analog I/O port on the back of the DATAPixx3 control unit. The advantages of streaming analog eye signals are:

Extremely short latencies between eye movements and streamed data (~1.7 ms)
Permits a single recording interface for neurophysiological data, gaze data and digital triggers
Can be used in conjunction with a conventional TRACKPixx3 schedule for more comprehensive offline analysis, if needed

This short guide shows how to configure TRACKPixx3 analog streaming output.

The analog I/O interface

In a conventional TRACKPixx3 setup, the TRACKPixx3 is connected to the DATAPixx3 control unit via a power cable and fibre optic link.

*Tabletop TRACKPixx3 installation example*

The DATAPixx3 control unit has several ports on the back, including a DB-25 analog I/O interface, shown in the image below. This port can be connected to your neurophysiological recording system and used to stream analog eye-tracking data.

*Zoomed-in view of the analog I/O interface on the DATAPixx3*

If you require a custom cable or a breakout box to connect this port to your recording system, please see our cabling solutions product page or contact our support team. Do NOT connect third-party cabling to our hardware without being certain of the cable wiring; this could irreparably damage your hardware.

There are four digital-to-analog converter (DAC) output channels on the analog I/O connector. The pin assignment is as follows:

Channel	Pin
GND	10
DAC0	11
DAC1	24
DAC2	12
DAC3	25
GND	13

These channels each have a range of ±10 V and 16-bit resolution. The maximum playback rate is 1 MSPS per channel. For full port specifications, see our DATAPixx3 product page.

The streaming capacities listed above are based on the DATAPixx3 output. Your analog receiver will have its sampling rate and resolution, which may be lower. If this is the case, you may notice some loss of precision (e.g., if your receiver only supports 8-bit resolution). Be sure to investigate the capabilities of your receiver and interpret your streamed data accordingly.

Channel options and how to interpret them

There are 4 channels dedicated to streaming analog data from the TRACKPixx3. Each channel can be independently configured to stream unique data. There are 16 channel configurations to choose from, which are described below.

If you have a DATAPixx3 purchased after September 2023 or an older DATAPixx3 Full, you can also configure one or more DAC channels for a custom-defined analog output schedule (e.g. an audio waveform). See this guide for more details.

Option number	Name	Description	Conversion
0	Default schedule-controlled analog	Default method of controlling analog outputs	N/A
1	Left eye screen X	Calibrated left eye x position in screen pixels, where (0,0) is the center of the screen	GazeX = Voltage × 819.670 pixels/V
2	Right eye screen X	Calibrated right eye x position in screen pixels, where (0,0) is the center of the screen	GazeX = Voltage × 819.670 pixels/V
3	Left eye screen Y	Calibrated left eye y position in screen pixels, where (0,0) is the center of the screen	GazeY = Voltage × 409.575 pixels/V
4	Right eye screen Y	Calibrated right eye y position in screen pixels, where (0,0) is the center of the screen	GazeY = Voltage × 409.575 pixels/V
5	Left eye pupil diameter	Left eye pupil diameter in camera pixels	PupilSize = Voltage × 51.190 pixels/V
6	Right eye pupil diameter	Right eye pupil diameter in camera pixels	PupilSize = Voltage × 51.190 pixels/V
7	Average eye screen X	Average calibrated x position of the left and right eyes in screen pixels, where (0,0) is the center of the screen. Note that if one eye fails to track, the data will be equal to the eye that is successfully tracked. If both eyes fail to track, the output will be -5.00 V.	GazeX = Voltage × 819.670 pixels/V
8	Average eye screen Y	Average calibrated y position of the left and right eyes in screen pixels, where (0,0) is the center of the screen. Note that if one eye fails to track, the data will be equal to the eye that is successfully tracked. If both eyes fail to track, the output will be -5.00 V.	GazeY = Voltage × 409.575 pixels/
9	Average eye pupil diameter	Average left and right eye pupil diameter in camera pixels. Note that if one eye fails to track, the data will be equal to the eye that is successfully tracked. If both eyes fail to track, the output will be -5.00 V.	PupilSize = Voltage × 51.190 pixels/V
10	Blink detection flag	Whether the blink detection flag is raised	-2.00 V = Left eye blink 0.00 V = Both eyes open 2.00 V = Right eye blink 4.00 V = Both eyes blink
11	Left eye raw X	The value of the X component of the vector between the center of the pupil and the corneal reflection for the left eye in camera pixel space. Used for custom image processing.	Raw = Voltage × 12.799 pixels/V
12	Right eye raw X	The value of the X component of the vector between the center of the pupil and the corneal reflection for the right eye in camera pixel space. Used for custom image processing.	Raw = Voltage × 12.799 pixels/V
13	Left eye raw Y	The value of the Y component of the vector between the center of the pupil and the corneal reflection for the left eye in camera pixel space. Used for custom image processing.	Raw = Voltage × 12.799 pixels/V
14	Right eye raw X	The value of the Y component of the vector between the center of the pupil and the corneal reflection for the right eye in camera pixel space. Used for custom image processing.	Raw = Voltage × 12.799 pixels/V
15	Left eye saccade & fixation flag	Status of the saccade and fixation flags for the left eye	-2.00 V = Saccade 0.00 V = No flag high 2.00 V = Fixation
16	Right eye saccade & fixation flag	Status of the saccade and fixation flags for the right eye	-2.00 V = Saccade 0.00 V = No flag high 2.00 V = Fixation

