How a cathode ray tube works, and why one is fussy about what you send it

A flat panel is a grid of cells and a memory holding what each one should be. Send it a picture and it keeps it until you send another. A cathode ray tube has one moving spot of light, and no grid, no cells and no memory. A picture exists because that spot goes round the whole screen sixty times a second, and because the coating keeps glowing for a few milliseconds after the spot has moved on.

Driving it requires a signal timed to the movement of that spot.

00 · a metal box with a handle on it

Switch it onSlow it down+−Reset viewFull screen

Eight layers, in the order they come off

The set

Scroll to take it apart. Drag to turn it, pinch or hold control and scroll to come closer, and pick any part to read what it is. Everything that comes off stays in view, where it was lifted to.

A monitor is a box of air around one glass bottle, and every other part in it is there to bend the beam inside that bottle or to feed it. Switched on, the beam draws the way the set does: 48.9 microseconds of picture, 2.4 off at the right, 4.7 flung back to the left, 7.6 waiting there. Slowed to one line a second, all four are things you can watch.

Explore the eight illustrated layers

00 · The set

70 cm of moulded plastic, 24 kg, and a picture 34 cm across.

01 · The back

Behind it, air. A set is mostly the space the bottle needs.

02 · The chassis

Mains, supply, tuner, audio, and the line output stage.

03 · The tube

Held by four lugs at the corners, with a steel band round the rim.

04 · The neck

Two pairs of coils. One bends left and right, the other down.

05 · The high voltage

The part that decides what signal the set will take. 15.7 kHz, no other.

06 · Inside the glass

A cathode at 800 degrees and a stack of grids. The first one is the picture.

07 · The beam

48.9 microseconds across, 4.7 back. 15,730 times a second.

01The beam

At the back of the neck is a cathode, heated to about 800 degrees, with a coating that gives up electrons easily when it is hot. Around it sit a stack of metal grids at different voltages. The first one is the important one: make it more negative and fewer electrons get past it, make it less negative and more do. That grid is where the picture signal goes. A line of video is a voltage on it, changing a few million times a second. What this set resolves is nearer four to six megahertz, measured on the chain rather than assumed.

The rest of the grids focus what gets through into a beam a fraction of a millimetre across. The coating on the inside of the funnel, held at twenty odd kilovolts, then accelerates it the length of the tube. The electrons leave the gun slowly and arrive fast. That is why the glass at the front has to be thick, and why a tube of any size is heavy.

02Bending it

The beam would land in the middle of the screen and stay there. What moves it is the yoke: two pairs of coils clamped around the neck, one pair bending it left and right, the other pulling it down the screen.

The horizontal pair determines the line rate this project must respect. To draw a line, the current through those coils ramps from one end to the other and then collapses back. That has to happen 15,731 times a second on this set. A coil resists a change in the current through it, so what drives it is a tuned circuit instead of a plain amplifier, built to resonate at one frequency with the coil's own inductance as part of the tuning.

This tuned circuit limits the frequencies a television can accept. Sending a different timing asks the deflection hardware to operate outside the conditions it was built for.

03The flyback

At the end of every line the current in the horizontal coils is at one extreme and has to get back to the other. It is not eased back. The drive is cut, the circuit is allowed to swing, and the beam is flung back across the screen in 4.7 microseconds, against the 48.9 it spent crossing. About a tenth of the time, going the other way.

Blanking and retrace are not the same thing. A line on this project's television is 63.6 microseconds long and only 48.9 of them carry picture. The other 14.7 are blanking, and the beam is not flying back for all of it. A front porch of 2.4 microseconds holds the beam off and still at the right. Then the sync pulse of 4.7, where it actually flies back. Then a back porch of 7.6, where it is off, at the left, and waiting.

The retrace is the sync pulse. The porches are the set being given time to settle and to clamp its black level, and a signal that skips them is one a television will not lock to.

