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How does a screen work?

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Re: How does a screen work?

#91

Post by myself248 »

rocqua wrote: post #82

I'm much more interested in the hardware driver. This thing gets digital encoded input, has to decode it, and then multiplex it to 8 million pixels. 60 times a second. Being able to hit at least 4 million different levels (talking about 4K 60fps 12 bit color). The input is roughly serial, so it takes a massive serial to parallel conversion.

But the output of the scaler chip is still serial, just now it's guaranteed to have the same dimensions as the panel itself, and includes whatever OSD overlay might be active, and any gamma/contrast/whatever adjustments. I believe in some cases, this is also where dithering takes place, to take a 6-bit panel and try to give it 7-bit (or, yikes, 8-bit) color depth by PWMing pixels that have intermediate values.

Look at the connector pinout of the panel itself. There's only 50 pins or so, and a lot of them are grounds. Whether the scaler-to-panel format is eDP, or LVDS (FPD-Link), or V-by-One, it's all still differential serial lanes at that point.

Around the perimeter of the panel, then, are the actual TCON and row/column driver chips, bonded right to the ITO traces on the glass, flip-chip-on-glass style. These have an outrageous number of pins, and directly connect to the gate (row) and source (column) traces. It's here that the serial becomes parallel, and the next stop is the transistors themselves (hence the MOSFET signal terminology of gate and source) in the individual pixels.

Older displays would have basically a bunch of serial-to-parallel register chips with each one's SO connected to the next one's SI. I ran across a fasincating video of replacing a bad chip in such a display, which happens to be gas plasma so the voltages involved are also pretty high too:

https://www.youtube.com/watch?v=6W3H5wOy5sY

Re: How does a screen work?

#92

Post by cubefox »

charcircuit wrote: post #78
layer8 wrote: post #56

Earlier quoted context omitted.

That HP reference is from 1970; CRTs did improve over time. The references I gave show that the intensity drops to below 10-15% within about a millisecond. The difference with LCD/OLED displays is that the latter are sample-and-hold, meaning that they show the image at full brightness for the duration of the whole frame. Their pixel response time may be faster than CRT phosphor persistence, but that is less relevant.…

>The problem with LCD/OLED is that they hold the picture for the duration of the frame Not necessarily. For example on VR headsets the LCD/OLED will only hold the picture for 10% of the frame.

Yeah, they do backlight strobing (LCD) or black frame insertion (OLED), to reduce blurring during smooth eye movements, at the cost of overall screen brightness. I actually think small CRTs would be perfect for VR headsets in this regard, as they are naturally have very short frame persistence.

One likely problem for battery powered headsets is the (I believe) relatively high CRT power draw. Another is probably the fact that they aren't used for anything else anymore, meaning CRT development has stopped a long time ago. There were quite small CRTs in the past for special applications, but probably not as small as is optimal for modern VR headsets. Both for optics and weight and space reasons.

Re: How does a screen work?

#93

Post by cubefox »

hinterlands wrote: post #45
layer8 wrote: post #38

Earlier quoted context omitted.

The phosphor still drops off very quickly [0][1][2], roughly within a millisecond. That’s why you would need a 1000 Hz LCD/OLED screen with really high brightness (and strobing logic) to approximate CRT motion clarity. On a traditional NTSC/PAL CRT, 1 ms is just under 16 lines, but the latest line is already much brighter than the rest. The slow-motion recording showing roughly one line at a time therefore seems accu…

I'm not quite sure what you're saying here. My assertion is that a visible image persists on the screen longer than it appears in the slo-mo clip. You can just point a camera with an adjustable shutter speed at a CRT and see it for yourself. Here's an example (might need to copy the URL and open in a new tab, they don't like hotlinking): https://i.sstatic.net/5K61i.png The brightly-lit band is the part of the frame s…

> The part above is the afterimage, which, while not as bright, is definitely there.

Yes it's there, but it's much less bright than the the scanned area, so it will be hardly perceptible relative to the bright part. The receptors in the eye will hardly respond to it after being excited so strongly by the bright part.

Re: How does a screen work?

#94

Post by cubefox »

kec wrote: post #35
ksec wrote: post #17

LCD on paper you see lots of drawbacks, in practice modern state of the art LCD for TV is pretty damn good. We will soon have RGB LED Backlight LCD with WHVA+ Panel that is about as wide angle as IPS, 95%+ REC 2020 colour, and 1-2ms response time. Phosphorescent blue OLEDs should reduce current OLED display energy usage by 20-30%. But it still seems to be way off for phones and mass usage.

None of that really helps LCDs primary downsides of poor contrast ratio and relatively high energy consumption. Backlit displays will always inherently score worse on these metrics vs self emissive displays.

The energy efficiency of LCDs is very good, typically better than OLED screens, except on very dark content.

Re: How does a screen work?

#95

Post by ksec »

kec wrote: post #35
ksec wrote: post #17

LCD on paper you see lots of drawbacks, in practice modern state of the art LCD for TV is pretty damn good. We will soon have RGB LED Backlight LCD with WHVA+ Panel that is about as wide angle as IPS, 95%+ REC 2020 colour, and 1-2ms response time. Phosphorescent blue OLEDs should reduce current OLED display energy usage by 20-30%. But it still seems to be way off for phones and mass usage.

