The rises and decays of the TTL both signal frame acquisition events.
Hannah, I have seen occasional TTL drops too. They are easy to spot since the TTL is generally very regular (e.g., there will be one 67 ms gap between TTLs in a 30 Hz recording and the rest will be between ~31-35 ms), so I just fill them in with my best guess of when they would have happened and then avoid any analyses of that period of time that need precise timing. In one example I just looked at, the TTL drop didn't coincide with a large timestamp gap in the CSV file, so I'm inclined to think that the actual frame was not
dropped, just the TTL. Most of my 2.5+ hr recordings don't have dropped TTLs.