Frame insertions every 90 seconds and slightly slow incoming frame rate?
 

Haven't seen this one get talked about as much, but in my short test runs, I always get a slightly lower frame rate reported by the PC whether that's with an AG1980, JVC 9800, or cheap VCR through an AIW 9000. Doesn't matter if I add TBCs/TBC-Like items inline (Tried DMR-ES10 + DVK100, Hotronic Ap41, DPS-235).

Interestingly, I get zero dropped frames with all of those setups, but virtual dub will "insert" a frame every 90 seconds or so like clockwork and displays the incoming frame rate averaging more like 29.96. My understanding is that the slightly low incoming frame rate will cause a frame to be inserted every 2996 frames or so frames to bring the frame rate up to 29.97.

I kind of thought that the different TBCs would do any frame insertions on their own to bring the frame rate up to 29.97, but perhaps TBCs don't actually change the incoming frame rate if it is very close to what they are supposed to output?

Is there an actual harm to setting frame rate to 29.96 if that's what I'm averaging? That would solve the problem altogether I think, though it's possible would drop a frame here and there - which is something I could easily test.

Both the AP41 and DPS-235 has a frame sync which I believe means if I fed it a "digital" genlock signal at 29.97fps that it would increase the rate of the video to 29.97 to match, but I'm not clear if that - anyone here actually use the frame sync part of these units or similar and does it actually affect anything?

Last question would be what is the advantage of a TBC inserting frames if the frame rate is slow versus just letting virtual dub do it?

An old school frame TBC should not insert or drop frames, it just times the frames and insert a fresh VBI signal, But a frame synchronizer can insert or drop frames to keep the audio locked to the video if there is severe drop outs, a frame synchronizer is found in most DV and SDI analog to digital converters after the frame TBC, Such devices do the capturing internally and stream the digital video to computer.

If using a frame TBC with analog out and a capture device, the frame synchronization is handled by the capture device and capture software, They work hand in hand to make sure the audio and video are in sync.