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 Post subject: 240p NTSC
PostPosted: Sat Jun 25, 2011 5:52 pm 
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My TV interlaces when it displays a regular cable tv signal, and it interlaces on the "INPUT" screen, but when it's displaying the signal from the NES, it stops interlacing.

I wanted to know why this happens, so I turned to our good friend Google, and it said that a standard 480i NTSC signal is 525 scanlines per frame, with each frame being made up of 2 262.5 scanline "fields" which alternate between the odd and even scanlines of the full resolution 480-scanline picture. I knew this already, but it didn't mention how a composite NTSC signal can be non-interlaced 240p @ ~60hz, instead of the regular 480i @ ~30hz.

I dug a little more info up, and ntsc-tv.com says that the vertical sync signal is as follows:
For odd fields:
6 pulses of hblank at double rate
6 pulses of vblank at double rate (vblank being hblank with an inversed duty cycle)
6 pulses of hblank at double rate
This equates to 9 scanlines of sync signals, then what I'm guessing to be 11 regular "active" scanlines (including a normal hsync signal and colorburst) of just black, and then 242.5 active scanlines of visible data.

Note that the odd field ends halfway through its last scanline.

The even field begins with the same 6-6-6 sequence of sync pulses, but they start in the middle of a scanline, so the sequence is offset by half of a scanline (which is why the pulses are at double rate; each scanline is guaranteed to begin with an hsync pulse that way). Since the even field's vsync signal ends in the middle of a scanline, the black level is output to fill the remainder of the scanline. Then, 10 blank scanlines follow, followed by a scanline whose first half is blank and whose second half is visible data, followed by 242 visible scanlines.

The important information is that odd fields have their vertical retrace command aligned with the scanlines, and the even fields have their vertical retrace command half a scanline early.

To detect the vertical retrace command, ancient TVs use a simple lowpass filter. For the 6-6-6 sequence, the first 6 pulses will average out to "slightly lower than black", the second 6 pulses to "vertical retrace", and the last 6 pulses to almost black again.

Remember, on even fields, this sequence occurs half a scanline earlier than on odd fields, so the TV detects the vertical retrace command half a scanline earlier than it would on an odd field.

This is what causes interlacing, on even fields, the vertical retrace occurs half a scanline earlier, which offsets the scanning by half a scanline's width. Because of this constant alternation between scanline-aligned and half-scanline offset vblanks, each field's scanlines will be drawn in between the scanlines of the previous field.

Knowing this, the way to have a non-interlaced signal, you just make sure vblank occurs at the same time for each field, rather than alternating. This seems to be referred to as 526 or 524-line NTSC, which kinda hints on how this is accomplished.

Pretend like you're always drawing an odd field, and either remove the half-active scanline at the end of the field (524-line), or add an extra hblank double-rate pulse at the end of the field (526-line).

On another note, this also explains why most LCD TVs make the NES graphics look like ass: They're trying to deinterlace a progressive signal.


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 Post subject: Re: 240p NTSC
PostPosted: Sat Jun 25, 2011 7:06 pm 
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Joined: Sun Nov 14, 2004 11:24 am
Posts: 330
Quote:
On another note, this also explains why most LCD TVs make the NES graphics look like ass: They're trying to deinterlace a progressive signal.


Ah! This explains why a scan doubler makes my NES look so much better on my LCD screen.


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 Post subject: Re: 240p NTSC
PostPosted: Sat Sep 03, 2011 3:25 pm 
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Joined: Sat Sep 03, 2011 2:52 pm
Posts: 5
Quote:
Knowing this, the way to have a non-interlaced signal, you just make sure vblank occurs at the same time for each field, rather than alternating. This seems to be referred to as 526 or 524-line NTSC, which kinda hints on how this is accomplished.

Pretend like you're always drawing an odd field, and either remove the half-active scanline at the end of the field (524-line), or add an extra hblank double-rate pulse at the end of the field (526-line).


