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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

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 2y ago
For this, I recommend the Technology Connections video https://www.youtube.com/watch?v=l4UgZBs7ZGo 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 2y ago
> 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 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 2y ago
  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=PL4aHiwoXvLGA6lr13xGxkxFV3Y20pv6Nb&index=15 https://www.youtube.com/watch?v=HdEfo8jS3FI&list=PL4aHiwoXvL...
- dclowd9901 2y ago
  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 2y ago
    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 2y ago
    I believe there are timing pulses in the blank area of the signal that it syncs to. But it depends on the signal.
  - hagbard_c 2y ago
    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.
MrRadar 2y ago
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 https://youtu.be/Oeylkj1vap8?t=962
okanat 2y ago
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.

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 2y ago

For this, I recommend the Technology Connections video https://www.youtube.com/watch?v=l4UgZBs7ZGo 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 2y ago

> 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 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 2y ago
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=PL4aHiwoXvLGA6lr13xGxkxFV3Y20pv6Nb&index=15 https://www.youtube.com/watch?v=HdEfo8jS3FI&list=PL4aHiwoXvL...
dclowd9901 2y ago
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 2y ago
  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 2y ago
  I believe there are timing pulses in the blank area of the signal that it syncs to. But it depends on the signal.
- hagbard_c 2y ago
  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.

MrRadar 2y ago

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 https://youtu.be/Oeylkj1vap8?t=962

okanat 2y ago

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.