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How does a screen work?

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Re: How does a screen work?

#71

Post by perching_aix »

LocalH wrote: post #32

I have to take issue with the usage of the terms "pixel" and "subpixel" with regards to CRT. CRTs do not display discrete pixels . They display discrete scanlines , each one made up of a smoothly varying voltage across the line (and thus resolution is a function of both the DAC in the display device in the case of systems that generate a digital signal and then convert it to analog for display, and the hardware insid…

I don't think this is an entirely fair characterization either. Note that everything I lay out here is just based on accumulated information gathered over the years due to vague interest, I haven't worked on or with CRTs (did use them though).

Monochromatic CRTs were well and truly resolution agnostic, there were legitimately no pixels or subpixels or anything similar to speak of. That said, the driving signal still had to be modulated to produce an image, and so it's not magic either. You can conceivably represent [0] all the available information in them using just 720 samples per line, which is exactly why DVDs had that as their horizontal resolution (720 pixels).

This story changes a bit though with color CRTs, where you did have discrete sets of patches of different phosphor chemistries called triads. There was absolutely a fixed number of them on a glass, so you could conceivably consider that as the native resolution for that given display, with each triad being a pixel, and each patch being a subpixel. The distance between these was the aperture pitch, much like how you have a pixel pitch on a typical flatpanel display.

The kicker then is that as you say, there's no strict addressing. From what I understand there were multiple electron guns scanning across the screen simultaneously, only being able to hit the specific color they were assigned, but the patch they were hitting wasn't addressed, they just scanned across the screen like the single electron gun did in monochromatic CRTs. You'd then get resolution invariance by just the natural emission spread providing you with oversampling / undersampling without any kind of digital computational effort. It's not really true resolution independence like with the monochrome ones, I'd say. I even recall articles where they were testing freshly released CRT monitors, and discussing how sharp the beam was, resulting in what kind of resolution adherence.

[0] an earlier version of this comment said "extract from" here; for various reasons you might already know, that's a different thing, and would not actually be true.

Re: How does a screen work?

#72

Post by _kb »

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 certainly one the bits of tech that would be an insane pitch if invented today: “Yes, please stick your face in front of this particle accelerator. I assure you, it’s completely safe. You may experience some visual side effects but that’s completely intended.”

Re: How does a screen work?

#73

Post by peatmoss »

bitwize wrote: post #69
perching_aix wrote: post #67

Earlier quoted context omitted.

There were a few things I personally found lacking as well, albeit they're fairly minor. Regarding CRTs, at the vector CRTs section, they mention "they were mostly monochrome and so the phosphor dots could be tightly packed" - this is not true either I believe, monochrome CRTs had a uniform phosphor coat on the inside, no subpixel patches. I'd have also liked if they delved a bit into the decay times of the various p…

> Regarding CRTs, at the vector CRTs section, they mention "they were mostly monochrome and so the phosphor dots could be tightly packed" - this is not true either I believe, monochrome CRTs had a uniform phosphor coat on the inside, no pixel patches. This is one of the reasons why emulated versions of Asteroids (arcade game) can never match the real thing: the razor-sharp, perfectly straight lines with zero aliasing…

My first and only time playing Asteroids in its original vector CRT glory was a bit shocking to me. I remember the evolution of CRTs in the 80s and 90s. My family had a small B&W CRT that fascinated me with its image quality, in that it always reminded me more of a b&w photo than color CRTs reminded me of color photos.

Still, that vector CRT that I saw perhaps a dozen years ago was quite a surprise. Lack of rastering and the utterly insane brightness sent me down a rabbit hole. I ultimately concluded I'm not ever likely to own a basement Asteroids cabinet.

Re: How does a screen work?

#74

Post by massung »

I know this is HN, but I honestly clicked this link initially thinking it was going to be a detailed analysis of how screen plays are constructed and executed in football.

It’s late and time for bed! LoL.

Re: How does a screen work?

