Hello! I'm in the process of capturing a 3 hour VHS tape in approx 32 min chunks. I started appending the sections together but I noticed that despite appending each section accurately using the frames as a reference, the audio was jumping slightly (I can provide a clip if necessary).
I checked the File Information of each video I captured and noticed a few have different video/audio lengths. Below is one of the problematic videos:
VirtualDub gives me the following File Information:Video
ID : 0
Format : Lagarith
Codec ID : LAGS
Duration : 32 min 18 s
Bit rate : 59.3 Mb/s
Width : 720 pixels
Height : 576 pixels
Display aspect ratio : 5:4
Frame rate : 25.000 FPS
Standard : PAL
Bits/(Pixel*Frame) : 5.715
Stream size : 13.4 GiB (94%)
ID : 1
Format : PCM
Format settings : Little / Signed
Codec ID : 1
Duration : 32 min 18 s
Bit rate mode : Constant
Bit rate : 1 536 kb/s
Channel(s) : 2 channels
Sampling rate : 48.0 kHz
Bit depth : 16 bits
Stream size : 355 MiB (2%)
Alignment : Aligned on interleaves
Video length: 32:18:12
Audio length: 32:18:20
My theory is the audio "jump" effect is happening because some of the 32min chunks of video have audio which is out of sync (in the case above the audio is longer by 8 ms). Therefore, I want to re-sync the video and audio.
What's the least invasive/destructive way to sync the audio/video? I've read about the following options
A) Selecting "Change so video and audio duration match"
B) Applying "time stretch" filter.
When I tried option A), it said the audio/video would need to be changed to 24.999fps. When I exported it, the problem of audio "jumping" when appended was solved - hooray! However, I'm a bit concerned about changing the frame rate - I read that's a "quick and dirty" way to fix problems and I'm wondering whether appending 24.999 fps and 25fps video will cause compatibility issues later down the line (I intend on archiving the Lagarith AVI and also exporting the project to MP4 for streaming). Additionally, the playback within VDub looks a bit choppy, although it seems fine when I play the exported video in VLC.
When I tried option B), I used the time stretch filter (input> time stretch (0.6) > output), full process mode but the video had no audio! Mediainfo claims the file has PCM audio but when I load it into Audacity there is no waveform. Perhaps my maths is wrong - I divided 12ms by 20ms = 0.6.
What is the best option for solving this problem? Many thanks!
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Last edited by dave_van_damme; 11th Sep 2021 at 22:59. Reason: clarity
I don't resynch my captures in VDub, so I can't help there. Nor do I cap 32 minutes at a time.
I assume the audio had drifted slightly while capturing the first clip, hence the different in length between the video and audio stream in that file. Or maybe there's a different explanation?
What's the best way to go about fixing this? Matching the video/audio frame rate seems the easiest but are there any disadvantages of this method?
Try demuxing the audio and video to elementary streams. Then appending the audio and video separately. Check the appended audio. Assuming it's OK, mux the appended video and appended audio together.
Is the beginning of the second appended chunk at the very beginning of the second capture ? Perhaps it takes a few split seconds for the audio capture to “hit its stride” and get in sync with the video (as it seems to be past that point) ; right at the joining point there seems to be a slight distorsion, as if the tape was still speeding up. If you overlapped the end of the first capture and the beginning of the second, you could try to join them a bit further. Also, rather than sticking with an arbitrary duration of 32 minutes, if it is not possible to capture the whole thing at once, you may want to match the duration of the captures with silences, or applause, or at least something more forgiving than a close-up of the singer singing at the top of her lungs in in the middle of a line.
Having messed around with it, I think I've found a solution. I opened up the video in Audacity and time stretched the audio so that it matches the video length. I then remuxed the new audio to the video in VDub with the following settings:
Video > direct stream copy
Audio > Full processing mode
Audio from other file
Having done that, it seems to avoid the "jumping" audio effect. Thanks for the replies!