FAQ: How can I take advantage of the timing offset in sending messages?

[SR-SUPPORT]

Some stimulus presentation software (e.g., E-Prime) cannot send a message to the EDF file at the precise moment a stimulus appears. Instead, the message is sent retrospectively after the event has occurred, creating a timestamp mismatch.

Offset Message Format
Data Viewer can correct this mismatch by interpreting a specially formatted offset message. If a message begins with an integer, Data Viewer treats that integer as a temporal offset (in milliseconds) representing the delay between the event and the message command. The syntax of message events are as follows:
MSG [Time_Message_Was_Sent] [Offset] [Message_Text]

When Data Viewer reads this and detects an offset is present, it calculates the true event time using the formula: True_Event_Time = Time_Message_Was_Sent - Offset.

Step-by-Step Example (Retrospective Timing)
Consider a scenario where a stimulus appears at 500 ms, but the software can only send the message at the end of the trial, at 1250 ms.

• Get the known event time:
  
  Code:

  imageOnset = 500
• Get the current time (when you're sending the message):
  
  Code:

  timeEnd = 1250
• Calculate the offset (e.g. 1250 - 500 = 750):
  
  Code:

  offset = timeEnd - imageOnset
• Send the formatted message:
  
  Code:

  tracker.sendMessage "750 imageOnsetTime"
  Note: The syntax above is an example (similar to E-Prime). The exact implementation depends on your programming language.
  

Data Viewer Interpretation:

• Data Viewer reads the message timestamp (1250).
  
• It detects the leading integer (750) as the offset.
  
• It performs the subtraction: $1250 - 750 = 500.
  
• The imageOnsetTime marker is placed at 500 ms in the viewer.
  

Note: You can also use a negative offset to mark an event that will happen in the future relative to when the message is sent. The logic remains the same:
Time - (-Offset) = Time + Offset