#75

Post by MarcelOlsz »

massung wrote: post #74

I know this is HN, but I honestly clicked this link initially thinking it was going to be a detailed analysis of how screen plays are constructed and executed in football. It’s late and time for bed! LoL.

I was expecting a Ciechanowski link lol.

Re: How does a screen work?

#76

Post by conradev »

kurthr wrote: post #66
perching_aix wrote: post #64

There are some sentences in this that are technically vague enough to pass, but I don't think are strictly speaking correct, and I believe will likely lead to a mistaken understanding: > modern displays don't paint the image line-by-line (...) They light up each pixel simultaneously, refreshing the entire display at once. The entire screen area is lit all the time now, yes, but refresh still typically happens line by…

I was glad they at least mentioned how IPS (PLS) and VA differ from older TN. But you're right both LCD and OLED refresh a stored voltage on the cell (or caps) on a roughly line by line (OLED can easily be 5 clocks on the GIP to cancel internal transistor offset voltages). I was mostly annoyed that they didn't mention the circular polarizer on OLEDs. Although there is discussion of going to color filters with Quantum…

The article also assumes all LCDs are transmissive, and the bulk are, but reflective (and transflective) LCDs are a thing.

Re: How does a screen work?

#77

Post by userbinator »

perching_aix wrote: post #64

There are some sentences in this that are technically vague enough to pass, but I don't think are strictly speaking correct, and I believe will likely lead to a mistaken understanding: > modern displays don't paint the image line-by-line (...) They light up each pixel simultaneously, refreshing the entire display at once. The entire screen area is lit all the time now, yes, but refresh still typically happens line by…

I don't know how common it is now, but a lot of high-resolution LCDs with dual LVDS interfaces were essentially two separate panels, with one lane feeding the top half and the other the bottom half.

Re: How does a screen work?

#78

Post by charcircuit »

layer8 wrote: post #56
bgnn wrote: post #52

Earlier quoted context omitted.

I'm not sure about this calculation though. Phosphor decays exponentially with a time constant of roughly 5ms (according to HP [1]). This means when a new frame comes at 60Hz refresh rate there is still 10-15% of the previous frame related excitation is present. This means there is considerable amount of nonlinearity, hence the performance is even worse than 10ms LCD/OLED displays. Genuine question: why do you think…

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.

Re: How does a screen work?

#79

Post by kuhsaft »

eastbound wrote: post #11
jagged-chisel wrote: post #9

Earlier quoted context omitted.

When I learned how TV worked at the beginning of television history, I found it super cool that the camera and all the TVs across the country had their scanning beams synchronized. That camera was driving your TV, almost literally.

I only recently found out that the tech to save images wasn’t invented, so they couldn’t display a revolving logo between shows. So… so the BBC had a permanent real-life logo with a permanent camera in front of it. So yes, any image was extremely ephemeral at the time. PS: Apparently it’s called a Noddy, it’s a video camera controlled by a servomotor to pan and tilt (or 'nod', hence the name Noddy): https://en.wikipe…

I don’t think that was why the Noddy was used. At the time film and projection were available. They could have recorded film and projected onto a sensor for re-broadcast.

The Noddy was used since it was a live broadcast and “allowed the idents to be of no fixed length as the clock symbols could continue for many minutes at a time”.

So, it’s not really because they couldn’t store video. It’s because they needed an indefinite amount of video for the clock idents and couldn’t generate them digitally.

Re: How does a screen work?

#80

Post by krackers »

perching_aix wrote: post #64

There are some sentences in this that are technically vague enough to pass, but I don't think are strictly speaking correct, and I believe will likely lead to a mistaken understanding: > modern displays don't paint the image line-by-line (...) They light up each pixel simultaneously, refreshing the entire display at once. The entire screen area is lit all the time now, yes, but refresh still typically happens line by…

Assuming a 60Hz refresh rate, does it take ~16ms (± the vblank inteval) for a complete cycle from top-left to bottom-right? Or does the scan happen faster than that (with something else being the limiting factor on overall refresh rate)?
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