Was the picture area of a CRT a parallelogram (instead of a true rectangle)?












12















This may seem like an absurd question at first, but I've been giving it some thought and I'm genuinely curious about the design details of these devices.



I was reading an answer on an unrelated Stack Exchange site about the retrace/flyback details on old CRTs, and the image attached to that answer piqued my interest:



NTSC scan lines



I understand that both the horizontal and vertical deflection of the electron beam is controlled by two sawtooth waves, the vertical running at the refresh rate and the horizontal running a few hundred times faster than that. I also understand that both sawtooths are constant sweeps, and not "stair stepped" to hold at any particular voltage to accommodate the viewable lines or retrace periods.



Here's the premise of my question: In the context of one single horizontal trace across the screen, the vertical position is also constantly increasing (towards the bottom) in preparation for the next scan line. It then follows that the scan line's vertical position at the left edge of the screen is slightly higher than the position at the right edge, and the whole screen is a parallelogram with left and right edges perfectly vertical, and top and bottom edges both slanted down towards the bottom right.



Assuming the premise is correct, was it common (or even feasible) for the designers of CRT computer displays to counteract this effect and make the screen and its contents perfectly square? Would such a compensation have even been worth the effort?










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  • 2





    It's always been a (very) slightly distorted rectangle. ...and nobody cared.

    – tofro
    14 hours ago








  • 2





    If the deflection grids/magnets are not mounted exactly perpendicular to each other -- and there doesn't seem to be any reason why they'd have to be -- the "horizontal" scan can be given a slight upwards slant that compensates for the steady drop of the vertical deflection during the line. (That is, if one cares, which I'm not sure one does).

    – Henning Makholm
    11 hours ago


















12















This may seem like an absurd question at first, but I've been giving it some thought and I'm genuinely curious about the design details of these devices.



I was reading an answer on an unrelated Stack Exchange site about the retrace/flyback details on old CRTs, and the image attached to that answer piqued my interest:



NTSC scan lines



I understand that both the horizontal and vertical deflection of the electron beam is controlled by two sawtooth waves, the vertical running at the refresh rate and the horizontal running a few hundred times faster than that. I also understand that both sawtooths are constant sweeps, and not "stair stepped" to hold at any particular voltage to accommodate the viewable lines or retrace periods.



Here's the premise of my question: In the context of one single horizontal trace across the screen, the vertical position is also constantly increasing (towards the bottom) in preparation for the next scan line. It then follows that the scan line's vertical position at the left edge of the screen is slightly higher than the position at the right edge, and the whole screen is a parallelogram with left and right edges perfectly vertical, and top and bottom edges both slanted down towards the bottom right.



Assuming the premise is correct, was it common (or even feasible) for the designers of CRT computer displays to counteract this effect and make the screen and its contents perfectly square? Would such a compensation have even been worth the effort?










share|improve this question







New contributor




smitelli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
















  • 2





    It's always been a (very) slightly distorted rectangle. ...and nobody cared.

    – tofro
    14 hours ago








  • 2





    If the deflection grids/magnets are not mounted exactly perpendicular to each other -- and there doesn't seem to be any reason why they'd have to be -- the "horizontal" scan can be given a slight upwards slant that compensates for the steady drop of the vertical deflection during the line. (That is, if one cares, which I'm not sure one does).

    – Henning Makholm
    11 hours ago
















12












12








12








This may seem like an absurd question at first, but I've been giving it some thought and I'm genuinely curious about the design details of these devices.



I was reading an answer on an unrelated Stack Exchange site about the retrace/flyback details on old CRTs, and the image attached to that answer piqued my interest:



NTSC scan lines



I understand that both the horizontal and vertical deflection of the electron beam is controlled by two sawtooth waves, the vertical running at the refresh rate and the horizontal running a few hundred times faster than that. I also understand that both sawtooths are constant sweeps, and not "stair stepped" to hold at any particular voltage to accommodate the viewable lines or retrace periods.



