Hi Yoon,
If there are no other dynamic stimuli onscreen, there should be about one #stimDisplayUpdate event for each frame in the video. If your 400ms video runs at 60 frames per second, then 23 events sounds about right.
The one piece of data that varies across all the similar-looking announcements should be the current_video_time_seconds field. This gives the time, in seconds on the video’s timeline, of the currently-displayed frame. It should start at zero on the first frame and then advance roughly in steps of the inverse of the frame rate. (There’s a bit more info in this discussion.) If you just want to count videos played, then looking for events with current_video_time_seconds equal to zero should work.
For audio stimuli, you’ll need to look at #announceSound. Since sound playback isn’t tied to the display refresh cycle, each sound stimulus generates an independent #announceSound event whenever it plays, pauses, resumes, or stops. If you want to know when each sound starts playing, look for #announceSound events where the action field has a value of play.
Cheers,
Chris