Configuring your analog output channels

Click on the section below to see how to configure TRACKPixx3 analog output using LabMaestro and our API tools.

In LabMaestro

Right-click on the DATAPixx3 in your project panel and select ‘Add to project.' Next, double-click on the DATAPixx3. This will open a device settings interface in the central Timeline Workspace. Scroll down to the Analog IO section and select the channel options you want to implement in your project. Select ‘Apply All’ to apply these settings to your device.

*Analog I/O configuration in the Device Settings for the DATAPixx3*

When you launch an experiment, LabMaestro will verify your TRACKPixx3’s current settings match those specified in your experiment’s Project Requirements. If there is a mismatch, you will receive a warning and be invited to update your TRACKPixx3 settings.

In MATLAB

Use the following commands:

MATLAB

%Connect to TRACKPixx3
Datapixx('Open');
Datapixx('SetTPxAwake');

%Configure channel output. Use the option numbers in the table above as arguments
%For example, to record left x, right x, left y and right y:
DAC0 = 1
DAC1 = 2
DAC2 = 3
DAC3 = 4
Datapixx('EnableTPxAnalogOut' , DAC0, DAC1, DAC2, DAC3);

%Push settings to device
Datapixx('RegWrRd');

%To disable and disconnect:
Datapixx('DisableTPxAnalogOut');
Datapixx('SetTPxSleep');
Datapixx('RegWrRd');
Datapixx('Close');

In Python

PY

import pypixxlib._libdpx as dp

#Connect to TRACKPixx3
dp.DPxOpen()
dp.DPxSetTPxAwake()

#Configure channel output. Use the option numbers in the table above as arguments
#For example, to record left x, right x, left y and right y:
dp.EnableTPxAnalogOut(modeDAC0=1, modeDAC1=2, modeDAC2=3, modeDAC3 = 4)

#Push settings to device
dp.DPxWriteRegCache()
    
#To disable and disconnect:
dp.DPxSetTPxSleep()
dp.EnableTPxAnalogOut() #resets default of all channels option 0
dp.DPxUpdateRegCache()
dp.DPxClose()

Summary

The TRACKPixx3 can stream four independent channels of eye-tracking-related data to your neurophysiological recording system via the analog I/O interface on the back of the DATAPixx3. Custom cabling or a breakout box is generally required. Channels can be configured and support 16 unique options, including custom analog waveform schedules. These data can then be converted back into their relevant coordinate systems using the conversion formulae provided in this guide.

Note that for testing purposes, ADC loopback (e.g., as shown here) cannot record DAC streamed output from the TRACKPixx3. You must use a physical hardware loopback cable, an oscilloscope or your neurophysiological recording system to measure streamed TRACKPixx3 output.