That swing is a large, fast change of current in a coil, producing a large voltage across it. The transformer wound around the same core steps up that voltage, which is then rectified to supply the anode’s twenty odd kilovolts. One component draws the picture sideways and supplies the accelerating voltage, out of the same collapse, once a line.

That return motion gives the flyback transformer its name. I used the same name for this project’s compositor.

Switch the model on above and slow it to one line a second. The sweep is the part where the picture is, the flick back to the left is the flyback, and the two pauses either side of it are the porches. The same thing happens at the bottom of the frame, more slowly: 22 blanked lines, 1.4 milliseconds, to get the beam back to the top.

04The screen

The inside of the faceplate is coated in a phosphor. An electron lands, gives up its energy, and the coating emits light for a few milliseconds and then stops.

The tube has no frame buffer, scaler or panel timing controller. At any instant, the viewer sees the bright spot and the fading glow of the lines it has just drawn. The picture is an artefact of persistence, partly in the phosphor and partly in the eye.

This matters to the Flyback study, which measures latency up to the start of scanout. A flat panel can buffer the frame after that point. The CRT draws from the incoming signal; the study discusses the remaining unmeasured delay through the converter and phosphor.

05Why 15 kHz and not 31

A computer monitor of the same era runs its line rate at 31.5 kHz or higher, and draws 480 or more lines progressively. A television runs at 15.7 and draws 240 lines in a field.

The number is what a broadcast standard settled on in the 1940s with the parts available then, and everything downstream inherited it: the yoke's inductance, the transformer's core, the capacitor tuned with it.

Send a set built for 15.7 kHz a signal at 31.5, and the horizontal output transistor is switched twice as often as it was designed for, into a circuit that is no longer resonant at the driving frequency. The transistor dissipates what it cannot pass on.

Flyback checks timings before passing them to the kernel, to avoid driving that circuit outside the configured frequency range.

The response depends on the set. Some lose sync, blank or shut down; others have little protection. This project has not tested out-of-range signals on a television.

06What the model above does not have

The model simplifies or omits four components.

The shadow mask or aperture grille sits just behind the phosphor on a colour set. It makes sure the beam for one colour can only land on that colour's phosphor. A colour tube is dim next to a monochrome one for that reason. Drawing it needs three guns.

Convergence is the business of making three beams land in the same place at the corners as well as in the middle, the fiddliest thing about a colour tube. One beam has no convergence problem.

The degauss coil is the loop around the front of the tube that fires for a second at switch-on to demagnetise the mask, and it does nothing a model can show.

And the high voltage supply is drawn as one component here. On a real chassis it is a multiplier, a regulator and a focus divider, all of it hanging off the flyback.

07The imperfections were the medium

For about twenty five years, game artists and programmers worked with these display characteristics. Some effects depended on the tube; others depended on the composite signal used to reach it.

There is no grid of pixels. The spot is a fuzzy round thing with a brightness that falls off from the middle, and it does not aim at a particular phosphor: whatever timing arrives is native, and there is nothing to resample onto anything.¹ A modern panel has a fixed number of cells and a scaler in front of them, so a 240 line picture is 240 lines interpolated onto a thousand and something. Here it is 240 lines.

It is an impulse and not a hold. A panel lights a frame and keeps it lit until the next one. A tube flashes a line and lets it die. The blur you see when something moves across a flat panel comes from that holding rather than from the refresh rate. It is why panels sold for games strobe their backlight or insert black frames, which imitates a tube.²

The tube emits light as the signal is scanned out. The Flyback study uses this relationship to interpret its software measurements, while distinguishing them from an optical latency measurement.

Bright spots spread into neighbouring areas, softening edges. Artists could account for that when drawing sprites and backgrounds.

What that was worth to somebody drawing | what it was worth

Colour, out of timing. The Apple II has no colour hardware. Wozniak clocked its black and white dot stream at a multiple of the television's colour subcarrier, so certain alternations of on and off arrive at the decoder as colour. Two pixels give purple or green depending on the phase, and the eighth bit of every byte delays the clock by half a cycle to give blue and orange as well.³

The IBM PC’s CGA uses the same effect on the same pixel clock.⁴ These colours arise when the television decodes the deliberately timed signal.

