How does the raster scan frequency affect the accuracy of a CRT TV picture?

[artis]

I couldn't find any information on this but I do wonder, what was the average CRT raster scan line thickness in terms of pixels illuminated in each full scan from left to right?
Did the electron beam illuminate a line that is just a single pixel width on each pass or did it hit multiple pixel width as it passed along the screen?

My own guess would be that those were multiple pixels widths on each scan line because if we take interlaced picture as an example at 50hz frequency then there were 25 odd and 25 even lines drawn on the screen each second, but the way I see it I can't imagine that if each line was a single pixel width it would be capable of covering the whole screen area.Oh and while I am at it, I'm sure someone here like @sophiecentaur or others could answer this one also.
So the raster scan frequency of 15..something Khz was chosen to be in correlation to the vertical refresh rate frequency because in order to draw an accurate picture the electron tube's 3 individual color gun grid's need to change their potential so that within each scan line brighter pixels get more electrons while darker ones get less, so I suppose the color gun accelerating electrodes were directly driven by the video signal circuitry but in order for this to make an accurate picture the raster scan had to be fixed rather precisely in order for the beam to match each pixel in the required intensity, so far correct?

If this is correct then how did the CRT cathode-anode high voltage effect this picture accuracy?
From what I have experienced myself and seen is that increasing or decreasing this main accelerating voltage makes the whole screen and picture brighter or darker but doesn't the accelerating potential also change the speed of the electrons in the beam so that at the same raster scan frequency the electrons now would arrive at the screen mask and pixels later and hit the wrong pixels?
Or do minor changes in accelerating potential doesn't affect the electron speed in vacuum by much?

 

[mfb]

If you hit multiple pixels then all these pixels will get a brightness that only depends on the beam width, not on the input signal. There wouldn't be a point in having these individual pixels as your resolution would be worse. You would also hit the other colors at the same time.

I can't imagine that if each line was a single pixel width it would be capable of covering the whole screen area.

By ramping voltages very fast.

Or do minor changes in accelerating potential doesn't affect the electron speed in vacuum by much?

You can calculate it. Take a typical voltage, calculate the speed. Now take 0.1% more or less and calculate that speed. Calculate how much time difference that produces from emission to deflection.

 

[berkeman]

the electron beam illuminate a line that is just a single pixel width on each pass

This.

There are a couple subtleties, though. For a color CRT there are 3 beams that are scanned together (RGB), but they still illuminate only one color pixel width in the scan line. Also, TV CRTs used an interlaced scan, where even and odd scan lines are scanned in alternate frames. Have you seen that in your reading?

Also, I think there may have been some experiments with monochrome CRTs with multiple guns to scan multiple pixels wide during each scan line, but that never found an application if I remember correctly.

If this is correct then how did the CRT cathode-anode high voltage effect this picture accuracy?

The focus depends on the overall anode-cathode voltage, but there is usually a separate focus electrode whose voltage is fine-tuned to give you the best spot size at the face.

 

[hutchphd]

Does everyone understand why the interlaced scanning was necessary? (I ask and will elucidate because it was not clear to me for a long time)
The original BW tube faces were just coated with phosphor. It is not easy to producer a nice tight-packed 525 line raster scan. If inadvertently the electron scan even slightly overlapped, the portions that were even slightly rescanned would be much too bright where they overlapped and degrade the picture badly.
Using interlaced scan ameliorated this problem: scans neighboring in space were separated in time by 1/60 sec and because the decay time for the phosphor was less than this there was no problem produced by small (less than a line) deviations in the raster scan position. Damn those RCA guys were good.

 

[JT Smith]

I thought interlacing was a way to reduce flicker while still having reasonable resolution with a limited signal bandwidth.

