How Cathode Ray Tubes Work [video] | Hacker News Reader

How Cathode Ray Tubes Work [video]

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60 pointshggh41 comments

Great video. Seeing what it looks like inside with the phosphor and the mask was so enlightening. I also recently watched the one by Technology Connections, which was quite good: https://www.youtube.com/watch?v=l4UgZBs7ZGo

I feel like I have a much better basic understanding of CRTs now. They felt like utter magic before. Now it just feels a bit like magic. The only thing that still confuses me is how the electron gun itself actually works. I don't really get how electrons are being shot at all and why we can shoot them in a beam.

Think of the electrons not as being shot, but as being boiled off a cathode where there are a lot of them, and then suddenly they find themselves attracted to an anode which has nothing blocking them from reaching it.

Don't think of the bullet as being shot, but as being released and finding itself attracted to a low pressure space with nothing blocking it from reaching it. ;)

The basic idea is you have a source of electrons and a way to get at least some of them going where you want. A simple way to do this is to have a hot charged filament (source of electrons) and a plate with a hole to attract them. Check out a google image search for "electron gun schematic" to get a better idea.

You can also build a glow discharge gun and use an aperture which is way simpler than a hot cathode.

One thing that I still find magical is vector graphics on a CRT, even when playing games like Asteroids or Lunar Lander. AFAIK there is no way for any other display to replicate the intensity of vector graphics, except on a real CRT. It would be a sad day when they eventually die off.

Maybe not the same thing but you could do something similar with a lasers and I'll let this random video I just looked up show how it works: https://www.youtube.com/watch?v=MsaYcX5aIac

Looks like there are some commercial products like that as well.

Great video and explanation, but he still doesn’t really explain how the magnets in the monochrome CRTs actually draw the image. How do they know where to point? How does it move so fast?

For this, I recommend the Technology Connections video https://www.youtube.com/watch?v=l4UgZBs7ZGo. He actually demonstrates how the beam is redirected by the electro-magnets wrapped around the base of the tube. It's very cool.

> How do they know where to point?

With a VGA signal, a pulse on the vsync line tells the monitor to start at the top left. A pulse on the hsync line tells the monitor to move down a row. In between those pulses, the analog values of the RGB values determine the color as the beam scans left to right. The CRT is dumb -- it's up to the device sending the signal to time the pixel values correctly so they hit the screen as the beam scans over the correct spot on the screen. If you were making a device that needed to display analog video, you'd have to make sure the signal you were outputting would make the electron gun fire at the right time. (e.g.: http://www.tinyvga.com/vga-timing)

> How does it move so fast?

They're electromagnets, so they move in sync with the timed signal sent to them.

In theory the electromagnets controlling the vertical and horizontal sweep of the electron beam should react instantly to the incoming sync signals but in practice it's much more complicated (because the magnetic fields store energy which you need to deal with somehow). Bob Anderson has an entry in his series on the general process of restoring vintage televisions which goes in depth on the topic: https://www.youtube.com/watch?v=HdEfo8jS3FI&list=PL4aHiwoXvL...

With CRTs, how did the input device know when to fire in sync with the CRT? I assume the CRT was operating on some factor of mains power for its timing?

The input signal combines video amplitudes (positive voltages) and sync pulses (negative voltages) which tell the CRT when to move the beam around. Technically composite video is AC-coupled so I'm not sure how sync pulses are actually identified (but video brightness is relative to the porch voltage right after a sync pulse). Not sure if antenna TV RF signals are DC or AC coupled.

I believe there are timing pulses in the blank area of the signal that it syncs to. But it depends on the signal.

The CRT fired in sync with its input signal, not the other way around. Timing was done through HSYNC and VSYNC as well as the blanking intervals - the area after the sync signals where no image signal was to be sent. VSYNC was locked to the net frequency on televisions and early monitors but could be configured within certain bounds on later models, HSYNC depended on the intended vertical resolution ((Vres + blanking/shelf) * VSYNC = HSYNC), the dot clock decided the horizontal resolution. Positioning was relative and depended on how the monitor was adjusted - horizontal and vertical deflection, pillow deformation, trapezoidal deformation, etc. CRT tubes had no concept of 'pixels', those were a function of the dot clock/HSYNC/VSYNC. Colour tubes had RGB triplets in either delta shape - three dots in a triangle in combination with a shadow mask with round holes - or next to each other for in-line, those with either a shadow mask (a metal plate with loads of slits) or a wire mask (thin vertical metal wires strung between the top and bottom of the CRT. There was no fixed relation between display pixels and those triplets, especially on later high-bandwidth multi-sync monitors this could result in display pixels taking up less than a colour triplet which led to odd colour effects on small details. I used a 17" CRT at 1600x1200 which often showed such effects on the small fonts I tend to use so as to cram as much as possible on the screen.

