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PostPosted: Wed Mar 13, 2013 8:00 pm 
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I was discussing the NES video output and realized that it's actually not 240p. It's 480p with every other line black, and at double the brightness of a normal picture on the same TV. That is, if you had a 480i60 CRT and a 480p60 CRT, you could get the 480p60 CRT showing an identical image as the 480i60 CRT showing a NES only if you did the above two things for the NES image. And if you had a true 240p60 monitor, you would not be able to get an image that looks like a NES, as your scanlines would be way too fat vertically.

The NES clearly isn't merely 240p, because you get very noticeable black scanlines between everything. In a way, these black lines are part of the picture. And because each scanline is painted 60 times per second, rather than the usual 30 it would get for a normal interlaced signal, it's effectively twice the brightness a normal picture's scanlines would be.


Last edited by blargg on Wed Mar 13, 2013 10:11 pm, edited 1 time in total.

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PostPosted: Wed Mar 13, 2013 8:16 pm 
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I think my brain had an error... :lol:
The hsync is still 15.something KHz, there's no 480p
and if it wasn't 15KHz my TV would surely complain about it...

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PostPosted: Wed Mar 13, 2013 8:59 pm 
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The way a TV displays a so-called 240p signal is as if it has an extra black scanline between each displayed one, and this is integral to the appearance of old consoles that used it. Thus, they are really more like 480p with every other line black.


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PostPosted: Wed Mar 13, 2013 9:16 pm 
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Interlace mode is displayed by having a half line, which then "restarts" the horizontal ramp, which translates to a small "delay" and the lines of new field end up in the middle of old. The progressive mode has no half lines, or even number of them so they cancel out each other and the lines of new field go on top of old.
In both cases the Hsync signal that defines line length (and amount of lines within vertical ramp, which is restarted by a Vsync pulse) is the same.
Operation of a CRT is so deviously simple... 2 ramp generators and some high voltage (and a bit of amplification).

For 480"p" you need 526 or 524 lines per vertical ramp, you only get 262 or 263, depending on source device.
Hsync speed = lines * fields
Since Hsync is around 15.something Hz you can only have around ~260 lines in case of 60 fields.

...and is that a serious topic or something I have missed at some point....?

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PostPosted: Wed Mar 13, 2013 9:39 pm 
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I... can't really tell if this is tongue in cheek or if you are being serious! The NES very surely puts out a 240p signal. As Tiido mentioned hsync is ~15Khz for sure; to be 480p it would have to be ~30khz. The black scanlines are just... scanlines; scanline is a poorly chosen name as it really describes the gaps between the lines of rendering. You'll notice that brighter lines often appear fatter, and on a well converged and focused CRT monitor or television the lines this gap won't be so visible.

I promise, it's a 240p signal :P


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PostPosted: Wed Mar 13, 2013 10:09 pm 
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Let's go to a different hypothetical example that's not the NES, so there's no preconceptions muddying things up.

PC monitors support interlaced and progressive signals. In interlaced mode, each field's scanlines are double spaced as compared to a progressive frame's scanlines. Each field scans every other line, leaving space for the interleaving lines that will be done on the next field. Let's say that this is a fixed-frequency monitor supporting 480i and 480p.

If you trick the monitor into displaying only one kind of field every time, your image's scanlines will be double-spaced. So if you were displaying an image with 240 scanlines, they'd take as much space vertically as 480 would in the monitor's progressive mode.

Now, let's say that you had another 240p-only monitor (no support for 480 at all). If you displayed the above image on it, it wouldn't look the same; the scanlines wouldn't have enough space between them. When you tricked the first monitor into displaying those 240 scanlines in the same vertical position every frame, you still had 240 black scanlines between them. But the 240p-only monitor's electron spot size is adjusted so that the scanlines mesh up against each other.

Now, let's say that you have a third monitor that only does 480p, no interlace support. Can you display the same thing you got on the first monitor? Yes, by showing a black scanline between each normal scanline. This will give you exactly the same appearance, since it's visually what happened on the first one when you tricked it to show the same field every time.

Thus, a 240-scanline image shown by tricking an interlaced monitor into showing the same field over and over is not the same as a 240p image on a 240p monitor, but is exactly like a 480p image made up of the 240 scanlines of the image interspersed with 240 black lines.

Thus, my claim that that the NES image is only properly represented as a 480p image, and that it's more than a mere 240p image. It's irrelevant what the video signal carries; what a TV tricked into displaying a progressive image displays is as if it were showing black scanlines between each one from the NES. If you want to focus on the video signal, then these are encoded into it by way of every frame having the same number of scanlines, without the marker every other one to tell the TV to alternate fields.