 

[256bits]

Does everyone understand why the interlaced scanning was necessary? (I ask and will elucidate because it was not clear to me for a long time)
The original BW tube faces were just coated with phosphor. It is not easy to producer a nice tight-packed 525 line raster scan. If inadvertently the electron scan even slightly overlapped, the portions that were even slightly rescanned would be much too bright where they overlapped and degrade the picture badly.
Using interlaced scan ameliorated this problem: scans neighboring in space were separated in time by 1/60 sec and because the decay time for the phosphor was less than this there was no problem produced by small (less than a line) deviations in the raster scan position. Damn those RCA guys were good.

One gets a bandwidth increase with interlaced scanning.
For a frame rate of 60Hz for NTSC ( 50Hz PAL ) with interlaced scanning at half the band width that the electronics have to deal with. If each frame was progressively scanned the electronics become more expensive all the way from camera, broadcastingand to the television set, reducing flicker through the persistence of vision of the eye and the level of phosphor luminescence decay rate. At the beginning of television, vacuum tube technology could not handle the sawtooth frequencies necessary for a suitable picture quality with progressive scan, so interlaced was introduced with an increase in the vertical lines of resolution.
and as a result, the picture detail is improved.

One side "benefit" is that those personalities on TV were asked to not wear stripped jackets, which would probably be not a problem with progressive scan, but with interlaced as each odd and even line would have to exactly and perfectly match up horizontally so as to not produce a jagged picture, which would appear to the viewer as a ghostly image.

 

[256bits]

I couldn't find any information on this but I do wonder, what was the average CRT raster scan line thickness in terms of pixels illuminated in each full scan from left to right?
Did the electron beam illuminate a line that is just a single pixel width on each pass or did it hit multiple pixel width as it passed along the screen?

B&W, the electron beam scanned each line at "pixel" width.
For RGB, a mask was put in front of the phosphor anode so that any overlap of pixel width hit the mask, but the electrons that went through the pixel width holes produced an illumination in the phosphor, either red green or blue, with no discernible overlap from one to the next of those phosphors or to the next pixel.

 

[hutchphd]

One gets a bandwidth increase with interlaced scanning.
For a frame rate of 60Hz for NTSC ( 50Hz PAL ) with interlaced scanning at half the band width that the electronics have to deal with. If each frame was progressively scanned the electronics become more expensive all the way from camera, broadcastingand to the television set, reducing flicker through the persistence of vision of the eye and the level of phosphor luminescence decay rate. At the beginning of television, vacuum tube technology could not handle the sawtooth frequencies necessary for a suitable picture quality with progressive scan, so interlaced was introduced with an increase in the vertical lines of resolution.
and as a result, the picture detail is improved.

Everything you say here is true. I simply mention a detail about about interlaced scanning which is not `sufficiently appreciated, and which was certainly known to the original engineers.

 

[256bits]

Everything you say here is true. I simply mention a detail about about interlaced scanning which is not `sufficiently appreciated, and which was certainly known to the original engineers.

I was just adding to your post with additional concerns.

I seem to recall, perhaps incorrectly, the overlap, evident on the green phosphor screens ( which a lot of people nowadays would not even know ever existed ). Light up a dot and the surrounding area is also illuminated faintly as well. Either that was due to issues involving focusing, turning on/off the beam, or the selection of a phosphor with a particular persistence I am not sure.

One thing I never investigated, is why the CRT tube for television used magnetic field deflection rather than electrostatic deflection to complete the image in the tube face. Another design choice to overcome a particular electronic hurdle?

 

[artis]

Ok, right , thanks folks.
Now that I think of it with a fresh morning mind I can totally see why the raster line couldn't be more than a pixel wide otherwise the picture would be very low resolution and inaccurate.

@mfb I used this calculator to estimate the difference in electron speed versus kV of accelerating potential.
https://www.ou.edu/research/electron/bmz5364/calc-kv.html
In the mid plane the velocity that is mentioned as Einsteinian is I believe the speed expressed as percentage of the speed of light or c.
Sure enough I put in 20kV and then 25kV of potential and there are some 3.. something % difference in electron velocity, but I assume that given the short distance between the electron gun and the anode (pixel screen) and the relatively slow raster scan frequency as compared to electron velocity in vacuum under those accelerating voltages the difference would not be felt by the eye?@hutchphd that is a good point you made, all in all I think the interlaced version was a compromise between many shortcomings of the technology of the day.
I mean as of today the CRT has been the longest display technology in use as it started out in the 1920's and went all the way up to early 2000's with devices like the Sony trinitron which when I was a kid I remember cost a small fortune when they came out.