Here's the premise of my question: In the context of one single horizontal trace across the screen, the vertical position is also constantly increasing (towards the bottom) in preparation for the next scan line. It then follows that the scan line's vertical position at the left edge of the screen is slightly higher than the position at the right edge, and the whole screen is a parallelogram with left and right edges perfectly vertical, and top and bottom edges both slanted down towards the bottom right.



Assuming the premise is correct, was it common (or even feasible) for the designers of CRT computer displays to counteract this effect and make the screen and its contents perfectly square? Would such a compensation have even been worth the effort?










share|improve this question







New contributor




smitelli is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












This may seem like an absurd question at first, but I've been giving it some thought and I'm genuinely curious about the design details of these devices.



I was reading an answer on an unrelated Stack Exchange site about the retrace/flyback details on old CRTs, and the image attached to that answer piqued my interest:



NTSC scan lines



I understand that both the horizontal and vertical deflection of the electron beam is controlled by two sawtooth waves, the vertical running at the refresh rate and the horizontal running a few hundred times faster than that. I also understand that both sawtooths are constant sweeps, and not "stair stepped" to hold at any particular voltage to accommodate the viewable lines or retrace periods.



Here's the premise of my question: In the context of one single horizontal trace across the screen, the vertical position is also constantly increasing (towards the bottom) in preparation for the next scan line. It then follows that the scan line's vertical position at the left edge of the screen is slightly higher than the position at the right edge, and the whole screen is a parallelogram with left and right edges perfectly vertical, and top and bottom edges both slanted down towards the bottom right.



Assuming the premise is correct, was it common (or even feasible) for the designers of CRT computer displays to counteract this effect and make the screen and its contents perfectly square? Would such a compensation have even been worth the effort?







crt-monitor display






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asked 23 hours ago









smitellismitelli

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  • 2





    It's always been a (very) slightly distorted rectangle. ...and nobody cared.

    – tofro
    14 hours ago








  • 2





    If the deflection grids/magnets are not mounted exactly perpendicular to each other -- and there doesn't seem to be any reason why they'd have to be -- the "horizontal" scan can be given a slight upwards slant that compensates for the steady drop of the vertical deflection during the line. (That is, if one cares, which I'm not sure one does).

    – Henning Makholm
    11 hours ago
















  • 2





    It's always been a (very) slightly distorted rectangle. ...and nobody cared.

    – tofro
    14 hours ago








  • 2





    If the deflection grids/magnets are not mounted exactly perpendicular to each other -- and there doesn't seem to be any reason why they'd have to be -- the "horizontal" scan can be given a slight upwards slant that compensates for the steady drop of the vertical deflection during the line. (That is, if one cares, which I'm not sure one does).

    – Henning Makholm
    11 hours ago










2




2





It's always been a (very) slightly distorted rectangle. ...and nobody cared.

– tofro
14 hours ago







It's always been a (very) slightly distorted rectangle. ...and nobody cared.

– tofro
14 hours ago






2




2





If the deflection grids/magnets are not mounted exactly perpendicular to each other -- and there doesn't seem to be any reason why they'd have to be -- the "horizontal" scan can be given a slight upwards slant that compensates for the steady drop of the vertical deflection during the line. (That is, if one cares, which I'm not sure one does).

– Henning Makholm
11 hours ago







If the deflection grids/magnets are not mounted exactly perpendicular to each other -- and there doesn't seem to be any reason why they'd have to be -- the "horizontal" scan can be given a slight upwards slant that compensates for the steady drop of the vertical deflection during the line. (That is, if one cares, which I'm not sure one does).

– Henning Makholm
11 hours ago












6 Answers
6






active

oldest

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13














IIRC, the electron gun was actually installed in a position where it was rotated slightly relative to the tube, to compensate for this effect, so the scan lines did end up being horizontal.






share|improve this answer



















  • 4





    ... which means that it is still a parallelogram, it's just the sides that aren't vertical rather than the top and bottom that aren't horizontal!

