Dropped or not dropped frame ?

[Music Fan]

Hi,
I'm doing some analog tapes captures (VHS and Hi8) with my PVR-150 (I use VVCR and Lagarith) and I noticed something strange ;
As Virtual Dub detected one dropped frame in a 1 hour file, I re-captured this moment to replace the missing frame and as there wasn't any dropped frame at this place in this new short capture, I believed it would be simple to solve.

The strange thing is that no frame is missing compared to the other file without dropped frame (I moved frame by frame to be sure), the dropped frame is in addition to other frames ! :confused:

In the file containing the dropped frame, all frames are there but shifted by one frame from the dropped frame.

How to explain that ? Could it be a false positive ?
Or is there also a dropped frame in the second capture at the same place but not detected by Virtual Dub ?
Or is it normal and a dropped frame is actually sometimes a doubled frame ?

As the sound seems ok in both files, I don't know what to do to throw away the dropped frame and ensure synch.
I prefer to let the sound untouched, thus I could maybe simply keep original file's audio, remove the dropped frame and remux audio and video.

[Sharc]

Hi,
Or is it normal and a dropped frame is actually sometimes a doubled frame ?

In my experience, when a frame is dropped, it is substituted by the repetition of the frame before in order to keep audio and video in sync. Therefore one will actually see the dropped frame as a duplicated frame.

[Music Fan]

Yes, but in this case it seems that the dropped frame doesn't replace any lost frame because all frames are there (when I compare with the other capture), it simply seems added.

[Sharc]

I suspect that this can happen when the audio and video clock of the capturing device are not derived from the same master clock and drift slightly over time, e.g. depending on temparature. When the audio lags the video an extra video frame may be inserted or skipped for A/V synchronization. What exactly happens depends probably on the capturing HW and how the SW tries to keep audio and video in sync.
After all, I think with only one such incident in 1 hour the capturing went pretty well.
Maybe somone else has a better explanation.

[Music Fan]

Interesting. In this case, I guess I should keep the doubled frame, but it will make a sort of stuttering.