So can this info help anyone make a scan converter for computer graphics?
Everyone wants to convert game graphics from let's say an emulator running on a PC or laptop, to a TV at true 240p low resolution!
Just the thought that it's almost impossible to do this baffles me extremely.

There are some scan converters that do this, but they are way too expensive or hard to find and weren't meant for the gaming community anyway.

Can't anyone make something like this?

Image

1. Any PC (or laptop) to small Converter via VGA.
2. Small Converter to TV via S-video.
3. A switch to convert the Interlaced image to a Non-interlaced image. (240p at 60hz)

Why the hell not??

I would pay anything to anyone who is willing to make me one like this.

Image

Oh, and it better have ZERO (0) lag.


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 Post subject:
PostPosted: Sat Sep 03, 2011 3:45 pm 
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The really easy way is to just use a Wii, it outputs 240p when running various emulators (including the Virtual Console).

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 Post subject:
PostPosted: Sat Sep 03, 2011 4:18 pm 
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Dwedit wrote:
The really easy way is to just use a Wii, it outputs 240p when running various emulators (including the Virtual Console).


Really?? I don't own a Wii... how well do the emulators for wii work?
What systems are emulated?
I am skeptical about finding a console that can emulate games better than a PC............................. and besides i want to run not just emus, but low res PC games as well, with the converter, all would be possible.


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 Post subject:
PostPosted: Sat Sep 03, 2011 4:27 pm 
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Formerly 65024U

Joined: Sat Mar 27, 2010 12:57 pm
Posts: 2257
On my friends TV playing Kid Icarus, the screen had a flicker too it. Maybe it was just me, maybe not. But it wasn't good.


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 Post subject:
PostPosted: Sat Sep 03, 2011 4:50 pm 
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It's worked perfectly for me, games run in Progressive 240p mode. I've never seen any flicker.

Unfortunately, the late 90's JVC TV I currently use for the Wii has a bad picture. When it's in interlaced mode, you can really see the interlacing, and it's annoying. When it's in 240p mode, you can see black scanlines on every other row. It looks sort of like an emulator with "TV mode" turned on, but for real. But at least the illuminated scanlines are bright enough to counteract the black scanlines. But this has nothing at all to do with the Wii itself, it's just a bad TV.

You can use SNES controller adapters that convert a SNES pad into a Gamecube controller. (Make sure you don't get the "newer" version of the Wii that drops Gamecube compatiblity) Retrousb.com sells the adapters.

There's also DOSBox available on the Wii, but I've never tested it out. Supposedly, it's too slow for games like Doom or Warcraft 1, but at least it supports USB keyboards and mice.

The NES emulator is FCEU, and it works as well as FCEU has ever worked. The SNES emulator is Snes9x. The Genesis Emulator works well too, but you need to make sure you redefine keys so you can get back to the main menu when using a SNES pad.

In order to modify the Wii to run homebrew software, you need a copy of Super Smash Bros. Brawl, or Lego Indiana Jones/Return of the Jedi/Batman, and an SD card.

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 Post subject:
PostPosted: Sat Sep 03, 2011 6:02 pm 
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Joined: Sun Sep 19, 2004 11:12 pm
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Location: NE Indiana, USA (NTSC)
Dwedit wrote:
In order to modify the Wii to run homebrew software, you need a copy of Super Smash Bros. Brawl, or Lego Indiana Jones/Return of the Jedi/Batman, and an SD card.

That or a letter bomb.


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 Post subject:
PostPosted: Sat Sep 03, 2011 7:24 pm 
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Nice, wiibrew hadn't listed that one yet.

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 Post subject: Re: 240p NTSC
PostPosted: Tue Mar 14, 2017 4:02 pm 
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Joined: Tue Mar 14, 2017 3:58 pm
Posts: 1
jay_are5 wrote:
Quote:
Knowing this, the way to have a non-interlaced signal, you just make sure vblank occurs at the same time for each field, rather than alternating. This seems to be referred to as 526 or 524-line NTSC, which kinda hints on how this is accomplished.

