Ghosting / fading signal in VHS captures

[Malcolm]

Hi,
while processing some old vhs captures i have encountered some artifacts which could be described as ghosting, but not like shifted copies of the signal but instead like a fading horizontal shadow of strong (black or white) amplitudes in the signal. I think the following pictures describe it better than my words:
http://img393.imageshack.us/img393/333/videoshadowing01cropog3.th.jpg (http://img393.imageshack.us/my.php?image=videoshadowing01cropog3.jpg) http://img90.imageshack.us/img90/7683/videoshadowing02cropmt1.th.jpg (http://img90.imageshack.us/my.php?image=videoshadowing02cropmt1.jpg)

I think these 'artifacts' are a typical result of the analogue signal processing in the vhs recorder or a form of degradation caused by the aging of vhs tapes.

Any idea of how these shadows can be removed?
Maybe it's a question of how the signal can be restored, which means it's best done in frequency space?

[R3Z]

Have you tried temporal filtering ? ie Motion Compensated Median Bluring (MedianBlurT is a good one)?

[Malcolm]

@R3Z
i'm not sure if i understand you correctly. Maybe we're talking about 2 different things: These shadows are no 'motion trails' or something like that. The areas where they occur are completely static. It's just that dark elements in the picture have these (fixed) 'shadows' to the right.
So i don't know how temporal filtering or MC Median blurring could help. Maybe you'd like to explain.

[yup]

Malcolm!
May be problem in video cable which connecting VCR and capture card? During fast transition and edge if You use not very good cable You get ringing (cable not have enough bandpass).
yup.

[2Bdecided]

I doubt it's the cable. I have tapes that look like that (Pingu mostly, so I'm not normally the one watching them!).

Cheers,
David.

[yup]

2Bdecided!
Then this restriction bandpass for VHS tape not cable and need try deringing filter or dehalo.
yup.

[Malcolm]

Malcolm!
May be problem in video cable which connecting VCR and capture card?

I tried both my Pioneer DVD/HD-Recorder as well as a capture card connected to the VCR. the DVD/HD-Recorder actually did a better job than the capture card. I have used a very good cable to connect them to the VCR. So i don't think i can improve this further. But thanks for the hint.

[yup]

Malcolm!
Try http://avisynth.org/vcmohan/FFTQuiver/F1Quiver.htm
In low pass with high value cut frequency.
yup.

[R3Z]

@R3Z
i'm not sure if i understand you correctly. Maybe we're talking about 2 different things: These shadows are no 'motion trails' or something like that. The areas where they occur are completely static. It's just that dark elements in the picture have these (fixed) 'shadows' to the right.
So i don't know how temporal filtering or MC Median blurring could help. Maybe you'd like to explain.

I wasnt sure if they were temporal or static, but you have just stated they are static so temporal filtering isnt going to help.

[Malcolm]

Try http://avisynth.org/vcmohan/FFTQuiver/F1Quiver.htm


Hi yup!
I played around a bit with F1Quiver and managed to seriously screw up my video! :D
Seriously, after processing by F1Quiver clean parts without 'shadows' looked a lot like the bad examples i gave in my first post! :rolleyes:
So i'm bit confused how to set up F1Quiver to deliver usable results. :confused:

[yup]

Malcolm!
Upload short unprocessed clip.
yup.

[Malcolm]

:thanks::thanks::thanks:
here are the 2 examples from above as video clips (o.k they are a bit strange, i know :D):
ghosting1_cut.m2v (1.2 mb, unprocessed) (http://rapidshare.com/files/102577130/ghosting1_cut.m2v.html)
ghosting1_processed_huffy.avi (12 mb, preprocessed) (http://rapidshare.com/files/102582856/ghosting1_processed_huffy.avi.html)
ghosting2_cut.m2v (5.6 mb, unprocessed) (http://rapidshare.com/files/102578941/ghosting2_cut.m2v.html)

Cheers,
Malcolm

[yup]

Malcolm!
If I right understandt we have low pass filtering before writing to VHS in luma and especialy in chroma space (chroma place in high frequency range and more filtered).
Low paas filter frequency responce:

pass
----------
********\
******** \
*********\ cut
And we need conctruct filter with responce:
****************increase level at high frequency
pass********/------------
********** /
**********/
saturation at high frequency need for nonsense to noise.
This filter can constructed in CCE during encoding. You need find only cut frequency.
yup.

[yup]

Malcolm!
Try:
ChromaShift(C=-2)
http://www.geocities.com/siwalters_uk/fnews.html
See little better than source.
yup.

[Malcolm]

@yup
Thanks for your effort. When using Chromashift(C=-2) i get the following output:
http://img504.imageshack.us/img504/2431/videoshadowingchromashiny6.th.jpg (http://img504.imageshack.us/my.php?image=videoshadowingchromashiny6.jpg)

So this filter does only change the chroma channels. But the shadowing is a problem in the luma channel. What do you try to achieve with this filter? :confused:

[yup]

@Malcolm!
I think that problem coupled shifting between luma and chroma planes and at edges chroma have small delay. I use chromashift on second clip and see small improvement. Also I do not know filter for increase high frequency (only in CCE), F1Quiver can not made it.
yup.

[2Bdecided]

Here's an idea. The problem appears to be like the application of a very strong horizontal low pass filter, and then mixing the output of this filter very slightly into the original video signal.

Maybe you could apply a very strong horizontal low pass filter, and then subtract the result (scaled down a lot) from the original video signal. If you can get the low pass filter and the scaling correct (i.e. to match the original problem) you should see a big improvement.

I'll leave it to the AVIsynth scrop gurus to figure out how to actually implement it ;)

Cheers,
David.

[Malcolm]

Here's an idea. The problem appears to be like the application of a very strong horizontal low pass filter, and then mixing the output of this filter very slightly into the original video signal.
Hi David,
that's what i have in mind too. But i see one problem: A low pass filtered clip would have the signal 'smeared' horizontally in both directions. Since these 'shadows' are only visible on the right side of strong black image elements, how could it be avoided to corrupt the image left of them?
Until now i wasn't able to create a filter/scripting combination that accomplishes this.

I have another unscientific view to the problem: If you think of these shadows as an analogy to harmonic waves or echoes or 'overtones' to the main signal in frequency space. Then maybe there's an algorithm that simply can 'deducted' them out of the signal. Maybe in frequency space this is an easy equation.

[2Bdecided]

You're assuming a symmetric impulse response low pass filter. There's no reason why you would have to use that - you could use something that matches the original analogue effect. For each pixel, create a weighted sum of the pixels to the left. It's just a different impulse response, but a valid filter.

I have no idea how to do this in AVIsynth. I may have heard talk of an arbitrary filter function, but I may have been dreaming!

Cheers,
David.