    – Tommy
    21 hours ago






  • 10





    @Tommy: I think that's arguable. If you consider the screen (i.e. what the user sees) as a rectangular mask that clips a larger parallellogram shape (the CRT projection), then I would still say that "the picture area is rectangular". Similarly, I would say that a modern day projector produces a rectangular shape even though its lens could project a larger circle/disc. You're right that the CRT could/would produce a parallellogram when not being clipped, but since it's being projected on a rectangular shape, it can only fill that rectangular shape and anything else is lost/not registered.

    – Flater
    14 hours ago













  • @Flater you've bested me there. I agree — one rectangular image, with horizontal scans, is produced rather than another.

    – Tommy
    12 hours ago













  • @Flater Is that right? I vaguely remember in the DOS age, the "screen" area (the pixel-addressable 320x200 or 640x480 or what-have-you) having a black border around it that could be controlled by the application, although it seldom ever was. If there was a mask, it would have to be at least wide enough to allow that to show through.

    – smitelli
    10 hours ago











  • @smitelli: It's possible that the electron gun was set up to not light up a pixel for any part of the paralellogram that is outside of the requested rectangle, I'm not sure how they handles the screen adjustments tbh. But I think that doesn't quite change the point that the screen output (i.e. the actually visible pixels) is rectangular.

    – Flater
    9 hours ago



















12














Canonically in NTSC standards the drawn lines are tilted slightly so that the start of the odd field starting at the left is perfectly level with the top of the even field starting in the middle horizontally. The actual angle though is so tiny, ~0.09 degrees, that it's negligible compared to the rotation error you'd get just from the earth's magnetic field and how well the factory calibration was done. TVs were generally set up with ~5-10% of the image being drawn outside of the edges of the screen anyway which would effectively crop the edges to be a rectangle no matter how far off the scanning pattern was.






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  • "[...] edges of the screen [...] crop the edges to be a rectangle [...]" - not exactly, the CRT screens I remember weren't rectangular, but had quite rounded edges and corners, even adding to the validity of your argument.

    – Ralf Kleberhoff
    3 hours ago



















7














Yes there are a lot of compensations in a CRT like:




  • magnets counteracting background magnetic fields

  • circuits counteracting curvature of CRT screen surface

  • circuits counteracting different length of the beam (edges/center)

  • "linearizations" of brightness (GAMA correction)


and probably much more I can not think of right now...



But back to your question the stuff you are describing is applicable only on monochromatic CRTs with analog sweeping. As the color ones got a luminofor masks presenting form of a "pixelated" or "lineated" grids preventing of any skew. But still the compensations must be done (by rotating the sweeping slightly) otherwise the beam could jump a line ...



However in digital era more recent CRTs are usually driven by a CPU and the sweeping is controlled digitally by DAC so no more skew as the vertical coil is no more changing during the vertical sweep ...






share|improve this answer































    5














    To expand further, it actually wasn't especially feasible — there is no easy solution that doesn't eliminate the interlacing.



    Interlacing works because the timing of the vertical retrace varies. On odd fields it is triggered so that scanning resumes at the beginning of a line. On even fields it is triggered so that scanning resumes in the middle. Because of the diagonal scan, that sets one field 0.5 lines higher than the other. If the scan weren't diagonal then the two fields would not enmesh in that manner — they would instead sit exactly on top of each other, just starting in different places.



    On a classic TV it's undesirable to make the diagonal scan anything other than diagonal because the flying spot during the capture process was diagonal. So you wouldn't be unskewing the image, you'd be skewing it.



