142
Appendix A: Using the EyeLink 1000 Plus Analog and Digital Output
Card
©
2013-2017 SR Research Ltd.
7.4.3 Scaling of Analog Position Data
Each of the types of position data is scaled to match the selected analog output
voltage range. Several variables in
ANALOG.INI
set what proportion of the
expected data range for each type will be represented at the output, and what
the total voltage range will be.
•
Total analog voltage range is set by
analog_dac_range
, followed by the
highest and lowest voltage required. The voltage range may be from -10 to
+10 volts, with other typical ranges being -5 to +5, or 0 to +10 volts.
•
The fraction of the total data range to be covered is set by the
analog_x_range
and
analog_y_range
variables. These are followed by the
data type, and the minimum and maximum range fraction. For example, 0
to 1.0 would cover the full range of the data, 0.1 to 0.9 would cover the
central 80% of the data, and -0.2 to 1.2 would add a 20% margin above and
below the expected data range. The polarity of the output signals can be
inversed by exchanging the lower end and higher end of the
analog_x_range
or
analog_y_range
data range.
•
For raw data, the default range is 0.1 to 0.9, because the pupil position will
never reach the edges of the camera image. It is possible that the scaled and
transformed pupil-CR data might exceed this range, but in general this
range will be similar to that of the camera image. Raw data should be
assumed to be in arbitrary units.
•
For HREF data, the entire data range is assumed to be -30000 to +30000.
This is about 127°. This should never be exceeded. The default range setting
is therefore 0.0 to 1.0. The HREF data may be recovered from the voltage by
the following formula:
HREF =(voltage-(minmaxvoltage)/2)*60000/(maxvoltage-minvoltage)
•
For gaze position data, the data range is scaled to the display coordinates,
which are 1024 by 768 at startup, and may be changed via link commands.
The data range setting is -0.2 to 1.2, allowing 20% extra range for fixations
that map to outside the display. This extra data range allows for
identification of fixations outside the display. Scaling to recover gaze
position data is more complex, as the numerical value is partially dependent
on the display coordinates. The following formulas do the conversion in
several stages, with R being the voltage range proportion, and S being the
proportion of screen width or height.
R = (voltage-minvoltage)/(maxvoltage-minvoltage)