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How does a screen even work?

https://www.makingsoftware.com/chapters/how-a-screen-works
196•chkhd•6h ago

Comments

p44v9n•5h ago
So fascinating!
qwertox•3h ago
Not only was the initial diagram all/explaining, but the "pop"-"pip" on zoom-unzoom of the image was just as nice as playing with a sheet of bubble wrap.

Wow, and that ruler on the right side, even with the sound.

One of the nicest pages I have been on.

And the landing page... https://www.makingsoftware.com/

It just keeps on giving.

consumer451•2h ago
This appears to be a lovely project. I wish the author all possible luck and success. I haven't joined a mailing list in a very long time, but I sure did in this case.
mrbluecoat•1h ago
Adding my congrats as well. The combination of well-written explanations for the semi-technical layperson combined with clear, intuitive graphics is a powerful instruction platform.
retrac•3h ago
CRTs are still slightly magical to me. The image doesn't really exist. It's an illusion. If your eyes operated at electronic speeds, you would see a single incredibly bright dot-point drawing the raster pattern over and over. This YouTube video by "The Slow Mo Guys" shows this in action: https://youtu.be/3BJU2drrtCM?t=190
YZF•3h ago
There is some persistence in the pixels/phosphor though so it's not a complete illusion. But yes, your eyes are integrating over the frame. There is also interlacing...

I read something interesting recent but I'm not sure if it's true or not. That as you age your integration frame rate decreases.

jagged-chisel•3h ago
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.
eastbound•3h ago
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.wikipedia.org/wiki/Noddy_(camera)

hinterlands•3h ago
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.

f1shy•2h ago
And still it was possible as a side attack, with just looking at the reflected brightness of a screen, to get a perfect image back.
layer8•1h ago
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 accurate.

[0] https://blurbusters.com/wp-content/uploads/2018/01/crt-phosp...

[1] https://www.researchgate.net/figure/Phosphor-persistence-of-...

[2] https://www.researchgate.net/figure/Stimulus-succession-on-C...

wincy•1h ago
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.
hinterlands•1h ago
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 scanned by the beam while the shutter was open. The part above is the afterimage, which, while not as bright, is definitely there.

bgnn•17m ago
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 CRTs are better?

[1] https://hpmemoryproject.org/an/pdf/an_115.pdf

snovymgodym•3h ago
> It's an illusion.

In a sense, all vision is.

grishka•3h ago
All our senses are.
grishka•3h ago
To me the magical part about CRTs is color. I don't quite understand how the shadow mask works. Like, yeah, there are three guns, one for each color channel, and the openings in the mask match their layout, and somehow the beam coming out of each gun can only ever hit its corresponding phosphor dots. Even after being deflected. But... how? Also, wouldn't the deflection coils affect each of the three beams slightly differently?
pulvinar•2h ago
Each hole in the shadow mask acts as a pinhole camera, giving an inverted image (in electrons) of the three guns. All three beams get bent nearly the same amount, but yes there is some distortion which is traditionally corrected for by a set of convergence coils and corresponding circuit with knobs for static and dynamic convergence [0]. A pain to adjust, BTW.

[0] https://antiqueradio.org/art/RCACTC-11ConvergBoardNewRC.jpg

Sharlin•2h ago
It's parallax, basically. The pigment dots and mask holes are positioned such that when you look from the perspective of the "red" electron gun (*), you only see red pigment dots. Move a couple cm to the "blue" gun and the parallax shift now makes you to see only blue pigment dots instead. Or from the other direction, no matter which "red" dot you stand at, you only see the "red" gun through "your" hole.

The exact sizes, shapes, and positions of the pigment dot triples (and/or the mask holes) are presumably chosen so that this holds even away from the main axis. Also, the shape of the deflecting field is probably tuned to keep the rays as well-focused as possible. Similarly to how photographic lenses are carefully designed to minimize aberrations and softness even far from the optical axis.

(*) Simplifying a bit by assuming that the beam gets deflected immediately as it leaves the gun, which is of course inaccurate.

somat•1h ago
For me it was the opposite. Learning how a monochrome CRT requires no mask sort of destroyed my world view of what a display had to have. pixels(even the quasi pixels as found in a color CRT mask) were not actually required or present.

As a result monochrome terminal text has this surprising sharpness to it.(surprising if you are used to color displays). But the real visual treat are the long persistence phosphor radar scopes.

grishka•25m ago
That's the cool thing about analog video, it doesn't really have the concept of horizontal resolution. Especially when it's monochrome. It's made up of lines that continuously change brightness as they're drawn.

Color composite video, as far as I understand, does have a limit to the horizontal resolution because in all three standards the color information is encoded as a high-frequency signal added to the main (luminance) one, so that frequency is your upper limit on how quickly the luminance can change.

S-video, VGA, and component should, in theory, allow infinite horizontal resolution and color.

charcircuit•3h ago
>The fact that they ever made it out of the research lab and into our homes is astonishing to me.

What is astonishing about LCDs? I don't mean to diminish the difficulty of scaling up the process, but if you think of early LCD displays they don't seem farfetched to be shipped to consumers.

YZF•3h ago
One random example is that your eyes are extremely sensitive to the tiniest defect or variation. So making a large display that looks good and is uniform is very challenging. Not to mention scaling up all the various processes like the photolithography and working with very large and thin glass panels.

It's all engineering but it's surprisingly hard to move things from the lab to manufacturing at scale. Years and years and lots of problem solving. Some efforts/approaches fail and you never hear of them.

charcircuit•2h ago
You are moving the goal posts from it being a consumer product to requiring it to be large, good, and uniform. Yes, there was engineering work to make such a consumer product, but it is something I would expect there to have been line of sight for.
YZF•2h ago
I was just giving one example.