    On a monitor life is slightly different, and true horizontals are likely accurately to reflect the image. But it's also generally the case that monitors have smaller scan lines in order to output a higher resolution, so the effect is less visible anyway — on a 14" 800x600 monitor you're already talking about the right hand side being less than 0.3mm lower than the left, but being almost 28.5cm to the right. With a multi-sync monitor, how far down the right is compared to the left is a variable function of the resolution.






    share|improve this answer

































      3














      The effect you are describing did not matter, mainly because in a television the effect is so small, and the TV camera also had a CRT tube that converts light to video signal in a matching scanning pattern so the picture is in fact not tilted due to the scanning. Computer systems usually used progressive scanning on CRT monitors so for example VGA has twice faster line rate than NTSC TV so the lines are more horizontal. Even with earlier computer systems such as CGA that was NTSC compatible, most likely other distortions of the CRT were more noticeable than scan lines not being exactly horizontal.






      share|improve this answer

































        0














        If the horizontal deflection circuit had no effect on the vertical positioning of the beam, and if the vertical deflection circuit had no effect on the horizontal positioning, then the shape would indeed be a parallelogram.



        In practice, however, it would be very difficult to design a CRT-based display in which the effects were cleanly isolated in that fashion. Horizontal and vertical deflection circuits interact with each other for a variety of reasons, and instead of trying to prevent such interactions, most displays instead try to compensate for them with a mixture of adjustable and non-adjustable compensation circuits that can independently adjust the width of the screen at the top, middle, and bottom. Further, the angle of the yoke assembly is often adjustable. While vertical motion during each scan line would cause a slight skew if horizontal and vertical deflection were independent, consumer-grade equipment is seldom close enough to perfect calibration to make that significant.






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          6 Answers
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          6 Answers
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          13














          IIRC, the electron gun was actually installed in a position where it was rotated slightly relative to the tube, to compensate for this effect, so the scan lines did end up being horizontal.






          share|improve this answer



















          • 4





            ... which means that it is still a parallelogram, it's just the sides that aren't vertical rather than the top and bottom that aren't horizontal!

            – Tommy
            21 hours ago






          • 10





            @Tommy: I think that's arguable. If you consider the screen (i.e. what the user sees) as a rectangular mask that clips a larger parallellogram shape (the CRT projection), then I would still say that "the picture area is rectangular". Similarly, I would say that a modern day projector produces a rectangular shape even though its lens could project a larger circle/disc. You're right that the CRT could/would produce a parallellogram when not being clipped, but since it's being projected on a rectangular shape, it can only fill that rectangular shape and anything else is lost/not registered.

            – Flater
            14 hours ago













          • @Flater you've bested me there. I agree — one rectangular image, with horizontal scans, is produced rather than another.

            – Tommy
            12 hours ago













          • @Flater Is that right? I vaguely remember in the DOS age, the "screen" area (the pixel-addressable 320x200 or 640x480 or what-have-you) having a black border around it that could be controlled by the application, although it seldom ever was. If there was a mask, it would have to be at least wide enough to allow that to show through.

            – smitelli
            10 hours ago











          • @smitelli: It's possible that the electron gun was set up to not light up a pixel for any part of the paralellogram that is outside of the requested rectangle, I'm not sure how they handles the screen adjustments tbh. But I think that doesn't quite change the point that the screen output (i.e. the actually visible pixels) is rectangular.

            – Flater
            9 hours ago
















          13














          IIRC, the electron gun was actually installed in a position where it was rotated slightly relative to the tube, to compensate for this effect, so the scan lines did end up being horizontal.






          share|improve this answer



















          • 4





            ... which means that it is still a parallelogram, it's just the sides that aren't vertical rather than the top and bottom that aren't horizontal!

            – Tommy
            21 hours ago






          • 10





            @Tommy: I think that's arguable. If you consider the screen (i.e. what the user sees) as a rectangular mask that clips a larger parallellogram shape (the CRT projection), then I would still say that "the picture area is rectangular". Similarly, I would say that a modern day projector produces a rectangular shape even though its lens could project a larger circle/disc. You're right that the CRT could/would produce a parallellogram when not being clipped, but since it's being projected on a rectangular shape, it can only fill that rectangular shape and anything else is lost/not registered.

            – Flater
            14 hours ago













          • @Flater you've bested me there. I agree — one rectangular image, with horizontal scans, is produced rather than another.

