User manual

126 Appendix A: Using the EyeLink 1000 Analog and Digital Output Card

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.

7.6.1 Strobe Data Input

The best time base method is to use the EyeLink 1000 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

active-low, 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.

7.6.2 Oversampling and Toggle Strobe

Another possibility is to over sample the analog output, by recording the analog

outputs at more than twice the EyeLink 1000 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 drive a gaze-

contingent display, as the time of each sample is unimportant and over

sampling will minimizes total data delay.