The first LCD products I remember were things like 7 segment digital watches and calculators where the LCD was passive and the "pixels" were large. I am not super familiar with how that went from lab to consumer product but I imagine even there it was non-trivial.

It took a long time to progress to modern LCD displays. It took years to get from small black and white displays, to small color, to larger and larger displays. Productizing this stuff includes building machines, factories, ASICs, and figuring out a lot of technology as you go along.

Some interesting history here: https://www.varjukass.ee/Kooli_asjad/Ylikool/telekom/displei...

charcircuit•1h ago
I'm not saying that it wasn't nontrivial, but that it wasn't surprising that it was able to happen.
YZF•3h ago
Put a magnifying glass on your LCD display and you can see the sub pixel pattern...

A few decades ago I worked on a huge machine that made LCD color filters.

ryandamm•2h ago
Or just a drop of water…
nu11ptr•3h ago
Am I the only one who read this as the terminal program "screen" (the terminal multiplexer)?
nyarlathotep_•3h ago
Ha, I did that too.
vicurve•3h ago
It was 50/50 for me as well but the screen source code is fairly readable and if I remember right eerily over-commented for Unix code! The function names actually make sense.
ksec•3h ago
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.

hinterlands•2h ago
I think it's fairly common for technologies to get really good just as they're becoming obsolete. Vacuum tubes, CRTs, optical disks, photographic film... in fact, they're often in some respects better than the early generations of the technology that replaces them.

But OLEDs just have too many advantages where it actually matters. Much lower power consumption, physically more compact (no need for backlight layers), etc.

kec•1h ago
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.
vicurve•3h ago
I can appreciate these articles as they are but I personally don’t like them. They are junk food level of infotainment to me. Something I’d find on a Wikipedia summary section that covers general points.

A CRT - to name one - is a device whose actual understanding will challenge people in profound ways. To ask “how does a screen even work?” and to begin to answer this question will require a bit more than a summary form of “thing goes from point A to point B”. The history of this discovery is a stack of books and in and of itself is fascinating - the experiments and expectations and failures and theories as to why and how. I suppose I just expect more of the site. The illustrations are nice. Oh and my moniker is just a coincidence.

meindnoch•2h ago
Agree. It is the equivalent of a coffee table book.
jorkingit•1h ago
A CRT - to name one - is a device whose actual understanding will challenge people in profound ways. To ask “how does a screen even work?” and to begin to answer this question will require a bit more than a stack of books of “the experiments and expectations and failures and theories as to why and how”. The history of this discovery is all of history leading up until that point and in and of itself is fascinating - the sociopolitical conditions and details of every single person's life and their astrological charts as to why and now. I suppose I just expect more of the site. Oh and my moniker is not a coincidence.
fabiensanglard•2h ago
The drawings are really good. Any idea what tool is used to make them? I emailed the author but have not heard back.
cyberlimerence•2h ago
From the FAQ on the main page:

> How do you make the illustrations?

> By hand, in Figma. There's no secret - it's as complicated as it look

benetttttt•2h ago
what a beautiful project. cant wait for this to be completed
Sharlin•2h ago
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.
pavlov•1h ago
Active matrix flat panels felt like incredibly cool technology when they became available in the 1990s.

Each individual pixel is driven by a transistor and capacitor that actively maintain the pixel state? Insane manufacturing magic.

Dead pixels used to be a big problem with LCD displays. Haven’t thought about that in at least twenty years.

Sharlin•45m ago
True – but on the other hand, it was "only" a few million elements, and very large ones, compared to, say, the DRAM chips of the time. Monitors certainly make the engineering feat more tangible, though!
LocalH•2h ago
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 inside the CRT monitor). Also, there is no mapping between any "pixels" represented within that varying voltage and the separate color phosphor dots.

Even "digital RGB" isn't digital in terms of the CRT. It's only "digital" because each color channel has a nominal on and off voltage, with no in-between (outside of the separate intensity pin). However, the electron gun still has a rise and fall time that is not instant.

Displays didn't truly become digital for the masses until the LCD era, with DVI and HDMI signals. Even analog HD CRTs could accept these digital signals and display them.

rahimnathwani•1h ago
Years ago I had an LG 32" wide-screen CRT TV. I chose that model because it had a VGA port. It advertised a resolution of 640x480.

I was thrilled when my computer let me choose a resolution of 848x480, and it worked perfectly.

Back in those days, the web was usable at that resolution.

mikepurvis•1h ago
It still basically should be, so long as well-designed sites give you the "small screen"/mobile layout.

Even apart from that, a lot of laptops still have 1280x800 as the default resolution, and that's only double the width of 640x480. Honestly, I'd actually be more worried about OS and browser chrome eating up the space than websites themselves being unusable.

rahimnathwani•58m ago
The 480 height is the bigger issue.

Try browsing on your phone in landscape mode.

swores•47m ago
> "It still basically should be, so long as well-designed sites..."

I believe that their point wasn't that "the web" has intrinsically changed, it was that too many sites are not well designed in this respect.

edit: they actually replied just before me and it seems that wasn't their point, but it would be my point (though I personally don't care about being able to use such a low resolution).

killjoywashere•1h ago
I happen to have a stereo microscope at my desk, so I put my Pixel 9 under there. At 100x mag (10x ocular x 10x objective) it looks like there are 3 layers: as I move my head around slightly (so the image is moving over my retinas), the blue moves faster and the red almost stays still, with green somewhere in the middle.
mikecarlton•1h ago
See also the author's page linked from the post. More very well-done content https://typefully.com/DanHollick
vojtechrichter•41m ago
Very cool looking website. Looking forward to the quality content
apricot•13m ago
This is a great project. I want to show it to as many teenagers as I can.

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