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Support   XID    Filters and Signal Flow

Introduction

The Cedrus RB-x40 response pads, StimTracker Duo/Quad, and Lumina controller are built on the XID 2 platform, a combination of tightly knit hardware and firmware. XID 2 includes features to control the signal flow and how it’s delivered via USB or m-pod. Here’s a summary, followed by more information and examples:

Most of these features can be set interactively using the Xidon 2 application. See Default Signal Flow Options section below for default values.

A powerful aspect of the signal flow features in XID 2 is that they are applied to each signal individually. For example, you can apply a hold-on filter to a specific light sensor while applying a single shot option to an fMRI scanner trigger.

The four diagrams below help visualize the signal flow:

How Does It All Work?

The following diagrams illustrate how the different options affect the event markers that are produced. They represent options used one at a time, e.g. the single shot diagram illustrates when that feature is used by itself. You can obtain more powerful signal flow control by combining the hold-on filter, hold-off filter, and single shot.

45ms Hold-On Filter

45ms Hold-Off Filter

45ms Single Shot

Some observations:


Applications

Here are some typical use examples.

LCD Monitors

The backlighting and brightness of LCD monitors is usually controlled by an LED bar. Varying the brightness is commonly done using a method called pulse width modulation (PWM). Changing the duration of the pulses causes the average voltage that’s supplied to the LED bar to change, and with it, the brightness of the monitor. The human eye will only notice a change in brightness, but the light sensors may pick up the individual pulses.

Older LCD monitors used a different backlighting method called CCFL, which is a fluorescent light. This also causes the light sensors to pick up flickering, at the rate of 50Hz or 60Hz.

Another situation is where researchers still use CRT monitors because they find them more predictable tools than LCD. On a 60Hz CRT, a light sensor will generate sixty pulses per second – too many.

Whether it’s the phosphor of a CRT or the backlighting of an LCD, all monitors rely on persistence of human vision to some extent. The optical inputs in XID 2 devices don't have any persistence. Applying a hold-on or hold-off filter helps distill the information provided by the light sensor.

Human Speech

The microphone input is heavily filtered towards voice input and the output predominantly maps to complete words (male or female) when tested with Harvard Sentences and no digital filtering.

Another situation is when you are playing a recording from your computer; human speech contains frequent pauses. Adding a hold-on or hold-off filter allows you to ignore these pauses.

fMRI Scanner Triggers (Lumina controller only)

MRI scanners can send a trigger pulse every TR to indicate the start of a scan, but some are programmed to send additional pulses to indicate the start of each slice within the TR period. Stimulus presentation software like SuperLab will only want a pulse every TR. Applying a hold-off filter or single shot can filter out the slice triggers.

External Input (StimTracker Quad only)

StimTracker Quad provides an option for connecting external switches. When a mechanical switch is pressed, its contacts will “bounce” a few times, causing several signals to be produced. The size and quality of the switch determines how long it takes for the bouncing to die down. You can “debounce” the switches using a hold-on filter.

Debouncing is needed only when connecting switches directly to StimTracker Quad. When connecting an RB-x40 response pad (or Lumina controller), debouncing is already done inside that device.

More Advanced Options

There are features that are meant to be used by a stimulus presentation program and cannot be set in Xidon 2. One of them is the digital switch that can turn output off or on to m-pod (and others outputs on StimTracker Quad); e.g. it can be used to turn off output to EEG after a participant’s vocal response and until the onset of the next trial. This would avoid having the recorders capture erroneous data such as a participant speaking in between trials.

The single shot feature also has a mode that is meant to be used only by a stimulus presentation program. Instead of a time period, a program can exercise more granular control by sending a command to the XID device to “arm it”. The XID device will then produce a single output on the onset of the next signal, but will not produce any more outputs until it is armed again.

Default Signal Flow Options

The following table summarizes the default hold-on and hold-off filter values:

_

Device

StimTracker

Quad

StimTracker Duo

_RB-x40

Response Pads


Lumina 3G

Audio

hold-on filter

4 ms

4 ms

hold-off filter

4 ms

4 ms

Light Sensors

hold-on filter

5 ms

5 ms

5 ms

5 ms

hold-off filter

5 ms

5 ms

5 ms

5 ms

Scanner Trigger

hold-on filter

5 ms

hold-off filter

0 ms

Keys

hold-on filter

0 ms

0 ms

10 ms

10 ms

hold-off filter

0 ms

0 ms

10 ms

10 ms

Microphone

hold-on filter

4 ms

hold-off filter

4 ms

Some notes:

Last Revision: July 30, 2018

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