Also the claim that it runs at twice brightness is related, since the TV normally only illuminates a given scanline 30 times a second, where here it's illuminated 60 times a second.


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PostPosted: Wed Mar 13, 2013 10:45 pm 
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Only difference between progressive and interlace is lack of half lines. 480p monitor is able to do 960i by design (if we speak about analog stuff), and so is 240p monitor doing 480i.

The so called progressive image happens only because of missing half lines, making your field go on top of old not inbetween in, leaving unscanned areas on the screen that we know as scanlines...
Scanlines only happen because the beam is not physically high enough to cover all the screen in the intended scan pattern. But you can de-focus the beam so it will cover up the non-scanned area... but it will look crappy with interlaced signal (lots of detail loss vertically).

If you intend to show 240 lines on a monitor that only takes 480 lines you have to add a blank line after every real line in the source material, but that is impossible in realitme, you got to add a (digital) buffering scheme...
You cannot make a 480 line only monitor show 240 line source material, the monitor will either go off sync or somehow shows 2 fields on top of each other, with half the effective frame rate.

And showing 480 line content on 240 line monitor results in something like this if you are extremely lucky :
http://www.tmeeco.eu/BitShit/VGAtoTV.JPG
But usually it is something like this :
http://www.tmeeco.eu/BitShit/VGAtoTV2.JPG

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PostPosted: Wed Mar 13, 2013 10:46 pm 
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It is true that conventional SDTV CRTs are only designed to display 480i, and that basically all 4th-generation and older consoles abuse them into displaying a progressive image. The electron beam diameter is tuned for 480i display: in this regard it is absolutely true that there are black lines between everything. However, smaller televisions can't really display 480i: there is a certain minimum size that the electron beam gets.

Similarly, end-of-generation multisync CRT monitors were tuned to display 1600x1200 or whatever, and when they were displaying particularly low resolution video modes (720x400 text, 320x200 "MCGA" graphics, 640x350 "EGA" graphics) you got the same "black lines" effect. It's misleading to say that these monitors' "true" resolution was anything in particular: they were continuous-space devices, with a blur kernel that was a function of the aperture grill and electron gun beam emission surface. In the same way, a SDTV CRT doesn't have a "true" resolution of 480p, and using any simple vertical-only blur filter on a 240p-upsampled-to-480p input is similarly inaccurate.


Last edited by lidnariq on Wed Mar 13, 2013 11:19 pm, edited 1 time in total.

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PostPosted: Wed Mar 13, 2013 11:16 pm 
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This is about TV standard. It describes that two neighbor frame fields must differs by one scanline, that enables interlace mode in TV set, which was conceived originally. But what if we feed to TV two exactly same frame fileds? We get progressive scan with half of resolution but at double framerate. NES PPU generate exactly same odd and even frame (we get this info by decap), so it is true 240p device. But every modern TVs (especially LCD) understand the original TV format is literally trying to deinterlace progressive signal and get ugly vertical stretch (instead of beauty scanline) at half of framerate.
Think about it.


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PostPosted: Thu Mar 14, 2013 7:32 am 
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I think what blargg is trying to say is that the CRT's electron beam is focused differently. It's focused one way for monitors designed for 240 lines (such as CGA and 8-bit home computers) and another way for monitors designed for 480 lines, be they interlaced or progressive (such as TVs and early VGA monitors). It's like how with the old CRT PC monitors, you can see the spaces between scanlines more clearly when running an 350p to 480p DOS game in full screen (in actual DOS, not a VM) than on your 768p Linux or Windows desktop, because the beam is focused for the 96 scanlines per inch that desktop environments assume.


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PostPosted: Thu Mar 14, 2013 9:19 am 
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I would not compare the quality of the mask in a regular TV and even the most simple monitor. Monitor has less size of grain of phosphors and the CRT beam is focused much better. I had the experience of using a CRT monitor tube in the TV same diagonal. The results are impressive.


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PostPosted: Fri Mar 15, 2013 8:53 am 
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Just to be sure, does that imply that the resulting NES display on a common CRT shakes a little vertically, due to interlacing?

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PostPosted: Fri Mar 15, 2013 8:59 am 
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The NES does not shake on a CRT unless it's run through a device with a frame buffer that converts everything to interlaced, such as my DVD recorder.


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PostPosted: Fri Mar 15, 2013 9:02 am 
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Jarhmander wrote:
Just to be sure, does that imply that the resulting NES display on a common CRT shakes a little vertically, due to interlacing?

No, because it tricks the TV into displaying the same field every frame, rather than alternating.


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PostPosted: Fri Mar 15, 2013 10:26 am 
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No, on top of old, instead of between it.

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