Transparency, out of a narrow channel. Composite video carries colour on a channel narrower than the one it carries brightness on, so two colours in alternating columns arrive averaged into one. The waterfalls in the first _Sonic the Hedgehog_ are a checkerboard of one blue over the rock behind it, and on the screen they were drawn for they are water.⁵ An RGB connection preserves that checkerboard, as does emulation without composite filtering.⁶

Half a colour, on a screen that averages.

Composite video carries colour on a channel narrower than the one it carries brightness on, so two colours in alternating columns arrive as one. The transparency is not in the picture. It is in the screen the picture was drawn for.

A program with no picture in it. The Atari 2600 has no frame buffer at all. There are 76 processor cycles in a line, and the program spends them changing the video chip's registers while the beam moves across the screen.⁷ Miss the timing and there is no picture at all. The beam is not where the program's output goes; the beam is the program's clock.⁸

The frame taken off. The Commodore 64's video chip decides to draw its border by comparing where it has got to against where the picture ends. Switch the machine to a shorter screen at exactly the right moment and it never starts, so the border is not there any more. On the sides it has to be done inside every line, to the cycle.⁹ Programmers used the timing of that comparison to keep the border open.

A gun that sees the beam. A light gun does not know where it is pointing. The console blanks the screen and flashes one target at a time while the gun reports whether it can see light, and the gun's filter is tuned to the fifteen kilohertz flicker of the beam itself.¹⁰ It works because the picture is being drawn rather than held, and it stopped working when screens stopped drawing: there is nothing at fifteen kilohertz on a panel to see.¹¹

These techniques used properties of the original display and video signal, which helps explain why the same image can look different on another screen.

08What this has to do with a compositor

These properties inform three parts of Flyback’s design.

The refusal guard. The tuned horizontal circuit needs a line rate within its supported band. Flyback checks the timing before sending it to the kernel: the line rate inside the band the configuration allows for that set, 15 to 16.5 kHz as shipped, the field rate between 40 and 90 Hz, and every timing in order.

The variable refresh rate. The horizontal rate must not move and the vertical one can. The vertical oscillator re-triggers on sync, so a longer frame is simply a longer wait before the beam is sent back to the top. The refresh rate can change without the mode changing, and it stops at about 55 Hz on this set, which is where the picture starts losing height.

The latency. The compositor controls when a frame starts scanning out. The study measures from a client’s commit to the start of scanout. It does not measure the converter’s delay or the time until light reaches the viewer.

References

The historical examples in section 7 come from the sources below; they were not measured for this project.

1. 1A tube draws with a round spot whose brightness falls off from the middle, and it never aims at an individual phosphor, so a non-native resolution costs it nothing: Display scaling, analog against digital, Blur Busters forums.
2. 2Impulse against sample and hold, and why strobing and black frame insertion exist: Why do some OLEDs have motion blur?, Blur Busters.
3. 3Colour without colour: Apple II computer graphics, Paleotronic, and Apple II composite artifact colour, Nerdly Pleasures, for the half cycle delay on the eighth bit.
4. 4Composite artifact colors, Wikipedia, for the shared 14.318 MHz pixel clock.
5. 5What do you see in Sonic the Hedgehog's waterfalls?, Time Extension.
6. 6Transparency, Raster Scroll, on what the Mega Drive's artists were drawing for.
7. 7Television Interface Adaptor, Wikipedia, for the registers holding one line at a time.
8. 8Atari 2600 hardware design: making something out of (almost) nothing, Big Mess o' Wires, for the 76 cycles.
9. 9Opening top and bottom borders on the Commodore 64, and the cycle exact version for the sides.
10. 10Zapper, NESdev wiki, for the high pass filter around the line rate and the one target at a time.
11. 11Yes, you can use lightguns on LCDs, sometimes, which is also where the filter frequencies come from.