Using External timebase for synchronization to a periodic TTL signal

The synchronization input of Qualisys camera systems can be used to synchronize the frames of the camera capture to an external source. This means that the camera frequency will depend on the input signal so that there is no drift between the two systems.

When synchronizing a Qualisys system to a period signal, set the Signal Mode under the External timebase options to Periodic. The synchronization signal from the external hardware must be a periodic TTL signal between 0 and 5 Volt. You can use a frequency multiplier or divisor to get the camera frequency that you need. For detailed information about the External timebase settings, see chapter External timebase.

The external timebase is connected to the Sync input port of a Camera Sync Unit. For an Oqus system, you can also use the Sync in connector of a Trigger/Sync splitter cable connected to the control port of one of the cameras.

Follow these instructions to use external timebase with a periodic signal.

Real-time

  1. Make sure that the settings for the External timebase on the Synchronization page in the Project options dialog are correct for your setup.

    Do not use External timebase settings that result in a camera frequency higher than 0.975*maximum frequency at the full image size.

  2. First time the periodic sync is activated you must wait for the camera to synchronize after clicking New on the File menu. QTM will report EXT ? Hz in the status bar for the camera frequency and the dialog below will appear. The following times a measurement is started the camera system will already be locked on and the real-time will start immediately.

  3. When the camera is synchronized the current camera frequency will be reported in the right corner of the status bar. The frequency will also be fixed for the camera system in the Camera settings sidebar and in the Project options dialog, so that the exposure time settings are limited to the correct values.
  4. The synchronized samples can now be sent with the RT protocol to another collection system.

Capture

The start of the capture needs to be known if there is an external system that collects data independently of QTM. There are two ways to know the time of the start.

  1. Record the Sync out signal from the camera on the external system. If you use external trigger on both of the systems then the start of the capture is synchronized with the first pulse in the pulse train after the external trigger signal is sent. Otherwise if the camera system is started without a trigger the start of the capture is after a very short pause in the pulse train between when you click on Start in the Start capture dialog and when the camera starts capturing.
  2. Start the camera system and the external system with the external trigger signal. Then the start of the camera system is in the default mode delayed 20 ms from the trigger pulse, which means that the camera system starts on following Sync in pulse. 20 ms is the minimum delay but the delay can be set to a higher number on the Synchronization page.