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PostPosted: Sat Nov 22, 2014 5:24 pm 
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I've seen a phenomenon when I play NES games on my CRT TV: In certain scenes, there are green stripes on the left side:

Image
("Ninja Gaiden")

Image
("Soccer League - Winner's Cup")


But this is not a general issue. Most scenes don't have it:

Image
("Ninja Gaiden")

Image
("Soccer League - Winner's Cup")

Image
("Zelda II - The Adventure of Link")

Also, until now, I've never seen this with scenes where black isn't the dominant color on screen.


Does anybody know what this is all about? Is it a phenomenon that's specific to my TV or have other people observed it as well? And what's the technical explanation that some scenes in NES games produce those green stripes, but most don't?

(Also, the phenomenon is not random. If a certain scene produces the stripes, then it will always produce them. And if a scene is fine, it will always be fine. So, it doesn't happen that one and the same scene sometimes has the stripes and sometimes doesn't.)

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PostPosted: Sat Nov 22, 2014 6:06 pm 
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Not-a-bug. It's the NTSC PPU's border being rendered. It uses the same color as the current background color, whereas on PAL PPU the border is always black. In many TVs the border is outside the visible portion of the screen, on some it might be visible. For example it's visible on the HDTV that I have. No emulators (to my knowledge) emulate the border. I did add it to my custom (yet unreleased) version of Nintendulator.

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Last edited by thefox on Sat Nov 22, 2014 6:20 pm, edited 1 time in total.

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PostPosted: Sat Nov 22, 2014 6:15 pm 
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I guess you misunderstood me. I'm not talking about the background color that's visible on the left and right side. I'm talking about this:

Image

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PostPosted: Sat Nov 22, 2014 6:20 pm 
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Oh, right, disregard the message then.

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PostPosted: Sat Nov 22, 2014 6:38 pm 
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DRW wrote:
Does anybody know what this is all about?
It looks like the electron beam is hitting stuff at the far left and bouncing back onto the screen. (This is probably not really what's happening, but that's what it looks like.)

DRW wrote:
Is it a phenomenon that's specific to my TV or have other people observed it as well?
It's specific to your TV, although it may happen on other CRTs if they are adjusted in a similar way. (It wouldn't be difficult to do this on a computer monitor and see what happens. Unfortunately, the only CRT monitor I have is in storage due to its limited usefulness since LCD monitors became affordable.)

DRW wrote:
And what's the technical explanation that some scenes in NES games produce those green stripes, but most don't?
There's a bright line on the left side of the screen, and a good chunk of it is outside the usual display area. Some of it reflects back into the display area, and the black background is dark enough that you can see the reflection.


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PostPosted: Sat Nov 22, 2014 9:58 pm 
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It happens on my CRT too. It is the currently-selected background color being drawn before horizontal-retrace has ended. It isn't physical light reflections or anything, it is legitimately an artifact being drawn on the screen by the electron gun. Thefox is correct in that the background color is drawn as a border around the visible portion of the frame, but if the border goes too far off the left side of the screen, it creeps into the horizontal-blanking portion of the scanline, which is supposed to be black, and creates that hazy glow on the left side of the picture if the offending content is bright enough. I've noticed this on standard cable tv too, so it's not a problem specific to the NES.


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PostPosted: Sat Nov 22, 2014 10:39 pm 
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So thefox was right the first time that it is "the NTSC PPU's border being rendered" after all. It just gets rendered where it shouldn't be because the TV is broken.


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PostPosted: Sat Nov 22, 2014 11:56 pm 
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The TV's not broken, it's just following the rules. :P


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PostPosted: Sun Nov 23, 2014 5:09 am 
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Alright, so let me summarize the whole phenomenon to make sure I understood it correctly:

My TV renders borders that are outside the usual "NES display area" and fills them with the global background color of the game.

In the current example, the global background color is very bright while the regular dominant color of the background tiles is black.

Now, according to Joe, the green stripes are kind of a reflection from the bright area on the left side that are visible because the dominant color is black.
According to Drag, it isn't reflection, but actually something that's drawn by the cathode ray due to some internal ways a CRT TV renders an image.

Is this correct so far?

So, this means two things:
1. The green light does not come from some magnetic interferences in my room or something like that.
2. The green light is not the result of my TV having been broken at one point in all the years.
The effect would still be visible with a brand new TV of the same brand in a room without any interferences.

Did I understand all this corectly?

Furthermore, is there any website on the internet where this phenomenon is described? If this is a side effect of NTSC CRT TV technology, shouldn't half of the "Ninja Gaiden" players experienced this? I'd really like to read more about it.

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German Retro Gamer article: http://i67.tinypic.com/345o108.jpg


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PostPosted: Sun Nov 23, 2014 7:43 am 
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Drag wrote:
The TV's not broken, it's just following the rules. :P

No, it's not, it should not shoot beams at all during the time the beam is going backwards. Though this would be really defective design rather than just being broken (and probably went unnoticed since you need a not-so-valid signal with an image that's dark enough which never happens with TV... or with most devices, really).


