Comment by laumars | Hacker News Reader

Comment by laumars | Hacker News Reader

What this article doesn't touch on but I have found an issue with classic consoles on non-CRT screens is:

1. Light guns

Even on consoles as late as the Sega Saturn, the light guns were design around the timings of CRT refresh rates and thus many (albeit not all) LCD screen are not compatible. I would imagine this could be a real issue for collectors / restorers of cabinets as shooting games (eg Virtua Cop) were quite popular in arcades.

2. Early 3D systems as used on Sega Scope for the Sega Master System; and the Nintendo 3D System for the Famicom (Japanese NES)

These devices are less common and as far as I'm aware only available on home consoles. For those of you who don't know what I'm talking about, it's active shutter glasses (ie disabled one eye at a time) and was timed around the interlacing on CRTs. At 50/60Hz (depending on your region) the effect can be a little unpleasant. But it's also pretty amazing to play when you consider the devices are approximately 30 years old.

There was an interesting post on hackaday a while ago about getting this to work using a Wiimote https://hackaday.com/2016/08/30/tricking-duck-hunt-to-see-a-...

Wonderful!

It would seem that if you're modifying the cabinet to use a different type of monitor, you could also modify the gun. Is there some kind of light gun-like technology that does work on LCDs?

The issue is that you'd also have to modify the software. A traditional lightgun can't tell the game where it recorded a hit, only when. It's up to the software to translate the timing information it got from the lightgun into a position on the screen. It accomplishes this by being synchronized with the electron beam of the CRT and thus having an accurate model for the position of the beam over time.

VLSI CRTC's like the 6845 had a light pen input and a register that captured the character position at the time it fired. You could read that and translate it. It was a small matter of integration to make the input work. This chip was meant for text display but the PC and other systems used it for pixel-based displays too.

WRT to the light guns, I never quite really understood why you couldn't somehow simulate this with LCDs. Isn't the refresh rate on LCDs higher? So couldn't you work around that somehow? Obviously the answer is prob. not since no one (that I know of) has done it, but I was genuinely curious of what precisely the technical limitation is.

LCDs cannot simulate this because their latency is too high. By the time the pattern is on the screen the lightgun has already recorded a miss.

The way light guns work, they rely on the fact that a white frame is not displayed all at once but "drawn" by the CRT ray - if you flash a single white frame, one location will flash white slightly later than other, and you can infer the location from the timing.

LCDs, well, change all the frame at once. The issue isn't with the frame rate but at what happens when the device is drawing a single frame change.

But following the same logic as GP, if you have a higher refresh rate, can't you just split the frame into several frames where you simulate that refresh scan as an animation?

Not in practice. To completely simulate the CRT refresh, you'd need to flash each pixel separately along the electron gun path. If your LCD has 1,000,000 pixels, you'd need 1,000,000 times faster refresh rate to do that.

>LCDs, well, change all the frame at once.

Is this true? It doesn't seem like it is, if it were then we'd not experience tearing in games, etc.

Tearing is, IIRC, a product of only part of the next frame being rendered when the refresh happens.

Screen tearing is, iirc from the gpu buffer only partially overwritten with the next frame when it's sent to the screen.

Tearing is a property of the output from the GPU. The data takes time to send, and the GPU can be directed to send different data halfway through the process (or the memory it's in the process of sending can be modified, etc.). The display just shows what it's sent... it shows it as it receives it (CRT), or it buffers it up and draws it later (LCD), but if there's a tear, you'll see it just the same.

There may be intermediate buffering, but the panel is still updated in a scanning fashion.

Obviously not, there is no memory inside the glass, and bandwidth is limited. LCD screens still scan in.

The difference is LCD keeps picture until told otherwise, whole picture is emitting light. CRT has limited phosphor persistence, there is only one point/pixel being lit at any given time.

https://www.youtube.com/watch?v=nCHgmCxGEzY

The latency on LCDs is much much higher, often as much as several frames. This could be removed but only at the cost of disabling all manufacturer's image post-processing including upscaling.

I have a "100Hz" CRT television that won't support light guns because the manufacturers decided they'd put this kind of image-"improving" technology in.

There are actually two techniques for light gun operation, one of which is "racing the beam" (unreliable in the presence of high retention phosphor) and the other of which is to draw a single frame of white squares for targets. https://en.wikipedia.org/wiki/NES_Zapper#Technical_details

I remember buying a Playstation2 light-gun as the console was already in its "sunset period" and when i got home i found out it wouldn't work on my TV. My TV was a late CRT model before flat screen LCD took-over and this particular CRT had 100Hz refresh rate.

Later than that; PS2 and Dreamcast.

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