[HN Gopher] How Cathode Ray Tubes Work [video]
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How Cathode Ray Tubes Work [video]
Author : hggh
Score : 46 points
Date : 2024-06-23 20:35 UTC (2 days ago)
(HTM) web link (www.youtube.com)
(TXT) w3m dump (www.youtube.com)
| Sakos wrote:
| 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.
| emchammer wrote:
| 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.
| willis936 wrote:
| 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. ;)
| Cerium wrote:
| 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.
| MisterTea wrote:
| You can also build a glow discharge gun and use an aperture
| which is way simpler than a hot cathode.
| Bluecobra wrote:
| 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.
| drivers99 wrote:
| 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.
| darajava wrote:
| 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?
| Sakos wrote:
| 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.
| kube-system wrote:
| > 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.
| MrRadar wrote:
| 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.you
| tube.com/watch?v=HdEfo8jS3FI&list=PL4aHiwoXvL...
| dclowd9901 wrote:
| 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?
| nyanpasu64 wrote:
| 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.
| sixothree wrote:
| I believe there are timing pulses in the blank area of the
| signal that it syncs to. But it depends on the signal.
| MrRadar wrote:
| 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
| okanat wrote:
| 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.
| lawlessone wrote:
| Be interesting if we kept developing this tech. Where would CRT's
| be now?
| ethagnawl wrote:
| 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.
| dylan604 wrote:
| 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.
| dclowd9901 wrote:
| I had a 32" 4:3 trinitron I used to use for vintage gaming
| and damned if that thing didn't weigh 130 lbs.
| bitwize wrote:
| 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.
| maxwell wrote:
| CRTs are still in production.
|
| https://www.thomaselectronics.com
| bitwize wrote:
| For military applications (e.g., fighter jet HUDs and
| instruments) only, at considerable expense.
| uyzstvqs wrote:
| 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...
| madspindel wrote:
| NanoLED will most likely replace OLED in the future.
| 1970-01-01 wrote:
| 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.
| ycombinatorics wrote:
| Nothing makes me feel older than seeing a youtube video trend
| about CRTs. This was part of my high school curriculum.
| z500 wrote:
| 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.
| dclowd9901 wrote:
| 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.
| sixothree wrote:
| That's a beast. Largest CRT I ever had on a desk was 21 and
| that was nuts.
| MisterTea wrote:
| 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.
| Clamchop wrote:
| 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.
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