For how they know where to point, that's something that's calibrated individually for each set. Typically the initial calibration is done at the factory (or, if you've done a major service on a set, by the technician who did the service) and then tweaked by the end-user to their own preferences. That's why all CRT displays have adjustments at least for hoizontal and vertical size and position (offset). Fancier displays have even more adjustments for other geometric corrections like pincushion and keystone.

Here's a video demonstrating how to do a full setup (calibration) on a vintage color CRT (timestamp 16:02 for the start of the procedure): https://youtu.be/Oeylkj1vap8?t=962

The answer depends on the level of the detail you are asking for. There is a generic explanation, slightly more engineering / scientific one, relying on basic electrical engineering knowledge with not much nuance one and finally full detailed engineering explanation with a whole bunch of calculus and nuances one.

CRTs are yet another circuit component. They rely on electrons moving or getting stored in them to "work". If you increase the voltage, the electrons will move faster through the gun and crash into the phosphor coating with more kinetic energy. Or one can increase the current i.e. the number of electrons hitting the CRT per unit of time. Both results in a brighter spot. Both requires one to increase voltage of some circuit at some point of the upstream.

Since the TV signals are also an observable piece of voltage information, which we receive via an antenna, we can create circuits that are capable of detecting that voltage and simply act on its strength. We can create a circuit that acts when a higher voltage level is observed and cause the dot to be brighter.

We can create a circuit that has repetitive behvaior like changing the magnetic field in the deflection coils such that it periodically moves left to right. So it will draw a line.

We can also create circuits that act on time-dependent behavior like the voltage dipping down for a tiny fraction of a second to a level that cannot produce a dot. So we can detect when we should move the deflection coil's magnetic field slightly downwards to draw the next line. Or even bigger dip to reset the position all the way to top.

When you combine all those circuits into one big Rube-Goldberg machine and adjust the thousands of literal and figurative knobs, you'll get a TV.

In the old days, our cars were powered by gasoline and every house had a ~25KeV particle accelerator in the living room.

There's an old site with a nice run down of building simple DIY CRT's using a glow discharge electron gun and laboratory flasks: http://www.sparkbangbuzz.com/crt/crt6.htm The glow discharge stuff is very easy to build as you dont need a filament/heater and power supply, it's a cold cathode gun. I wound up coming across another simple and fun project that demonstrates a very simple glow discharge gun in a glass bottle: https://hackaday.com/2011/08/30/diy-electron-accelerator/

I built one of those bottle accelerators but with some mods using a smaller Stuart's soda bottle. Cathode is a 0.5 in diameter Al spacer bushing mounted to one end of a 0.25 inch aluminum tube punched through a rubber stopper and the other to a tube adapter screwed to a KF25 vac flange. I then rigged up a stand to hold a vacuum manifold connected to my Alcatel 2008 vac pump and could pull a nice vacuum down to 0.4 Pa. The bottle gun's anode is a loop of 12 AWG copper wire inserted into a drilled hole and sealed with Faraday wax (safe and easy to make but messy and does not easily clean up.) Finally, wired it to my 200W Bertan +10kV 20mA supply. The +10kV lets me setup the gun as a common cathode putting the electron gun parts at ground potential. Only the anode is at high potential. I then decided to see if I could focus the beam and MacGyverd a lens out of a big 2 inch conduit bushing I had lying around in the electrics bin. Wound a bunch of 20 AWG magnet wire around it and found that building a big enough field to focus and sat it around the bottle neck. That big hunk of steel took a crap ton more power than I thought - I could focus the beam to midway of the bottle at around 300W (30V DC @ 10A)into the focus lens (yes it got very hot)! At full beam power I can melt holes in tin foil sheet using a focused beam.

Nothing makes me feel older than seeing a youtube video trend about CRTs. This was part of my high school curriculum.