I believe the shadow mask for the 3 gun color RGB screens was a way to make use of geometry as each gun's electrons had a slightly different angle with which they approached the screen so they each hit one of the 3 subpixels but those electrons that went astray for whatever reason were caught by the metal mesh in order for them to not strike neighboring pixels causing blur in the image.
I think the downside of this was that the electron beam intensity dropped compared to the black n white screens that had no mask so they had to adjust stuff ,

 

[artis]

@256bits my guess for why the magnetic deflection rather than electrostatic would be that for larger screens than those used in scopes they couldn't recharge/discharge the electrodes fast enough and that would blurr the beam , also a magnetic field doesn't do any work on the charged electron beam merely deflecting it while an electric field would do work on the beam and then beam would become uneven in strength from place to place as it would get closer to the positive electrode the electron acceleration would increase for example.

I guess it is also easier to build a current switching circuit than a high voltage fast switching circuit especially back in the day?

 

[sophiecentaur]

My own guess would be that those were multiple pixels widths on each scan

but they still illuminate only one color pixel width in the scan line.

The actual pixel height (i.e. resolvable part of the image) is precisely, by definition, the width of the beam. (Plus a bit of overlap) The phosphor spacing ( a different quantity) will be less than the spot size when possible, to avoid artefacts. Vertical and horizontal resolution are more or less the same if the beam modulation has sufficient bandwidth.

also a magnetic field doesn't do any work on the charged electron beam merely deflecting it

I was thinking about that statement. I guess, for a given deflection on the screen, it's the lateral Momentum change of the electrons (same transit time). The difference with magnetic scanning is that less force can be applied over a longer beam path. Electric Field deflectors can only be 'so long' because they need to be tapered outwards and field drops accordingly. So you couldn't do a wide screen TV without the neck sticking out through the wall into the next room (or having a very slow beam speed (= dim). The power required for a magnetic scan coil is high, though, because of the high energy in the scan coil fields. Life is made worse because of the terrible image distortion that would result without a lot of correction to the vertical and horizontal scan waveforms. And I don't think that any correction can be made without loss (= high scan power). Line and even frame output circuit were frighteningly big and sweaty.

I believe the shadow mask for the 3 gun color RGB screens was a way to make use of geometry as each gun's electrons had a slightly different angle with which they approached the screen so they each hit one of the 3 subpixels

The phosphor dots were deposited photographically. The shadow mask was put in place and removed several times so that light from a source would hit only the desired spots on the photo resist on the tube face. Then the phosphor dots (each colour at a time) were deposited in (hopefully) the same places that the appropriate electron beam would hit it. Three light sources were used and adjusted to the same position as the virtual image of the electron beam sources.
The shadow mask tube was a great demonstration of just how much the manufacturers 'needed' to sell receivers and the lengths they were prepared to go to get Colour TV out to the public. Big business! As soon as Sony came up with the PIL tube( Edit ; I think it was called Trinitron and was earlier than PIL,) the whole problem of colour purity and convergence became much easier. Double whammy; the old shadow mask sets were so rubbishy that people went out and bought better ones, as soon as they were available. Then the public went wild about flat screen etc. etc... Home TV now mimics 1950s SciFi.
...and now a prayer for the LCD TV display.

 

[hutchphd]

One thing I never investigated, is why the CRT tube for television used magnetic field deflection rather than electrostatic deflection to complete the image in the tube face. Another design choice to overcome a particular electronic hurdle?

