sending TTL code while playing a video file

Hello,

We need to time-lock varying points in our stimuli to external hardware (by sending a TTL code). The stimuli are videos about 2000ms in length. We want to record ERP data with time zero at about 500ms into the video. Problem: sending the code (which is essentially like playing another stimulus) causes the video to stop at that point. How do we send the code at, say, 500ms after stimulus onset time without interrupting the video?

Many thanks, motomibo

Would it be ok to send the signal in synch with the onset of the video (time 0) and account for that in your data analysis?