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PostPosted: Sun Nov 23, 2014 1:07 pm 
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DRW wrote:
My TV renders borders that are outside the usual "NES display area" and fills them with the global background color of the game.
Correct, and it's the NES that generates this border, your TV's just showing it.

Quote:
Now, according to Joe, the green stripes are kind of a reflection from the bright area on the left side that are visible because the dominant color is black.
This could happen, but it's not likely that this is what's happening here.

Quote:
According to Drag, it isn't reflection, but actually something that's drawn by the cathode ray due to some internal ways a CRT TV renders an image.
Correct, and to elaborate, it's because the electron gun scans left to right, top to bottom. Once a scanline is drawn from left to right, the gun has to return to the left side of the screen, which means it quickly sweeps back across the screen, and the gun is not deactivated during this time (the NTSC spec doesn't require it to be, either), which is why the signal needs to be black during h-blank, and why h-blank is even called h-blank in the first place.

In summary, the scanline terminates with a signal to trigger horizontal retrace, and during horizontal retrace, the scanline data must be blank so the gun doesn't draw anything while it's sweeping back to the left side of the screen, hence why this area of the scanline is called horizontal blanking.

Quote:
So, this means two things:
1. The green light does not come from some magnetic interferences in my room or something like that.
2. The green light is not the result of my TV having been broken at one point in all the years.
The effect would still be visible with a brand new TV of the same brand in a room without any interferences.

1 and 2 are correct. The last statement depends on the TV. The NTSC signal does not require a television to disable the electron gun during h-blank and v-blank (and indeed, a lot of TVs don't because it's just extra circuitry), but that doesn't mean that there aren't TVs that do it anyway.

Quote:
Furthermore, is there any website on the internet where this phenomenon is described? If this is a side effect of NTSC CRT TV technology, shouldn't half of the "Ninja Gaiden" players experienced this? I'd really like to read more about it.

I don't know of any name for this phenomenon, but I said it happens on my TV too, so I'd have to imagine it's pretty common. Most players just don't notice or care.

Sik wrote:
Drag wrote:
The TV's not broken, it's just following the rules. :P

No, it's not, it should not shoot beams at all during the time the beam is going backwards. Though this would be really defective design rather than just being broken (and probably went unnoticed since you need a not-so-valid signal with an image that's dark enough which never happens with TV... or with most devices, really).

You're correct, the gun should not shoot beams during horizontal nor vertical retrace, which is why the horizontal and vertical blanking portions of the scanline and frame signal exist to ensure that the gun is not active during this time.


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PostPosted: Sun Nov 23, 2014 1:43 pm 
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DRW wrote:
If this is a side effect of NTSC CRT TV technology, shouldn't half of the "Ninja Gaiden" players experienced this?

I think those of us growing up with this technology in existence simply didn't care (read: didn't give a shit). We cared more about what it was we were playing/doing (i.e. the game). I remember playing NES games on all sorts of setups, especially TVs which were misaligned or "out-of-whack" in some way or another, including 4" black-and-white portable TVs -- what mattered is that we were enjoying the game.


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PostPosted: Sun Nov 23, 2014 1:46 pm 
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Drag wrote:
The NTSC signal does not require a television to disable the electron gun during h-blank and v-blank (and indeed, a lot of TVs don't because it's just extra circuitry), but that doesn't mean that there aren't TVs that do it anyway.

TVs that don't add their own blanking will show artifacts on authentic copies of VHS and DVD videos that use Rovi's Macrovision analog copy protection.


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PostPosted: Sun Nov 23, 2014 2:04 pm 
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Drag wrote:
Quote:
Now, according to Joe, the green stripes are kind of a reflection from the bright area on the left side that are visible because the dominant color is black.
This could happen, but it's not likely that this is what's happening here.
I disagree. From experience, it is really easy to deliberately construct video signals on CRTs to cause the electron beam to "skate" along the outside edges of the CRT and cause the kind of depicted blur.

Quote:
Correct, and to elaborate, it's because the electron gun scans left to right, top to bottom. Once a scanline is drawn from left to right, the gun has to return to the left side of the screen, which means it quickly sweeps back across the screen, and the gun is not deactivated during this time (the NTSC spec doesn't require it to be, either), which is why the signal needs to be black during h-blank, and why h-blank is even called h-blank in the first place.
Retrace lines are a lot more obvious and a lot less blurry. They're also aligned to the phosphor mask, so they'll be whatever color's being intentionally transmitted, not greenish-white as depicted.

This hypothesis also requires that the video signal enable video even earlier than necessary for internal reflection.


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PostPosted: Sun Nov 23, 2014 9:43 pm 
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My mistake. I thought he meant the picture was physically reflecting off of the screen's bezel and back onto the glass. I see what you mean though, and that makes more sense. I remember playing with an old B/W TV's horizontal-hold knob, and seeing crisp picture data crawl back onto the screen (backwards and stretched) when it went too far off the side, and I honestly didn't think the NES's signal was SO off-spec that it'd do that. Now that I understand, I agree with you.

Also, as a side note, my "haze" artifact has a rainbow effect to it instead of a solid color, but it otherwise looks the exact same as the screenshots in the first post, including it being stripey. My TV is a Panasonic though.


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