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continuous output differs from most eye-tracking systems with analog outputs,
which either output continuous analog data (such as limbus-tracking systems)
or output samples at a lower rate, such as 50/60 Hz video-based tracking
systems. This causes the EyeLink II analog output to rapidly step between data
values, which means that sampling at fixed intervals makes it likely that
samples might be missed, sampled twice, or the transition between samples
might be recorded instead. Since the EyeLink II tracker and most dataacquisition systems rely on interrupt-driven software sampling and output, it is
possible that time base jitter could result in missed samples, or repeated
recording of a single eye-position sample. This would appear as a "step" artifact
in rapidly-changing eye-position data, such as saccades or pursuit.
8.5.1 Strobe Data Input
The best time base method is to use the EyeLink II analog output strobe, which
is assigned to digital output D7 on the analog card connection board. This
signal can be configured to be a short or long trigger pulse, which can be used
to trigger hardware data acquisition on analog input devices equipped for this,
or to trigger interrupt-driven acquisition. The characteristics of this strobe
pulse may be set in the analog.ini file, with the strobe being active-high or activelow, and with duration between 5 and 2000 microseconds.
The onset of the strobe is also delayed from the time that analog outputs
change, in order to allow outputs to settle to the new voltages. A delay of 400
microseconds is standard, allowing the use of signal-conditioning low pass
filters as discussed earlier.
8.5.2 Over sampling and Toggle Strobe
Another possibility is to over sample the analog output, by recording the analog
outputs at more than twice the EyeLink II sample rate. This will prevent missed
samples, but will still result in steps in the data. Recording the digital strobe
output (on an analog or digital input channel) in combination with the analog
data allows the first data from each sample to be selected, by detecting the
change in value of this output. By setting the duration of the strobe pulse to 0,
the strobe output can be set to toggle between high (4 to 5 volts) and low (0 to 1
volt) for every sample, which produces the best signal. Over sampling can also
be used without the strobe when the analog data is being used to a drive gazecontingent display, as the time of each sample is unimportant and over
sampling will minimizes total data delay.
EyeLink II User Manual version 2.12 (07/02/2006) © 2002-2006 SR Research Ltd. 101