None of that really helps LCDs primary downsides of poor contrast ratio and relatively high energy consumption. Backlit displays will always inherently score worse on these metrics vs self emissive displays.

>downsides of poor contrast ratio

In terms of TV. LCD have higher peak brightness. The Sony Bravia 10 will be out soon, hopefully it will showcase the world what LCD could be.

Not to mention cheaper at larger size panel.

Re: How does a screen work?

#96

Post by timw4mail »

nedt wrote: post #85
Sharlin wrote: post #31

CRT displays are one of those analog technologies that are arguably much cooler than their digital successors. Think – a literan raygun, a particle accelerator, inside your monitor, creating the image you're looking at.

It's much cooler if you haven't used it. That mass of a 19in was crazy. Then you had all of those that had the high pitched noise. And if you were unlucky you had to degauss them. Cool concept, but in practice the digital successors are better.

Mostly. But multi-scanning to different resolutions is something I'll miss.

Re: How does a screen work?

#97

Post by wincy »

BenjiWiebe wrote: post #58
wincy wrote: post #42

Earlier quoted context omitted.

I definitely like my new 240hz 4k oled HDR monitor, though. They're getting there! The data rate it's pushing through the displayport cable for uncompressed 4k HDR is something 80gb/s though. Absolutely mind boggling. Huge upgrade from my 1440p 165hz IPS monitor that had huge amounts of smearing when playing games.

What model is your new monitor?

The ASUS PG27UCDM 26.5" 4K UHD (3840 x 2160) 240Hz Gaming Monitor [0] paired with an RTX 5090 for my home desktop, but I got a USB switcher (for peripherals), and keep it on my standing desk that I plug in my work laptop too with a USB-C to DisplayPort cable. Only 60hz on the work laptop but I really like having a quad monitor setup in a T shape (3 27” monitors and the laptop plugged in with screen open below the central monitor, which is the OLED). It’s great for both productivity and for gaming. I turned off HDR for work, though.

The only annoying thing is every couple hours it asks me to run a 7 minute pixel refresh cycle to avoid burn in, but according to the dashboard I run it every 2.5 hours or so when I go on breaks, so I think I’m good.

Overall the monitor is just fantastic, my LAN party buddies and I dreamed about OLEDs like this back in 2003 and kept saying it was “just around the corner”. The biggest thing is in dark scenes in games there’s absolutely zero noticeable smearing.

[0] https://www.microcenter.com/product/689939/asus-pg27ucdm-265...

Re: How does a screen work?

#98

Post by perching_aix »

layer8 wrote: post #38
hinterlands wrote: post #12

Earlier quoted context omitted.

That slo-mo video is somewhat misleading, though. The phosphor glows for a good while, so there is a reasonable chunk of the image that's visible at any given time. The problem in that video is that the exact location the beam is hitting is momentarily very bright, so they calibrated the exposure to that and everything else looks really dark.

The phosphor still drops off very quickly [0][1][2], roughly within a millisecond. That’s why you would need a 1000 Hz LCD/OLED screen with really high brightness (and strobing logic) to approximate CRT motion clarity. On a traditional NTSC/PAL CRT, 1 ms is just under 16 lines, but the latest line is already much brighter than the rest. The slow-motion recording showing roughly one line at a time therefore seems accu…

> The phosphor still drops off very quickly [0][1][2], roughly within a millisecond.

It's phosphor chemistry dependent. Different color patches on the same glass would decay at different rates even. But yeah, 1 ms is a good lower bound, although when I last researched this, it was definitely the best case scenario for CRTs. I'm fairly sure the ~500 Hz OLEDs that are already floating around are beating the more typical CRTs of old already.

> That’s why you would need a 1000 Hz LCD/OLED screen with really high brightness (and strobing logic) to approximate CRT motion clarity.

At 1000 Hz you wouldn't need the strobing anymore (I believe?), that's the whole point of going that fast. We're kinda getting there btw! Hopefully with HDMI 2.2 out, we'll see something cool.

> On a traditional NTSC/PAL CRT, 1 ms is just under 16 lines, but the latest line is already much brighter than the rest.

That doesn't really math for me. NTSC would be 480 visible lines at 60 Hz, and so 480 lines / ~16.6 ms = 28.8 lines/ms (6% of the screen). Note that of course PAL works out to the same number: 576 lines / 20 ms = 28.8 lines/ms (just 5% of the screen here though!).

Re: How does a screen work?

#99

Post by Sharlin »

nedt wrote: post #85
Sharlin wrote: post #31

CRT displays are one of those analog technologies that are arguably much cooler than their digital successors. Think – a literan raygun, a particle accelerator, inside your monitor, creating the image you're looking at.

It's much cooler if you haven't used it. That mass of a 19in was crazy. Then you had all of those that had the high pitched noise. And if you were unlucky you had to degauss them. Cool concept, but in practice the digital successors are better.

Trust me, I remember. Cool does not equal convenient. Once a friend and I dragged three extra CRTs to a demo party just so we could put them side by side and write a program that displayed random scrolling messages. Controllable via an IRC bot, even! Today you could do that with a single ultrawide…
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