CRT raster scan both ways

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  • #1
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Someone surely has though of this before but I still wonder.
The original electron gun raster scan pattern was from top left to right and then the gun is switched off while the deflection coils reset so that the electron beam would start again at the left side but now a pixel row or two (as in interlaced) lower.
It takes time for the magnetic field to change and so the electron beam cannot do useful work at that moment.
Were there any ideas to have the raster scan pattern from left to right and then in the next row from right to left back and then again from left to right etc. This way instead of moving the beam back and starting a new line a new line would simply be drawn from the side where the beam left the previous line. Magnetic deflection coils would not have to be reset instead they could just go continually from left to right and back and the resetting would only need to happen at the end of each full frame from bottom to top.

This would mean that each frame could be drawn quicker so more frames could have been packed in a given time and less flicker. Probably increased bandwidth would also be needed.

In modern flat panel technologies like LCD or OLED etc this is probably not a problem because there is no beam to reset and the dispaly is driven electronically from a driver chip, so each next pixel scan line starts as soon as the last one ends with some small delay probably.
 

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  • #2
berkeman
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I looked at this a long time ago, and there is a big issue with it. With the current system the scan lines are parallel. If you just turn the beam around and head back the other way while the vertical deflection is continuing, you do not have parallel lines anymore.

You could try to come up with a vertical deflection system that paused at each line while it was traced out and then jumped down one line spacing and paused again, etc. But that vertical scanning circuitry would be pretty complicated to do with magnetic deflection.
 
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  • #3
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@berkeman you are I think referring to CRT scan?
Why would the next line be not parallel to the previous one? Surely the scan line that starts from left to right is horizontal , why couldn't a scan line that starts from right to left also be horizontal.


Although not to do with CRT's but in theory a flat panel TFT system could be made where in each frame instead of scan lines the whole frame was lit up alltogether at the same time , one would then need to control each pixel individually as is done now but control all of them at the same time. In theory done this way even with current frame/refresh rates of a couple of 100hz the picture would probably look even more natural, although it already does given the fast framerates and limitations of human vision.
The control circuitry would then probably need to be much more complicated.
Maybe @sophiecentaur has something to say about this whole deal, I've noticed you to be knowledgeable about radio and Tv issues.
 
  • #4
berkeman
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Why would the next line be not parallel to the previous one? Surely the scan line that starts from left to right is horizontal , why couldn't a scan line that starts from right to left also be horizontal.
The raster scan lines are tilted slightly, due to the downward scan rate at the same time the horizontal scan is happening. If you turn around and scan back the other way, you will end up with a zig-zag pattern.

And please don't call me "Shirley". :wink:

1625601660378.png

https://retrocomputing.stackexchang...t-a-parallelogram-instead-of-a-true-rectangle
 
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  • #5
tech99
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When John Logie Baird made the first demonstration of television in 1926 he found that scanning left to right gave less flicker. This is thought to be because we read in that direction and the eye is trained accordingly.
Another objection to the double scan proposed is that the slight non linearity of scan caused by the charge and discharge of the timebase capacitor would shift the pixels on alternate lines sideways, giving a ragged appearance..
 
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  • #6
Averagesupernova
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There may be other issues but non-parallel lines to me should not matter. If the camera tube is scanned in the same manner, it should reproduce the image the same way. That is not to say there weren't other technical/practical issues. I worked in the NTSC video test equipment field for a while and such a thing had not occurred to me.
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That system would have made the comb filter that used a delay line to delay the luminance signal by one horizontal line worthless.
 
  • #7
tech99
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There may be other issues but non-parallel lines to me should not matter. If the camera tube is scanned in the same manner, it should reproduce the image the same way. That is not to say there weren't other technical/practical issues. I worked in the NTSC video test equipment field for a while and such a thing had not occurred to me.
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That system would have made the comb filter that used a delay line to delay the luminance signal by one horizontal line worthless.
Please correct me if I am wrong but I thought the delay line was part of PAL and SECAM but not used in NTSC.
 
  • #8
Averagesupernova
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Please correct me if I am wrong but I thought the delay line was part of PAL and SECAM but not used in NTSC.
The phase of the chroma signal is 180 opposite from one line to the next. Since all information changes very little from one line to the next, the video is delayed by one line and added to the undelayed to get the chroma and luma seperated.
 