I will guess that one reason is that the magnetic deflection would not change the speed of the electron which could complicate things. Also the requirement to drive bending magnets at 15.750kHz was easier to meet than a similarly deflective (high voltage) electrostatic rig, and the magnets are external to the vacuum envelope

 

[berkeman]

One thing I never investigated, is why the CRT tube for television used magnetic field deflection rather than electrostatic deflection to complete the image in the tube face. Another design choice to overcome a particular electronic hurdle?

From the CRTs I've seen over the years, I think that electrostatic deflection requires a longer overall tube length for the same screen area. I'm not sure why that is, but certainly for commercial TVs, a shorter overall tube length is desirable. For the CRTs in oscilloscopes, the length is less of an issue, and getting rid of the stray magnet fields from the deflection yokes is a good thing when you have all of that low-level analog processing circuitry in the o'scope.

https://en.wikipedia.org/wiki/Cathode-ray_tube

 

[artis]

Not to mention that modern scopes also don't use CRT's anymore but still i guess a small tube like that within a scope was within the practical "scope" of electric field deflection... pun very much intended. :D
@sophiecentaur so you did describe the way in which color CRT's were built back in the day?
I guess it was easier to build the black n white screens where one would only have to apply phosphor to the screen and that's it, no dots no etching etc.

 

[sophiecentaur]

I'm not sure why that is,

I mentioned why in that long rambling post. Electrostatic deflection plates would need a lot of space between them for a wide deflection. Fields / voltages would need to be massive. CROs used / use electrostatic deflection because it's more linear. To get fast deflection bandwidth, the beam has to be slow and is dim. I believe that Post Deflection Acceleration was used to beef up the beam brightness in high speed CROs.
The magnetic deflection fields cover a much bigger region. The only snag is that the geometrical distortion is a nightmare. The Earth's field was enough to mess with purity and convergence.
On the whole. the CRT is past its sell by date. `(Someone may tells us of an application that still demands analogue displays.)

 

[mfb]

Sure enough I put in 20kV and then 25kV of potential and there are some 3.. something % difference in electron velocity, but I assume that given the short distance between the electron gun and the anode (pixel screen) and the relatively slow raster scan frequency as compared to electron velocity in vacuum under those accelerating voltages the difference would not be felt by the eye?

You don't need to assume, you can calculate it.
A HV supply that changes from 20 kV to 25 kV is dangerously broken. Consider a 0.1% difference.

 

[rcgldr]

For CRT monitors, beam width and scan rate are adjusted to handle different resolutions. My CRT monitor (Viewsonic G225F) has a 1920x1440 mask, but can display just about any resolution down to 640x480. In addition to a VGA connector, it also has RGBHV inputs, (separate horizontal and vertical sync signals), which can display just about any resolution.

The beam width and the transitions do not have to be exact multiples of the 1920x1440 pixels. Close up image of a white arrow on a CRT screen, note that the edge "pixels' are only partially lit.

 

[sophiecentaur]

note that the edge "pixels' are only partially lit.

That involves spatial filtering. It's very common for text characters, to make them look nicer and to avoid artefacts. If you choose an arbitrary number of lines per inch and didn't do the filtering, then some choices could give disturbing - moiré patterns for instance. Different line frequencies are the equivalent of re-sampling of a signal that's already been sampled once, without the right filtering.

 

[Keith_McClary]

Lazily resorting to wiki :

Additionally, magnetic deflection can be arranged to give a larger angle of deflection than electrostatic plates; this makes the CRT and resulting television receiver more compact. The angle of magnetic deflection, for a given deflection current, is inversely proportional to the square root of the CRT accelerating voltage, but in electrostatic deflection, the angle is inversely proportional to the accelerating voltage (for a particular value of deflection plate voltage). This has the practical effect that high accelerating voltages can be used without greatly increasing the power of the deflection amplifiers.[1]

 

[rcgldr]

note that the edge "pixels' are only partially lit.

