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Old 24th September 2002, 14:56   #6  |  Link
Xesdeeni
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Join Date: Aug 2002
Posts: 467
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I think that two different things are being intermixed -
NTSC Broadcast Standard and consumer grade VHS recording.
Broadcast NTSC has a 4.2MHz bandwidth. This equates to about 320 lines of (horizontal) resolution (4.2MHz / (29.97 * 525)) * 2 * .8 * 3/4). [However, the 3.57MHz color modulation requires a notch filter that throws a wrench into the actual resolution.]
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For NTSC Broadcast, its 485 (525 - 40 service lines) lines correspond 340 "pixels" of subjective resolution (Kell Factor)...
You are mixing your metaphors. Lines of resolution is an ANALOG measurement. Pixels are a DIGITAL measurement. The Extended Kell Factor is used to estimate their relationship.
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...because TV signal is an interlaced one, and human eye somehow can detects the drop of resolution.
No, interlacing and human perception do not figure into this. It's as I described in my previous post: If the lines are in phase with the scanlines, you can display 480 lines of resolution. If they are not, then you can display fewer, and in the real world, they are NEVER in phase :-). Using the Extended Kell Factor you can estimate that 480 lines can represent about 370 lines of (vertical) resolution with all phase relationships.
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As for horisontal resolution, according to my book sources, the bandwidth is chosen such a way that it makes a square "pixel", which correponds to 453 x 340 effective resolution.
Just because it's in a book, don't believe it. And of course, don't believe it just because I say so...do some research and experiments on your own, and you'll find something very similar to what my research and experimentation has revealed.

Besides, your book is confusing DIGITAL pixels with ANALOG lines of resolution, too. You said it yourself above, 480 lines gives an effective 340 lines of (vertical) resolution. For all intents-and-purposes, analog scanlines are exactly the same as vertical pixels. So 480 vertical pixels gives an effective 340 lines of (vertical) resolution. That's what the Extended Kell Factor does, and that's the difference between pixels and lines of resolution. (720x480 digital resolution (4:3) ~= 415x370 (382x340) lines of resolution)
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Resolution of the consumer-grade VHS VCR (perhaps used by 99% of the people reading this forum) must be substantially lower. Just record some high-quality TV broadcast and play it back. The degradation in signal quality is noticable.
Of course it is. First, you are going from analog to analog. That brings in a whole new list of issues, such as transmission interference, multipath interference, signal-to-noise ratios, impedence mismatches, etc. But above all that, the 4.2MHz bandwidth transmitted signal is being squeezed into a 3.2MHz bandwidth VCR. Of course there is a loss of quality. But that is almost exclusively in the horizontal direction, due to bandwidth limitations. There can be some crosstalk between adjacent fields on a VCR, but that only reduces the signal-to-noise ratio. It has no significant effect on vertical resolution.
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How bad is the resolution of your VCR depends on particular model. But 352 x 240 effective "pixels" sound reasonable.
How can 240 pixels capture 340 lines of (vertical) resolution? It can't. So while you may find that 240 lines is sufficient for your purposes, you should also be able to see that you are leaving some vertical information on the table.

Horizontally, you should do some tests yourself, because anything I can say won't show you. But when I do the tests, there is most definitely a difference. I used a worse example than even the theoretical model mentioned above. I used an eight year old tape (complete with degradation dropouts), recorded on a cheap VCR, in SLP (6 hour) mode.

Even with this extremely suboptimal source, 720 and 480 were absolutely noticably better than 352. That is empirical proof that 352 pixels cannot represent all the information from a VHS tape. The theory says that 352 pixels can only handle about 203 lines of (horizontal) resolution, and that correlates closely.
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Let me enphasize what I mean by effective "pixels". An average person looking at the signal played by consumer-grade VCR on standard TV and at 352x240 resolution picture played on progressive PC monitor will make a conclision that both pictures have the same resolution
Apples and oranges. TVs have a different effective resolution, they have a different phosphor decay rate, and they have a different gamma response curve. PCs will interpolate the 352x240 up to whatever resolution they are asked to display (some video cards do a better job than others), thus smoothing some of the worst of the video (although they are still not creating any extra information). Compare the same video on the same screen and I don't think you will come to that same conclusion.
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The fact that VHS signal has some particular effective resolution doesn't mean that your capturing resolution should be the same.
Absolutely. There are a number of reasons that you would want to use a different capture resolution.
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In vertical direction the choice is clear. Either you capture at full (480 for NTSC) or half.
Sure, you could throw out one field, but realize that you are actually throwing out data. Of course, depending on what your source is and what your eventual destination format is, this might be the best thing you can do.

If you are taking a truly interlaced source (camcorder, live sporting event, soap opera :-), etc.) and going to VCD, then this is probably the best.

But if you are going from a telecined film source, then you would be better to inverse the telecine and then scale down the 480 lines to 240, so you don't introduce needless aliasing (a whole other rathole could be inserted here about filtering a signal prior to subsampling it).

And, if your ultimate destination is interlaced (SVCD, DVD, even VHS), then you should not throw out that field.
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In horizontal direction your signal is purely analog and the capturing process represents the convolution of video signal with the sampling voltage of the capture card. For example, if you capture your analog 352 x 240 signal with 352 x 240 capture card resolution, your effective horizontal resolution will be only 352/Sqrt(2)= 250.
[Crap, now I have to dig into the rathole I mentioned above.]

What you actually capture will depend heavily on your capture device. Most capture devices only have one filter on their input, optimized for their maximum capture resolution. When you want a lower capture resolution, they may decimate (throw out samples), or they may vary the capture clock. But in either case, they are introducing aliasing because they are not modifying the filter for the lower sample frequency. For this reason, you are almost always better off capturing at the highest resolution that the device can go, and post-processing to scale the image down with an algorithm that effectively filters the image. (This holds true for most analog to digital devices, including scanners.)
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To achieve a reasonable quality you have to capture at least at 352*1.41= 496 pixels of horizontal resolution.
Ugh. "Pixels of horizontal resolution." Pixels...period.

But after all that, once we remove the confusion between pixels and lines of resolution, we are saying almost exactly the same thing: The digital exivalent of VHS is about 423x480.

Xesdeeni

[Edited to correct reference to NTSC color burst frequency.]

Last edited by Xesdeeni; 24th September 2002 at 21:36.
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