Possible to fix analogue TV echo/halo problem?

[Pate]

Hi!

Sorry for the somewhat vague title, but the first problem is that I don't know the correct term for the problem I am curious about. It is sort of edge-enhancement halo, but seems to only happen after bright areas (on a scanline) switch to dark areas. This seems to cause the next luma values to drop below the base level, then jump back up, and only after that the luma value levels to the correct (dark) value.

I believe this was present already in the analogue TV antenna signal when I originally (in 1985) recorded this VHS tape I am trying to digitize.

Is there a way to remove (or minimize) this kind of a picture problem, without blurring or otherwise destroying the picture quality otherwise?

Here attached is an example capture image, see for example the edge where the white shirt switches to black jacket. There is also some halos around the hand and the microphone stand, around his hair, etc.

Any hope for improving this problem?

Thanks!

[kpmedia]

That white edge on the black lapel -- the left side, from his point of view -- is a halo. And in a way, it is an echo.

The technical term for this is "ringing".

This is most commonly found on video or still images that have been artificially sharpened. In the digital domain, most sharpening methods add the appearance of sharpness by putting lighter pixels next to darker ones. In the analog domain, it's caused by oscillations in frequency -- and may be intentional or accidental. In both cases, it's faux sharpening. It's not possible to actually add detail, but it can give the appearance of sharpening. The added contrast along edges is what gives this appearance.

Fixes for it tend to be limited or impossible, because it's an embedded image/signal damage. It's not really reversible.

There are times when what appears to be ringing isn't actually ringing, but chroma shifts. Remember that luminance data has the full resolution (i.e., 728x480), while chroma has half to a quarter of the resolution. Because of resolution differences of the actual color data, any mild chroma offset can create a mix of color bleeding and ringing-like artifacts.

However, based on other points in the image, I don't see any chroma offset.

Now then..

If you're determined to correct this, then there may be some methods worth trying by using Avisynth.

• I've attached the FixVHSOverSharp() filter to this post.
• There's the BlindDeHalo() script that can be found, with some usage instructions at this doom9 post.
• Finally, there's the HQDering() script, also discussed in some older doom9 posts.

Overall, these tend to be more poorly documented than some of the more common Avisynth filters and scripts, so experimentation will likely be required to find the perfect settings.

Deringing is, at the core, blurring the video in those areas. It's not magical removal. So keep that in mind. The scripts essentially search for the ringing issues, then attempt to blur the halos away, possibly even clone them out. As with any other filter that attempts to correct isolated portions of an image, it's possible to run into false positives on the detection, resulting in a video with added artifacts elsewhere.

Proceed with caution.

[Pate]

Thanks for the very informative and thorough reply! (I decided to become a premium member while reading your reply. If even such badly formulated questions get such an expert handling, your site is well worth supporting! ).

I do not want to introduce additional artifacts, even though this ringing is a bit distracting. I'll experiment with the options you gave, thanks for digging them up!

Pate

[kpmedia]

Quote:

Originally Posted by Pate

Thanks for the very informative and thorough reply! (I decided to become a premium member while reading your reply. If even such badly formulated questions get such an expert handling, your site is well worth supporting! ).

Thanks very much.

Quote:

I'll experiment with the options you gave, thanks for digging them up!

Feel free to attach a short sample clip (16MB max) to this thread, and we can try to run a few tests on it, too.

The most complicated aspects will be:
- Getting Avisynth up and running properly, if you've never done it before.
- Finding the exact versions of plugins. Some DLLs, like Masktools, may be a bit of a nuisance.
- And then finally, playing with settings will take experimentation.

Avisynth is setup here, and several versions of Masktools are available, so I can try to run a quick test between some other projects.
I'm somewhat curious myself, as to how well this may work.

Generally speaking, ringing looks worse the larger the picture gets. A 13" CRT tube TV won't be anywhere near as noticeable as a 60" 1080p HDTV. So be sure that you're judging video fairly -- especially when VHS is the source. I prefer to monitor on a 30-36" CRT or EDTV (sub-HDTV LCD). If that looks good, then it's tested against 55-60" 1080p screens, just for the sake of curiosity.

[Pate]

Yeah, I actually only noticed the problem after looking at the result of my avisynth conversion full-screen on my 90" projector screen.

I think I will let it be, I experimented with some settings but did not find the result to have sufficient improvement over the original to warrant spending more time with it.

I am somewhat familiar with Avisynth in general (used it many years ago when converting HDTV captures to XviD), but I am not sure whether I have yet found the best filters and plugins for VHS enhancement work. Though, based on what I have managed to get done already, I am pretty surprised at how good the result turns out to be. I keep forgetting it is not a DVD playing, as the result almost looks like one. I suppose this is mostly due to my finally using a JVC S-VHS deck with TBC/DNR, instead of the old poor normal VHS deck I used before.

Pate

[juhok]

My comment on Avisynth denoising (was doing it anyway for elsewhere) http://cdn4.digitalFAQ.com/juhok/Play7_noise_ULY0.avi (UTVideo 4:2:0).
MVTools via SMDegrain with strong prefilter and one of the noisiest sources I've seen. It's not perfect but dimishing returns and stuff.