That involves spatial filtering. It's very common for text characters, to make them look nicer and to avoid artifacts. If you choose an arbitrary number of lines per inch and didn't do the filtering, then some choices could give disturbing - moiré patterns for instance. Different line frequencies are the equivalent of re-sampling of a signal that's already been sampled once, without the right filtering.

I should have noted that although the mask is 1920x1440, max resolution is 2048x1536, and there are no moiré patterns, despite the beam and transitions occurring on partial "pixel" boundaries. This also occurs on lower resolutions that are not integer divisions of 1920x1440. There is an adjustment to get rid of moiré patterns if they do appear, but I don't know what this adjustment does.

 

[artis]

The interesting part about CRT's is that you can learn some high energy physics from them while on the other hand all LCD's whether CCFL or LED backlit or OLED's are more like IC's since essentially the whole screen is nothing but a single layer chip architecture being digitally controlled, fascinating all in all but the idea of electrons accelerated by high voltage etc bring some unique feel to the CRT.Now one thing I have always sort of had trouble with is how did the deflection fields not mess up the beam precision , I mean an electron passing through a magnetic field experiences the lorentz force which makes it to gyrate , so how come the beam was kept "spot on" as it was dragged from the maximum angle down to the zero angle in the center and back?
Maybe the deflection fields were bit different than that from a field found within a solenoid core, I'm not sure

 

[mfb]

The magnetic fields are orthogonal to the direction of motion and the radius of curvature is larger than the size of the magnetic field. You only get a small fraction of a circle within the magnetic field.

 

[sophiecentaur]

how did the deflection fields not mess up the beam precision

The answer is "with great difficulty" The controls needed to converge an old shadow mask tube consisted of at least a dozen knobs, on a pull-out unit, so you could take it out and bring it in front of the screen. In addition, there were static controls which could be moved mechanically. Monitors were lined up (daily), using a grid of white lines on a black background.

Not a good system but the only way at the time.

 

[sophiecentaur]

You only get a small fraction of a circle within the magnetic field.

Hardly "small" for a wide angle tube and, of course, because the fields were not uniform and there was line / field scan crossover effects, the path was not a simple circle.

 

[artis]

Well the way I see it is that the speed of the electrons has to be or was large enough so that each electron as it passed through the perpendicular to it's trajectory B field got a change in it's direction so an angle from it's straight trajectory but not enough to get caught up in the B field lines and make a full circle and then spiral around the B field until it hits the inner glass surface close to the deflection coils.

I assume that if the electron speed/energy was low enough or the deflection field high enough this could happen?But I'm still sort of amazed at how the fields were made homogeneous enough so that each electron got deflected just the right amount and still the beam was precise for the later 90's early 2000 rather large diagonal crt screens , and not just large diagonal but also short tube length from screen to electron gun neck.
I remember some samsungs that were rather slim for a CRT yet the screen was somewhere around 30 inch IIRC

 

[sophiecentaur]

Affair the field was far from homgeneous. Just like with a wide angle glass lens with geometric corrections, the fields were deliberately distorted to make the beam follow horizontal lines across the screen.

 

[Keith_McClary]

beam was precise for the later 90's early 2000 rather large diagonal crt screens , and not just large diagonal but also short tube length from screen to electron gun neck.

Some CRT TV screens were actually flat. So it was quite a trick to make it focus to a dot in the corners as well as the center. Also they needed to make relativistic corrections (I can't find a better reference, but do you really want to do the math?).

 

[hutchphd]

Also they needed to make relativistic corrections (I can't find a better reference, but do you really want to do the math?).

Yes...rest energy of electron is 511keV and the accelerating Voltage is 30keV.$$\frac {v^2}{2c^2}=30/511$$ So $$v\approx0.3c$$ More than I would have guessed !