  • #9
tech99
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The phase of the chroma signal is 180 opposite from one line to the next. Since all information changes very little from one line to the next, the video is delayed by one line and added to the undelayed to get the chroma and luma seperated.
Is that not the PAL (Phase Alternate Line) system?
 
  • #10
hutchphd
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The "just move the electron beam down a pixel and scan it the other way" is nontrivial to implemaent. The existing interlace scan (developed for analog tubes) requires only a single vertical top to bottom sweep for each even frame (at 60 Hz) and an equivalent one for each odd frame.
The stairstep approach required by this proposed scheme is much more difficult to implement and requires vertical steps synched at the horizontal scan rate. Not easy to do. And this is to "improve beam efficiency"?
The folks who did TV (much of it invented at RCA a mile from my present abode) wrung pretty much every last bit out of that system.
 
  • #11
Averagesupernova
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Is that not the PAL (Phase Alternate Line) system?
No idea. If you do the math of NTSC you will find what I say to be true. Chroma subcarrier is 3.579545 Mhz. Horizontal scan is 15734.266.
 
  • #12
tech99
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Is that not the PAL (Phase Alternate Line) system?
Another objection I can see is that at the edges of the picture the lines converge, giving only half the vertical resolution. Ideally the scan lines should be as wide as the spacing, so they disappear.
 
  • #13
Klystron
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Electromagnetic fields provide many tasks inside a 'cathode-ray tube' (CRT) including deflection as described. Electrostatic deflection of the beam provides precise placement and rapid recovery depending on the application and design specifications including the phosphor coating and the expected display.

Early radar screens such as the UPA series combined electrostatic and electromagnetic circuits to collimate, direct and 'fire the gun'; that is, pulse the CRT.

1625605589091.png UPA-35 CRT mounted for radar display (note the orange phosor).

CRTs to display data from rotating antennae often draw a radius line cursor from the origin that rotates similar to the receiver antenna, placing the operator at a simulated center. The rotating cursor appears to paint series of pixels representing processed target returns, symbols, numbers, characters and related information such as identity and time stamps.

NTSC provided a compromise vision system including the raster scan rate, broadcast television limitations, physics and technology, with safety standards. I have worked with displays somewhat as @artis describes, most not suitable for long term home use. Children sat directly before the screen staring into the CRT. Safety concerns may have discouraged manufacturers from employing otherwise sound ideas.

Electromagnetic theory as applied to NTSC television, home and citizen-band (CB) radio transmitter/receiver rx/tx; combined electronic and magnetic fields, usually visualized as normal (orthogonal) to each other, as an EM field. An EM field propagates normal to the combined electronic and magnetic components even as each periodically zeroes.

EM field response times generally exceed information update rates. For instance, persistence of vision varies near a seventh of a second. A frame rate of 28-30 hertz provides the appearance of continuous motion; well inside raster flyback and EM field recovery time rates.
 
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  • #14
Keith_McClary
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The stairstep approach required by this proposed scheme is much more difficult to implement and requires vertical steps synched at the horizontal scan rate. Not easy to do.
It might require more than 17 vacuum tubes 😲 !

Also
In very early vacuum tube television sets, the EHT was derived directly from a high voltage winding on the mains transformer using a half wave rectifier. In later television sets, the EHT supply was invariably generated by rectifying the flyback pulses from the scanning circuitry rather than directly from the mains supply (a practice that survived the transition to transistor circuits). Although this provided a greater degree of safety, the reason for the change was that the mains transformer had been eliminated from sets produced from the 1940s onwards. Wikipedia
 
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  • #15
Baluncore
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I have always wanted to set up a Lissajous figure scan pattern. The advantage there is that the beam deflections are fundamental sinewaves. The system would be more efficient because the deflection coils could be resonant. Each pixel would be scanned twice, sort of like interlaced pictures, but once on each diagonal. The outside edge would be darkened.

Select two mutually prime integers with a ratio close to the aspect ratio. Multiply those by 25 or 30 Hz depending on the 50 or 60 Hz supply frequency, so the frames repeat. The centre of the screen will then be diagonally cross-hatched.

CRTs to display data from rotating antennae often draw a radius line cursor from the origin that rotates similar to the receiver antenna, placing the operator at a simulated center. The rotating cursor appears to paint series of pixels representing processed target returns, symbols, numbers, characters and related information such as identity and time stamps.
Early small boat radar had only one deflection coil, it rotated about the neck of the tube, synchronously with the antenna.
 
