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07-03-2017, 05:18 AM
Bobban Bobban is offline
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Hi!

I have a PAL VHS tape I'm working on and there are a couple of places that appear to have tracking problems (at least I think they are). After recapturing the tape, the problem appears in the exact same places again and nowhere else, so I believe it's a problem with the source tape and not a tracking problem of the VCR.

Please see the attached clip (in Lagarith lossless), where the problem is obvious for a few seconds (in both audio and video).

Since I can't fix it at the capture stage, is there any AVIsynth filter or something else I could use to improve it in the editing stage? Any help would be very much appreciated!


Attached Files
File Type: avi Problem Clip.avi (15.29 MB, 33 downloads)

Last edited by Bobban; 07-03-2017 at 05:34 AM.
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  #2  
07-03-2017, 11:52 AM
sanlyn sanlyn is offline
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Thanks for the sample. Looks like damaged tape.

The video is animation crated at 23.976 film speed with duplicate frames, and some form of pulldown (telecine) has been applied for 25fps playback. In this case the duped frames and pulldown are to your advantage. You can replace bad frames with good duplicates. Unfortunately IVTC (inversese telcine) won't work with bad frames, so I used QTGMC to deinterlace, fixed bad frames, then used srestore() to get the original 23.976 fps. It's possible that the same pulldown algorithm might not be used for the entire movie.

You have to deinterlace the clip to move frames around and to use Stab() to stabilize a little. It's a pretty messy clip with buzzing edges and other noise. Mount the video in Virtualdub and add its yadif deinterlacer, then observe the clip frame by frame. The frame numbers quoted below are deintelace frame numbers, which are double the original field numbers in the original clip.

There are frame hops in deinterlaced frames 14, 23, 90, and 99. You also have horizontal dropouts ("rips") all over the place and blotchy distortion in the bricks of the background door and at the big guy's feet, notably frames 15, 43, 44, 91, 98, 101, 114, and 115, and more distortion on the door and floor in some of the 80's and 90's. I also noticed before any frames were fixed that the bottom border flutters slightly up and down. I deinterlaced with QTGMC and corrected these glitches manually by replacing bad frames with good frames using Avisynth's built-in FreezeFrame() function.

The syntax for FreezeFrame is:
FreezeFrame (clip, int first_frame, int last_frame, int source_frame)
http://avisynth.nl/index.php/FreezeFrame

An example would be the first frame hop in frame 14 and the distortion on the floor in frames 13 and 15. I replaced 13-14-15 with good frame 12:
Code:
FreezeFrame(13,15,12)
The script I used:
Code:
AviSource("Drive:\path\to\video\Problem Clip.avi")
ConvertToYV12(interlaced=true)
AssumeTFF()
QTGMC(preset="medium",border=true)

FreezeFrame(13,15,12)
FreezeFrame(22,24,21)
FreezeFrame(86,87,88)
FreezeFrame(89,89,88)
FreezeFrame(90,90,89)
FreezeFrame(90,90,89)
FreezeFrame(91,91,92)
FreezeFrame(97,98,96)
FreezeFrame(99,99,98)
FreezeFrame(100,100,101)
RemoveSpotsMC()
Stab()
Crop(10,0,0,-16).AddBorders(8,4,2,12)
sRestore()

return last
The attached mpg sample was re-encoded with 2:2 pulldown flags applied for 25fps DVD.

What VCR and external frame tbc are you using? Usually, although not always, a line and frame tbc can help prevent this sort of thing. The horizontal frame shift at the start and end look as if they might be intentional, not unusual for anime. If not, the only way to fix it is to take all the shifted frames at once and shift them one way or the other to match the start and end frames, which means cropping of all the frames in the entire sequence -- that would be rather drastic.


Attached Files
File Type: mpg Problem_rework_25fps.mpg (1.88 MB, 6 downloads)

Last edited by sanlyn; 07-03-2017 at 12:03 PM.
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  #3  
07-04-2017, 06:46 AM
Bobban Bobban is offline
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Quote:
Originally Posted by sanlyn View Post
I deinterlaced with QTGMC and corrected these glitches manually by replacing bad frames with good frames using Avisynth's built-in FreezeFrame() function.
Thank you so much! I will have a closer look at FreezeFrame(), but your code already made the clip much better!


Quote:
Originally Posted by sanlyn View Post
What VCR and external frame tbc are you using?
I am using a JVC HR-S8500 SVHS Player with built-in TBC, and I have also sent the signal via a Panasonic DMR-ES15 dvd recorder as a pass-through for extra TBC. None of this corrects the problem though, so I think you're right that it's a bad tape.

Do you know of any way to improve the audio distortion as well?
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07-04-2017, 07:04 AM
sanlyn sanlyn is offline
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When damaged tape mistracks it often causes audio distortion, in this case a warp in pitch. You would have to very cleverly isolate that distortion in something like Audacity or another audio editor and correct the pitch for just a few seconds of audio, then remix it back into the frames. That would require an unusual level of patience, since the pitch frequency appears to vary from start to finish. But an audio app like Goldwave or the free Audacity is the only way to fix it.
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