Trying to Understand Inserted Frames
I recently tried to capture something from my AG1980 to my PC with no TBC using AmarecTV (gave up on VirtualDub due to sync issues). Overall, I will get a recording with maybe 1 inserted frame every 10 minutes so I am not really able to notice it visually. Audio sync is not an issue.
Comparing the software to VirtualDub, I believe VDub will by default play with the audio to line up with the video rather than the reverse, which is what Amarec does. Since these frames are being inserted, I have the following questions:
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On the other hand, I'm using some type of frame-sync tbc. |
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Thank you for the reply. |
I have no idea what a null frame would look like. A blank frame of static? Black? Orange? Blue? I've seen inserted frames before, in one bad case up to three in a row -- they are exact duplicates inserted by the software. The sound track continued as if nothing happened.
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In any case; if the video is being recorded slower than the audio, inserting extra frames wouldn't "catch up" with the sound. Rather, the software drops or skips frames that it didn't have time to process. This effectively speeds up the frame rate. Inserted frames (i.e, null frames, aka duplicates) are used to slow the video frame rate to maintain the desired target fps. Neither dropped nor inserted frames are directly connected to adjusting audio sync. The process is directed at maintaining a frame rate reasonably close to the target frame rate. You can have a visually perfect 2-hour capture with no dropped or inserted frames at 29.97fps, but the sound track could still be out of sync. This is probably why VDub capture has separate frame timing options and audio sync options. If you had no adjustment of audio signals whatever, and the audio input always proceeded at a constant timing rate, then dropped frames would speed up the video frame rate to the point where there might be enough dropped frames to allow audio to fall behind the video. If enough inserted frames are added to slow the frame rate, there might be enough insertions over time to make the audio jump ahead of the video. Neither case is certain. But I've always turned off the drop/insert options because I don't seem to have needed them with a proper playback chain at work. The timing option that I leave enabled is to adjust the audio rate to the video rate. If that adjustment results in wobbly or sour/chirpy sound, it means you need to improve something somewhere in the capture chain, software installation, or OS operation so that video and audio timings are closer together, either before they enter the actual capture device or while they're being processed. If I recall correctly from my film production classes, the camera is supposed to maintain the video frame rate. The sound system adjusts audio timing to keep pace with the film. With high quality hardware, the video and audio timing are usually close enough so that massive adjustments aren't necessary. |
My guess is that frames get inserted due to audio video sync issues. You video drifts ahead of the audio so you get a frame duplicate to throw the video 30 milliseconds back. I know for sure Canopus ADVC-300 were inserting frames and I could later easily spot them. Canopus would not report inserted frames. But that's probably how they kept audio and video more or less in sync.... Although audio would always be ahead of the video in Canopus.
Blackmagic Design would drop frames to keep audio and video in sync and also wouldn't report dropped frames to the user. I myself use to be taking a different approach. I would let audio drift away and would manually sync it by resampling. |
Inserts/drops can happen due to many things.
My recent issue on a new build is looking to be due to the HDD I/O, either drivers or hardware (motherboard) issue. I may scrap it, and replace with another mobo (and RAM, maybe keep CPU). |
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Since you own multiple TBCs, why not test for yourself with this tape and compare to find out? You could also record a test tape containing audio and a video with a running frame counter, then capture that through multiple workflows and compare. I did a ton of testing with such a tape (albeit silent) but analyzing the results became too complex to summarize. I can say that the DVD recorder passthroughs skipped frames entirely with my test tape. |
If the inserted frames appear at specific interval, remove them without touch audio. Then just squeeze the audio file. I can give you details how to squeeze the WAV file without audible quality loss.
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Thank you all for the feedback. I can say with some certainty that this probably now the most detailed thread on inserted frames out there.:)
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