Quantcast Trying to Understand Inserted Frames - digitalFAQ Forum
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04-15-2017, 04:32 PM
premiumcapture premiumcapture is offline
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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:
  1. What are the consequences, if any, of having a capture with a stray inserted frame?
  2. Since it seems that the audio is not being changed, can these frames be pulled out later with no ill-effect?
  3. If I had a TBC hooked up, would it be inserting frames on its own, which would then be unreported by VirtualDub but effectively done in the same way?
  4. There is an option to disable inserted frames - should they be turned off or do they serve an important purpose in the final capture file?
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  #2  
04-15-2017, 05:47 PM
sanlyn sanlyn is offline
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Quote:
Originally Posted by premiumcapture View Post
I recently tried to capture something from my AG1980 to my PC with no TBC
Why no tbc? It seems like an inefficient use of time and effort to discuss all these issues, none of which come as a surprise with no tbc.

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Originally Posted by premiumcapture View Post
gave up on VirtualDub due to sync issues
I can't say anything about this, as it has always baffled me why users have sync issues with Virtualdub (except for those users who won't use a TBC and then complain about it). I'm baffled because I've never had audio sync issues with VirtualDub capture and recommended hardware.

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Originally Posted by premiumcapture View Post
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.
Would you believe no inserted frames and no sync issues during my last 37 hours of capture? Maybe there were issues over the years since 2002 that I never noticed even after going over every frame during post-processing and authoring/burning, but the last 37 hours of tapes are the last that I post-processed and saw none of the issues mentioned. So I'm still baffled.

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Originally Posted by premiumcapture View Post
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.
No. It depends on the Virtualdub settings. VDub's defaults will drop and insert frames if you have no frame-level tbc but should maintain sync. But, then, I don't use the default settings.

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What are the consequences, if any, of having a capture with a stray inserted frame?
Video stutter. Inserted frames repeat themselves. Audio isn't affected.

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Originally Posted by premiumcapture View Post
Since it seems that the audio is not being changed, can these frames be pulled out later with no ill-effect?
The frame's audio is lost with the frame. The total length of video and audio is shortened for every every frame removed.

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Originally Posted by premiumcapture View Post
If I had a TBC hooked up, would it be inserting frames on its own, which would then be unreported by VirtualDub but effectively done in the same way?
Frame-level tbc's don't insert frames and don't drop frames. They correct the frame timing signal for either 25 or 29.97fps, depending on the format they detect or the format they're configured for. For example, the AVT-8710 auto-detects NTSC or PAL, but you can also override it manually.

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Originally Posted by premiumcapture View Post
There is an option to disable inserted frames - should they be turned off or do they serve an important purpose in the final capture file?
I've always turned off VirtualDub's drop/insert and some default features. Haven't you seen seen these timing option posts? Capturing with VirtualDub [Settings Guide] . Discussed are options for both legacy and newer USB-based devices.

On the other hand, I'm using some type of frame-sync tbc.

Last edited by sanlyn; 04-15-2017 at 05:59 PM.
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  #3  
04-15-2017, 06:10 PM
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Quote:
Originally Posted by sanlyn View Post
Why no tbc? It seems like an inefficient use of time and effort to discuss all these issues, none of which come as a surprise with no tbc.
I have a TBC-1000 I purchased from Datavideo, a TBC-100, and the newer glitchy AVT 8710. I do not like what the Datavideos do to the picture and have used them as a last resort and the AVT 8710 acts fairly spotty sometimes. I am looking for information more than anything so I don't see it as inefficient at all.

Quote:
I can't say anything about this, as it has always baffled me why users have sync issues with Virtualdub (except for those users who won't use a TBC and then complain about it). I'm baffled because I've never had audio sync issues with VirtualDub capture and recommended hardware.
I get it working and start to trust it and then it acts wonky on me. I had an older machine on XP and it worked wonders but with the newer Windows it has never been consistent for me.

Quote:
Would you believe no inserted frames and no sync issues during my last 37 hours of capture? Maybe there were issues over the years since 2002 that I never noticed even after going over every frame during post-processing and authoring/burning, but the last 37 hours of tapes are the last that I post-processed and saw none of the issues mentioned. So I'm still baffled.
The commonly used TBCs will spit out consistent framerates so I don't know how it works on the hardware level. My experience has been using the same source with no inserts on a TBC could have some without it, which means if it does happen, it isn't through the software. Looking back on the Amarec logs, it's possible to spot each frame once I know to look for it, but watching it and seeing it is next to impossible. I say this because I had one today in a slow-mo scene that was visually imperceptible.

Quote:
Video stutter. Inserted frames repeat themselves. Audio isn't affected.


The frame's audio is lost with the frame. The total length of video and audio is shortened for every every frame removed.
Can you clarify this a little bit? My initial understanding had been that the video is recorded slower than the audio, creating a gap which requires a null frame to catch the video up to the audio. Does an inserted frame mean both null audio and video are placed?

Thank you for the reply.
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04-15-2017, 06:52 PM
sanlyn sanlyn is offline
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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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04-15-2017, 07:06 PM
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Quote:
Originally Posted by sanlyn View Post
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.
I should stick to one phrase - inserted frame is short for 'insert null frame.' In Amarec's case, it's a duplicate of the prior frame, as you said.
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04-16-2017, 05:04 AM
sanlyn sanlyn is offline
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Quote:
Originally Posted by premiumcapture View Post
My initial understanding had been that the video is recorded slower than the audio, creating a gap which requires a null frame to catch the video up to the audio.
How Amarec handles this sequence, I wouldn't know. I never had to use it but what from what I've seen of the interface it doesn't seem to offer the flexibility of Virtualdub.

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.

Last edited by sanlyn; 04-16-2017 at 05:47 AM.
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04-16-2017, 03:06 PM
metaleonid metaleonid is offline
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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.
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04-16-2017, 03:15 PM
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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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04-16-2017, 10:32 PM
msgohan msgohan is offline
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Quote:
Originally Posted by premiumcapture View Post
Since it seems that the audio is not being changed, can these frames be pulled out later with no ill-effect?
If the audio is synched in the version with the inserted frames, and you remove them without removing the associated audio chunks, the audio will gradually drift out of sync. If you do remove those audio chunks, expect to hear little "blips".

Quote:
If I had a TBC hooked up, would it be inserting frames on its own, which would then be unreported by VirtualDub but effectively done in the same way?
It really depends. This is covering some of the same ground as a previous thread of yours: Myth or fact: Timebase corrector (frame synchronizer) should always be used? .

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.
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04-17-2017, 05:57 AM
metaleonid metaleonid is offline
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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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  #11  
04-17-2017, 06:59 PM
premiumcapture premiumcapture is offline
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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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