Trinitrons and workalikes were very good, but the shadow mask displays that this video focuses on to my memory didn't have the excellent contrast and black levels cited. I remember most of them looking pretty dull, and downright silvery if there was much light in the room.

When I was a kid we had a Trinitron. I used a magnet on it once and pretended not to know anything about it as my dad called the cable company. Apparently it was able to degauss itself because the color went back to normal eventually.

Those trinitrons looked amazing. I had a 22” I had bought myself when I was a senior in high school.

But god were they heavy.

That’s a beast. Largest CRT I ever had on a desk was 21 and that was nuts.

Anyone knows why they were so much heavier? Just size difference or some other interesting reason? :-)

Be interesting if we kept developing this tech. Where would CRT's be now?

What's interesting and possibly surprising is that the core tech didn't evolve much past the 50-60s. Sony's Trinitron were the pinnacle, prior to modern inputs and HD. The original patent expired in the mid 90s and everyone else raced to implement their version of it.

So, if evolution had continued, _I think_ we'd have marginally lighter and more efficient displays with HDMI et al and the surrounding electronics and the "smart" features seen on modern screens.

Not sure about them being lighter. The vast majority of the weight of a CRT was the thick glass on the front. CRTs that claimed to be "flat" used very thick glass that was curved like a lens so that the front the viewer saw was flat. The larger the screen, the thicker the glass would need to be to keep the edges/corners from bowing. That glass was very heavy. The 32" 16x9 broadcast Sony monitor we had was extremely heavy and required two people at a minimum to carry/place it.

I had a 32” 4:3 trinitron I used to use for vintage gaming and damned if that thing didn’t weigh 130 lbs.

I've got a 36" in my office and it's a monstrosity. I need to vacate my office in the near future and moving it out is the part I'm least looking forward to.

Granted, I was a much younger man when I moved it in but I kind of can't believe I did it by myself with only the help of a skateboard and some movers' straps. It's a wonder I didn't kill myself or the set. There's no way I'm doing that again.

How long has it been in it's current position? I ask, as ours had spent years in the exact same spot without moving until the studio was moved into a new location. The new location was 90° from where it had been previously. During that time the unit had become aligned to the earth's magnetic field so that in the new location we had to do something specific to it to "undo" that. I wasn't around when it actually happened, but it took some serious research to get it resolved. It was something that was not unheard of, and once the right person was contacted and had the issue described he immediately asked if it had been recently moved. Sorry I can't give you the procedure, but at least you'll be aware of potential thing that can happen when you move it

Wow. That's not an issue I'd heard of either. How long was it there?

Mine has been in its current position for about five years. I was being slightly hyperbolic about being much younger -- but it was pre-40 and pre-kids, so not too much.

It was there at least 5 years as well. We might be talking about different monitors though. This one was the flagship broadcast reference monitor not a consumer TV if that has anything to do with it.

There has been one innovation, the SED: https://en.m.wikipedia.org/wiki/Surface-conduction_electron-...

In this technology, each pixel has its own tiny electron emitter, thus the display is like a grid of tiny CRTs. It allowed for CRT-quality contrast and color reproduction but with the size and weight of an LCD system, with similar power draw if not less.

But LCDs are cheaper to produce, and they just sucked all the oxygen out of the market.

CRTs are still in production.

https://www.thomaselectronics.com

For military applications (e.g., fighter jet HUDs and instruments) only, at considerable expense.

CRT could've definitely been preferred over LCD by many gamers if it was innovated on at the same pace, and for watching video as well. CRT has true blacks and very low input lag. HDR on CRT could probably be possible as well. The downside is that many others don't want to deal with the size of CRT displays, that's why this never happened.

But it's not needed as OLED combines the benefits of both but better. OLED just has to go down in price...

NanoLED will most likely replace OLED in the future.

CRTs only had 'true blacks' in darkened rooms, otherwise you'd see the phosphors which, especially on older models were more 'gray' or 'greenish' than black. They also had quite a bit of light bleed from neighbouring image areas due to reflections in the tube front which by necessity had to be thick because it had to be strong enough to a) keep its shape and b) not implode under the near vacuum in the tube.

How much more size and resolution did you want? A 4k 16:9 CRT would probably cost you $3999 today, and the weight would require you to also buy an entirely new desk.

Cheaper than an Apple Pro Display XDR ($4999 to $5999).

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