            – Tommy
            12 hours ago













          • @Flater Is that right? I vaguely remember in the DOS age, the "screen" area (the pixel-addressable 320x200 or 640x480 or what-have-you) having a black border around it that could be controlled by the application, although it seldom ever was. If there was a mask, it would have to be at least wide enough to allow that to show through.

            – smitelli
            10 hours ago











          • @smitelli: It's possible that the electron gun was set up to not light up a pixel for any part of the paralellogram that is outside of the requested rectangle, I'm not sure how they handles the screen adjustments tbh. But I think that doesn't quite change the point that the screen output (i.e. the actually visible pixels) is rectangular.

            – Flater
            9 hours ago














          13












          13








          13







          IIRC, the electron gun was actually installed in a position where it was rotated slightly relative to the tube, to compensate for this effect, so the scan lines did end up being horizontal.






          share|improve this answer













          IIRC, the electron gun was actually installed in a position where it was rotated slightly relative to the tube, to compensate for this effect, so the scan lines did end up being horizontal.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 23 hours ago









          rwallacerwallace

          10k450149




          10k450149








          • 4





            ... which means that it is still a parallelogram, it's just the sides that aren't vertical rather than the top and bottom that aren't horizontal!

            – Tommy
            21 hours ago






          • 10





            @Tommy: I think that's arguable. If you consider the screen (i.e. what the user sees) as a rectangular mask that clips a larger parallellogram shape (the CRT projection), then I would still say that "the picture area is rectangular". Similarly, I would say that a modern day projector produces a rectangular shape even though its lens could project a larger circle/disc. You're right that the CRT could/would produce a parallellogram when not being clipped, but since it's being projected on a rectangular shape, it can only fill that rectangular shape and anything else is lost/not registered.

            – Flater
            14 hours ago













          • @Flater you've bested me there. I agree — one rectangular image, with horizontal scans, is produced rather than another.

            – Tommy
            12 hours ago













          • @Flater Is that right? I vaguely remember in the DOS age, the "screen" area (the pixel-addressable 320x200 or 640x480 or what-have-you) having a black border around it that could be controlled by the application, although it seldom ever was. If there was a mask, it would have to be at least wide enough to allow that to show through.

            – smitelli
            10 hours ago











          • @smitelli: It's possible that the electron gun was set up to not light up a pixel for any part of the paralellogram that is outside of the requested rectangle, I'm not sure how they handles the screen adjustments tbh. But I think that doesn't quite change the point that the screen output (i.e. the actually visible pixels) is rectangular.

            – Flater
            9 hours ago














          • 4





            ... which means that it is still a parallelogram, it's just the sides that aren't vertical rather than the top and bottom that aren't horizontal!

            – Tommy
            21 hours ago






          • 10





            @Tommy: I think that's arguable. If you consider the screen (i.e. what the user sees) as a rectangular mask that clips a larger parallellogram shape (the CRT projection), then I would still say that "the picture area is rectangular". Similarly, I would say that a modern day projector produces a rectangular shape even though its lens could project a larger circle/disc. You're right that the CRT could/would produce a parallellogram when not being clipped, but since it's being projected on a rectangular shape, it can only fill that rectangular shape and anything else is lost/not registered.

            – Flater
            14 hours ago













          • @Flater you've bested me there. I agree — one rectangular image, with horizontal scans, is produced rather than another.

            – Tommy
            12 hours ago













          • @Flater Is that right? I vaguely remember in the DOS age, the "screen" area (the pixel-addressable 320x200 or 640x480 or what-have-you) having a black border around it that could be controlled by the application, although it seldom ever was. If there was a mask, it would have to be at least wide enough to allow that to show through.

            – smitelli
            10 hours ago











          • @smitelli: It's possible that the electron gun was set up to not light up a pixel for any part of the paralellogram that is outside of the requested rectangle, I'm not sure how they handles the screen adjustments tbh. But I think that doesn't quite change the point that the screen output (i.e. the actually visible pixels) is rectangular.