 

[tech99]

Does everyone understand why the interlaced scanning was necessary? (I ask and will elucidate because it was not clear to me for a long time)
The original BW tube faces were just coated with phosphor. It is not easy to producer a nice tight-packed 525 line raster scan. If inadvertently the electron scan even slightly overlapped, the portions that were even slightly rescanned would be much too bright where they overlapped and degrade the picture badly.
Using interlaced scan ameliorated this problem: scans neighboring in space were separated in time by 1/60 sec and because the decay time for the phosphor was less than this there was no problem produced by small (less than a line) deviations in the raster scan position. Damn those RCA guys were good.

The purpose of interlacing was to reduce flicker. Flicker arises because the frame rate is not fast enough for the eye to be completely fooled. The persistence of the TV phosphor was not enough to avoid flicker. By using two fields to form each frame, the refresh rate is doubled.
The width of lines can be adjusted by using focus control, but it is very difficult to make them just touch, and the normal arrangement is for the lines to be somewhat narrower than the spacing. Some early TVs used spot wobble to widen the lines so they exactly touched.
Unfortunately, although interlacing reduces brightness flicker, it creates inter line flicker, and the modern view is that interlacing is not useful.

 

[tech99]

I couldn't find any information on this but I do wonder, what was the average CRT raster scan line thickness in terms of pixels illuminated in each full scan from left to right?
Did the electron beam illuminate a line that is just a single pixel width on each pass or did it hit multiple pixel width as it passed along the screen?

My own guess would be that those were multiple pixels widths on each scan line because if we take interlaced picture as an example at 50hz frequency then there were 25 odd and 25 even lines drawn on the screen each second, but the way I see it I can't imagine that if each line was a single pixel width it would be capable of covering the whole screen area.Oh and while I am at it, I'm sure someone here like @sophiecentaur or others could answer this one also.
So the raster scan frequency of 15..something Khz was chosen to be in correlation to the vertical refresh rate frequency because in order to draw an accurate picture the electron tube's 3 individual color gun grid's need to change their potential so that within each scan line brighter pixels get more electrons while darker ones get less, so I suppose the color gun accelerating electrodes were directly driven by the video signal circuitry but in order for this to make an accurate picture the raster scan had to be fixed rather precisely in order for the beam to match each pixel in the required intensity, so far correct?

If this is correct then how did the CRT cathode-anode high voltage effect this picture accuracy?
From what I have experienced myself and seen is that increasing or decreasing this main accelerating voltage makes the whole screen and picture brighter or darker but doesn't the accelerating potential also change the speed of the electrons in the beam so that at the same raster scan frequency the electrons now would arrive at the screen mask and pixels later and hit the wrong pixels?
Or do minor changes in accelerating potential doesn't affect the electron speed in vacuum by much?

A monochrome TV screen is not made up of pixels, but is continuous.

 

[artis]

@tech99 a black n white screen yes but i was referring to a color screen

I tried to find any field line presentations from a deflection yoke but can't find any.
It seems like each yoke had two coils that were similar to ordinary pole coils being straight for the longer part and oval at ends and then it has this toroidal shaped ferrite with a winding around it.
although it's rather hard to imagine what the fields were like.

 

[sophiecentaur]

The purpose of interlacing was to reduce flicker.

The main purpose / great advantage of interlace is that it improves Motion Portrayal. It would be easy enough to introduce some 'extra' flicker as with the rotating Maltese cross screen in movie projectors which does reduce flicker but doesn't eliminate the jerky motion. A 25/30 Hz picture rate causes very jerky motion portrayal. The 50/60 Hz field rate that interlace provides a significant improvement - it's almost the same effect as doubling the temporal sample rate in a very significant point in the spectrum of moving images. The downside is pretty minimal. Yes, the lines appear to creep down the screen but at 'Five times Picture Height' (the recommended viewing distance for 625 line TV) you can't actually spot that. None of that is relevant once you up convert in the receiver and provide motion interpolation.

A monochrome TV screen is not made up of pixels, but is continuous.

Well . . . . . . It does have exactly the same vertical 'pixellation' that you get with a 2D sampled image. The continuous coverage of the monochrome phosphor is a different issue and Pixels are not inherently associated with phosphor dot spacing and sizes.