CRTs and Strobe crosstalk / Double image

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Joined: 04 May 2019, 09:12

CRTs and Strobe crosstalk / Double image

Post by Talrivian » 15 Aug 2019, 21:11

I've been researching and absorbing everything I can about monitors and all of the different artifacts and technologies and I recently procured two CRTs and am interested in CRTs and how they draw images vs. LCDs. I watched a youtube video of a CRT drawing mario at 380,000 FPS! It was incredible. I never knew that they drew an image basically one pixel at a time like a laser pointer. Very fascinating.

My question is this:

Seeing how modern LCDs can create double images if you strobe the back light and use a refresh rate that is double your actual FPS, say 30 FPS and 60hz, what happens with a CRT?

What I find interesting is that a CRT creates an image so thinly that it's almost like a strobed backlight that also strobes a portion of the screen, sort of like an OLED, except it does it one pixel/pixel line at a time!

If you use a video source at 30 FPS on a 60hz CRT, does it flash the image twice as you track it with your eye, causing a double image? Or do CRTs work differently from LCDs? Do they only draw the image once as needed? Do CRT refresh cycles act like Gsync in a way, sort of like a "maximum refresh rate"?

I am very fascinated by this and I am slowly becoming with monitors and the technology behind it! I can't believe I've been playing PC games obsessively for years and only just know exploring this side of technology, which is arguably the most important part of your PC!

Also, on a side note, if OLEDs can be strobed, why not just draw images one pixel line at a time like a CRT? Clearly this would give the best possible motion blur reduction imagineable, since there's no strobe light to sync with a refresh rate since the pixel ITSELF illuminates?

Sorry for the rambling!

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Re: CRTs and Strobe crosstalk / Double image

Post by Jason38 » 15 Aug 2019, 21:45 Chief talks about the double images of CRT in this post.

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Re: CRTs and Strobe crosstalk / Double image

Post by Jason38 » 15 Aug 2019, 21:47 ... -and-hold/ This is also a mandatory read from Chief.

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Re: CRTs and Strobe crosstalk / Double image

Post by Chief Blur Buster » 16 Aug 2019, 16:23

Two different things with different causes...

There's an important distinction between strobe crosstalk and duplicate images.

The Double Image Phenomenon

All impulsed displays CRTs, plasmas, strobed LCDs -- anything that uses pulsing, flicker, strobing, flashing, phosphor, etc -- will be prone to this.
How To Fix: Use frame rate matching refresh rate, even if you have to lower your refresh rate, upgrade GPU, switch from console to PC, and/or reduce detail level, in order to achieve this.


The Strobe Crosstalk Phenomenon

This occurs when pixel transitions (GtG) is incomplete between strobe flashes. LCDs takes time to refresh pixels. For zero strobe crosstalk, GtG needs to be completed in the black interval (backlight turned off) before the screen is flashed again. Check High Speed Videos Of LCD Refreshing to see how a non-strobed LCD refreshes. Observe how the GtG fade zone can interfere with the ability to strobe the LCD. For more information see Strobe Crosstalk FAQ.
How To Fix: Get the fastest LCD you can get, TN panel technology, with the best strobe-tuning (e.g. NVIDIA ULMB), and use a slightly lower refresh rate to make sure that there is enough refreshtime for GtG to complete in the blanking interval between refresh cycles. And obviously, make sure framerate = refreshrate too, to avoid the other double-image phenomenon too.


The best strobe backlight LCDs will be closer to 1%-3% (e.g. NVIDIA LightBoost default setting, or NVIDIA ULMB with intentional digital contrast-range reduction) and the worst strobe backlight LCDs will be closer to almost 25% (e.g. poorly tuned VA LCDs)

Can Both Of The Two Causes Combine?

Unfortunately, yes.
Doing both at the same time (half frame rate on a strobed LCD display) such as 60fps at 120Hz on ULMB will create 3 images:
- The usual universal double image phenomenon
- Followed by a faint strobe crosstalk copy

TL;DR: Strive for framerate=Hz on impulsed displays. And strive for one of the better brands of strobing (and on a TN LCD) if you want strobing.
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