Quantcast Errors on the edges of converted VHS tapes? - digitalFAQ Forum
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11-15-2008, 09:38 PM
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There's been some concern lately from a few folks, both customers and otherwise, regarding "tracking noise" and "odd edges" on VHS tapes that they (or I) have converted. So I wanted to address this issue, shed some light on what it is, what means, and how it can be removed (if required).

The issue is that of "overscan" on an analog signal. Some of this has been explained on the site, currently located on this page, but I want to re-explain it again, a little differently.

Let us begin...

Analog video is a very chaotic mechanism, compared to the relatively variable-free or variable-controlled digital realm. Once upon a time, some many decades ago, broadcasters and television makers decided that it was acceptable to have noise in the video signal.

overscan-noise.jpg

Notice the errors on all sides of the image. These were present on the original video tape. It's a mix of closed-caption noise, head-switching noise, and a simple lack of image for the black areas. Processing the tape through a proc amp (a device that can tweak and improve color, contrast and brightness) has added more errors to the edges, because it also does not cover 100% of the image, just most of it. The proc amp boundaries, an Elite BVP-4 Plus in this case, made nasty rainbow effects and added a hard border just a few pixels off the left and right screen edges.

What is seen on a television set is maybe 93% (on average) of the image, starting from the center. The outer edges are never seen aside from special monitors designed to show edge-to-edge information. There have been a few cheaply-made HDTV sets that show the overscan area, but it is a small minority, mostly confined to early-version screens. A good television, be it traditional, SDTV, EDTV or HDTV, will properly crop down your image.

If you really, really want to hide this noise, because you or somebody else is likely to view it on an edge-to-edge device (most likely a computer screen), then yes, it can be masked in most video editors and some video encoders. TMPGEnc Plus, Adobe Premiere, VirtualDub and Sony Vegas Video come to mind. Note that I said "mask" and not "crop". Rarely would you want to chop off part of your image, covering it over with black is the best option. Just cover it enough to hide what is bad. If you cover too much, the black borders may shown on screen!

You would only crop if the video is not intended for tv anymore, such as uploading to Youtube, or for other web-style distribution.

This is what a mask would look like. Please note that I've masked about 7% of the image, just to show what the entire overscan area would look like, in comparison to the edge-to-edge data.

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This is the real image that was shot in the consumer camera, or created in the professional editor (and displayed on your television). It was created knowing that the excess edge data would never be shown - or so they thought. Few folks could have foreseen the way digital video and computers have developed so fast, and become so mainstream. It wasn't too many years ago that folks like Bill Gates were telling us that computers wouldn't need to go faster or have more memory, it was "more than anybody would ever need".

This is what you'll see on your tv screen:

overscan-matted.jpg

Was fixing the image worth adding more noise to the edge area? You bet! Just check out how bad the original image was, before the proc amp improved it:

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Hopefully this will explain what you're seeing, and put aside any concerns. I would imagine the only aspect not covered here is software-specific methods to mask, should it really be needed.



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04-20-2012, 01:24 AM
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As a follow-up to the above post, there are ways that the video can be further cleanup up, using more advanced scripting and plugins from Avisynth. For example, notice how the man's yellow uniform and the red detailing on the jet is bleeding. This can be somewhat repaired when the damage is uniform through the video, as it was on this VHS tape.

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Here's the script:
Code:
ImageSource("c:\overscan-final-television.jpg")
ConvertToYV12()
ChromaShift(C=-4, L=-4)
^ Note that ImageSource() was used here, but it would be AVISource() or ffvideosource() for videos.

The image is also sharper when the chroma more properly aligns itself on top of the luma.

While this post is unrelated to the topic of the first post, I didn't want it to be implied that nothing else could be done to improve the video beyond the "final" image from the first post. That was only "final" in regards to overscan noise.


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File Type: jpg overscan-final-television.jpg (50.1 KB, 521 downloads)
File Type: jpg overscan-final-television-avisynth.jpg (46.7 KB, 518 downloads)

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