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  • #16
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And please don't call me "Shirley".
not quite sure what you mean by that.
But I got the idea after the picture you posted, right so the original scan lines are not horizontal either, well in that case zig zagging them indeed would not produce a good result
 
  • #17
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It might require more than 17 vacuum tubes 😲 !
Yeah, it's easy to forget that the development of TV sets was a process, and it started with minimal electronics (and constant requirement with backward compatibility).

Ps.: I guess zig-zag scanning would be easier with electrostatic deflection, since the voltage has less constraint against sudden changes than current. So this tube actually could be able to do such trick (no wonder: it was coming from oscilloscopes). As the story went, electrostatic deflection was inconvenient due the long neck of the tube and the low deflection angle, so by the time things settled (40's, I believe) it was out of question.
 
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  • #18
Baluncore
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not quite sure what you mean by that.
Shirley, surely that joke is from "Airplane" = "Flying High".
 
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  • #19
Baluncore
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Here is a Lissajous Raster with frequencies of 101 and 139, drawn as a yellow line on black.
Lissajous_raster_101x139.png
 
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  • #20
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Not sure why but looking at that picture @Baluncore is painful for my eyes even on a flat panel screen
 
  • #22
Baluncore
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Why did we have top to bottom?
Because that is the way we read an English language book.
 
  • #23
Baluncore
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Not sure why but looking at that picture @Baluncore is painful for my eyes even on a flat panel screen
There are several reasons for the crazy visual effects. A low-resolution original was drawn in pixels. That is displayed on your screen pixel grid, which shows many different sampling beats. To focus your eyes, your brain then tries to identify range and to sort out the parallax. That is why you can see many layers in 3D.

But none of that is relevant to the scan in a grid. There is only one line in that plot, with the x and y deflection being fundamental sine and cosine waves. Higher mutually prime scan frequencies would fill in the black bits, just like a normal raster.

Modulation of the line brightness would produce an image without the optical confusion in the parts of your visual brain that still remain after viewing the image.
 
  • #24
Klystron
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Not sure why but looking at that picture @Baluncore is painful for my eyes even on a flat panel screen
Tuned Lissajou patterns tend to be easy on the eyes. Moire patterns baffle the eye; the perceived pain likely from muscles in the eye attempting to focus on an 'object' comprised of interference.

Though difficult to make out, the 'hourglass' shape in @Baluncore 's example is a typical Lissajou pattern. Adjusting one or both input sources leads to an 'eggbeater' pattern until the next tuned state. Mike's raster patterns in post #4 would produce Moire patterns if printed with the lines closer together.
 
  • #25
sophiecentaur
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There's one word which I can't yet find in this thread and that's 'Compatibility'. (I lied about that - just found it mentioned higher up.) The first TV systems had problems enough in producing the first low resolution monochrome systems. The move to the higher resolution monochrome system (525 and 625 lines) was a significantly hard jump and pushed the technology to the limit in many ways. The really clever available advance was to use interlace, which helped motion portrayal because they could double the rate that pictures could be transmitted and the vertical resolution is the same for stationary pictures. This was 'doable'.
The monochrome system was then pretty much set in stone and there were many existing TVs (around the US, at least) but no extra transmitters or spectrum space for a brand new Colour system. So moving to colour required a system that was (almost) undetectable. NTSC, PAL and don't forget SECAM did their best to deal with limited channel spec; phase distortion was mitigated by moving from NTSC to PAL and early PAL didn't even use a delay line. The eye averaged out colour errors from line to line. SECAM was a basically FM colour subcarrier system (I believe that was for the benefit of viewers in mountainous regions who had to deal with worse phase distortion)

I don't think any of the ideas aired in this thread hadn't been considered but it was basically too late to implement any of them. I would't mind betting that any of the alternative scanning systems would have made life even harder to allow extra colour information to be squeezed in. Anything other than a regular scan rate and consistent video frequencies would have made it impossible to place the spectral components of the colour subcarrier into the spaces in the comb of the luminance spectrum, which is why it's possible to shoe-horn the colour information in amongst the luminance information without too much distortion and crosstalk.
This thread could go on for many pages before we manage to deal with all the clevernesses of (particularly) PAL.
 

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