Knowledge Base

Determining The Difference In Offset

After you have imported all data (such as physiological data or a video file) into the corresponding observation in The Observer XT, you can determine the difference in offset by carrying out manual synchronization.

When You Used One Trigger Pulse

Through manual synchronization you make sure that the time-stamped trigger comment logged in the event log file in The Observer XT coincides with the corresponding signal recorded in a video file, an audio file, or a physiological data file. After you have carried out manual synchronization, you can see the difference in offset in the File Synchronization window in The Observer XT. Next time you carry out an observation, you need to enter this difference in offset as the delay for starting up an observation for this specific application in the Add External Programs window in The Observer XT. As a result, the start of the observation and the start of the external application are now synchronous.

When You Used A Series Of Trigger Pulses

When you used a series of trigger pulses in the case of a long observation, you first determine the delay in starting up the external application by using a pulse at the start of the observation. Next, you use the pulse at the end of the observation to determine how far the observational and external data have started running out of sync. Upon re-import of the external data you can stretch the external data to match the duration of the observation by changing the end time of the external data file.

Determining The Difference In Offset Between An Event Log File And A Video File

The procedure below assumes that the video file has already been imported into the corresponding observation.

1. Open the observation.
2. In the Event log, click on the row with a trigger comment.
3. Click the Offset button on the toolbar.
4. Select Manual offset, read the instructions in the Synchronize window, and click the Start Synchronization button.
5. Click in the Video window. The video window turns red.
6. Use the Playback Control to go forward or backward to find the onset of the corresponding visual or auditory signal in the video, or the keycode signal recorded in the screen capture. Note: If you have The Observer XT 11.5 or higher and MediaRecorder 2.5 or higher and you start video recording with commands from The Observer XT, you do not have to determine the offset between the observation and the video. The Observer XT does that for you and synchronizes event log data and video automatically.
7. Click the Offset button again to confirm the offset.
8. To view the offset, click the Offset button again and select Numerical Offset. The File Synchronization window displays the offset for the video file relative to the event log file.
9. In the Project Setup, click Devices, then locate the video recording program item. Enter the offset value (converted to ms) in the fields ms before Start Observation and ms before Stop Observation in the Add External Program window.

Determining The Difference In Offset Between An Event Log File And A Physiological Data File

1. Open the observation containing the physiological data.
2. In the Event log, click on the row with the trigger comment.
3. Click the Offset button on the tool bar.
4. Select Manual offset, read the instructions in the Synchronize window, and click the Start Synchronization button.
5. Click in the External Data and Audio window that displays the physiological data. The border of the External Data and Audio window turns red.
6. In the External Data and Audio window, click on the graph and drag the graph so the peak created by the TTL pulse coincides with the vertical hairline, which indicates the corresponding position of the trigger comment in the Event Log file.
7. Click the Offset button again to confirm the offset. The event log and the external data file are now synchronized.
8. To view the offset, click the Offset button again and select Numerical Offset. The File Synchronization window displays the difference in offset for the file.
9. Do one of the following:
   • When you used one trigger pulse — This is the value you need to enter in the fields ms before Start Observation and ms before Stop Observation in the Add External Program window next time you carry out an observation.
   • When you used a series of trigger pulses — After you have determined the offset difference at the start and end of the observation, you need to re-import the physiological data file and, upon import, change the Start and Stop time of the physiological data set. In the Import External Data window you can change the Start and Stop time by double-clicking the corresponding cell.

Source: The Observer XT 17 - SyncBox Technical Note, Noldus Information Technology