Pretend like you're always drawing an odd field, and either remove the half-active scanline at the end of the field (524-line), or add an extra hblank double-rate pulse at the end of the field (526-line).


So can this info help anyone make a scan converter for computer graphics?
Everyone wants to convert game graphics from let's say an emulator running on a PC or laptop, to a TV at true 240p low resolution!
Just the thought that it's almost impossible to do this baffles me extremely.

There are some scan converters that do this, but they are way too expensive or hard to find and weren't meant for the gaming community anyway.

Can't anyone make something like this?

Image

1. Any PC (or laptop) to small Converter via VGA.
2. Small Converter to TV via S-video.
3. A switch to convert the Interlaced image to a Non-interlaced image. (240p at 60hz)

Why the hell not??

I would pay anything to anyone who is willing to make me one like this.

Image

Oh, and it better have ZERO (0) lag.


Hi, where did you get this picture?


I’m handling, doing some mods and would help me a lot to know more about the silver hardware in your picture.

Regards


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 Post subject: Re: 240p NTSC
PostPosted: Tue Mar 14, 2017 4:23 pm 
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Joined: Sun May 27, 2012 8:43 pm
Posts: 1298
[quote=]
3. A switch to convert the Interlaced image to a Non-interlaced image. (240p at 60hz)[/quote]

You can't just do that, as the duration of a frame of an interlaced image and a non-interlaced image are different. You might be able to get away with it if your monitor/TV doesn't mind the vsync pulses being +/- 1 line every other frame, but that's all you can really do there.


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 Post subject: Re: 240p NTSC
PostPosted: Tue Mar 14, 2017 4:42 pm 
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Joined: Sat Sep 03, 2011 2:52 pm
Posts: 5
mikejmoffitt wrote:
3. A switch to convert the Interlaced image to a Non-interlaced image. (240p at 60hz)

You can't just do that, as the duration of a frame of an interlaced image and a non-interlaced image are different. You might be able to get away with it if your monitor/TV doesn't mind the vsync pulses being +/- 1 line every other frame, but that's all you can really do there.


I'm talking about doing it the same way an Extron Emotia does it. This hardware already exists and can do it, but there should be a simpler, smaller version of it.

akmodan wrote:
Hi, where did you get this picture?


I’m handling, doing some mods and would help me a lot to know more about the silver hardware in your picture.

Regards


Lol, I "photoshopped" that.


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 Post subject: Re: 240p NTSC
PostPosted: Wed Mar 15, 2017 9:45 am 
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Posts: 1298
The Super Emotia uses a framebuffer, as well as a so-so quality ADC. There's nothing exciting in there, unless a frame of latency is exciting. Furthermore, the Super Emotia takes 480p and spits out 240p/480i, instead of taking 480i and producing 240p.


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 Post subject: Re: 240p NTSC
PostPosted: Wed Mar 15, 2017 10:25 am 
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Joined: Sat Sep 03, 2011 2:52 pm
Posts: 5
Actually, you're right. It is 480p that will get converted, not 480i, because the intended source is a PC :mrgreen:
We don't want a composite / s-video / etc being converted. I want a signal coming from a VGA or HDMI being converted.
(Though truth be told, I've played PS2 / GC era games that were interlaced, through a capture card, then through the Extron Super Emotia just to convert them, and it wasn't bad at all! Some obvious latency was there, but very minimum. I was able to play / finish every game I tried just fine)

Framebuffer, 1 frame of latency? This seems perfectly fine with me!
When I play PC games, on a CRT, through the Super Emotia, things seem to be identical to how they are on a computer monitor, so that frame of latency really is nothing to me if its even there.

mikejmoffitt wrote:
as well as a so-so quality ADC. There's nothing exciting in there

This is actually a strong reason to need an upgraded device! Imagine if it WAS exciting?!


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