            – Flater
            9 hours ago








          4




          4





          ... which means that it is still a parallelogram, it's just the sides that aren't vertical rather than the top and bottom that aren't horizontal!

          – Tommy
          21 hours ago





          ... which means that it is still a parallelogram, it's just the sides that aren't vertical rather than the top and bottom that aren't horizontal!

          – Tommy
          21 hours ago




          10




          10





          @Tommy: I think that's arguable. If you consider the screen (i.e. what the user sees) as a rectangular mask that clips a larger parallellogram shape (the CRT projection), then I would still say that "the picture area is rectangular". Similarly, I would say that a modern day projector produces a rectangular shape even though its lens could project a larger circle/disc. You're right that the CRT could/would produce a parallellogram when not being clipped, but since it's being projected on a rectangular shape, it can only fill that rectangular shape and anything else is lost/not registered.

          – Flater
          14 hours ago







          @Tommy: I think that's arguable. If you consider the screen (i.e. what the user sees) as a rectangular mask that clips a larger parallellogram shape (the CRT projection), then I would still say that "the picture area is rectangular". Similarly, I would say that a modern day projector produces a rectangular shape even though its lens could project a larger circle/disc. You're right that the CRT could/would produce a parallellogram when not being clipped, but since it's being projected on a rectangular shape, it can only fill that rectangular shape and anything else is lost/not registered.

          – Flater
          14 hours ago















          @Flater you've bested me there. I agree — one rectangular image, with horizontal scans, is produced rather than another.

          – Tommy
          12 hours ago







          @Flater you've bested me there. I agree — one rectangular image, with horizontal scans, is produced rather than another.

          – Tommy
          12 hours ago















          @Flater Is that right? I vaguely remember in the DOS age, the "screen" area (the pixel-addressable 320x200 or 640x480 or what-have-you) having a black border around it that could be controlled by the application, although it seldom ever was. If there was a mask, it would have to be at least wide enough to allow that to show through.

          – smitelli
          10 hours ago





          @Flater Is that right? I vaguely remember in the DOS age, the "screen" area (the pixel-addressable 320x200 or 640x480 or what-have-you) having a black border around it that could be controlled by the application, although it seldom ever was. If there was a mask, it would have to be at least wide enough to allow that to show through.

          – smitelli
          10 hours ago













          @smitelli: It's possible that the electron gun was set up to not light up a pixel for any part of the paralellogram that is outside of the requested rectangle, I'm not sure how they handles the screen adjustments tbh. But I think that doesn't quite change the point that the screen output (i.e. the actually visible pixels) is rectangular.

          – Flater
          9 hours ago





          @smitelli: It's possible that the electron gun was set up to not light up a pixel for any part of the paralellogram that is outside of the requested rectangle, I'm not sure how they handles the screen adjustments tbh. But I think that doesn't quite change the point that the screen output (i.e. the actually visible pixels) is rectangular.

          – Flater
          9 hours ago











          12














          Canonically in NTSC standards the drawn lines are tilted slightly so that the start of the odd field starting at the left is perfectly level with the top of the even field starting in the middle horizontally. The actual angle though is so tiny, ~0.09 degrees, that it's negligible compared to the rotation error you'd get just from the earth's magnetic field and how well the factory calibration was done. TVs were generally set up with ~5-10% of the image being drawn outside of the edges of the screen anyway which would effectively crop the edges to be a rectangle no matter how far off the scanning pattern was.






          share|improve this answer








          New contributor




          Jamvanderloeff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
          Check out our Code of Conduct.





















          • "[...] edges of the screen [...] crop the edges to be a rectangle [...]" - not exactly, the CRT screens I remember weren't rectangular, but had quite rounded edges and corners, even adding to the validity of your argument.

            – Ralf Kleberhoff
            3 hours ago
















          12














          Canonically in NTSC standards the drawn lines are tilted slightly so that the start of the odd field starting at the left is perfectly level with the top of the even field starting in the middle horizontally. The actual angle though is so tiny, ~0.09 degrees, that it's negligible compared to the rotation error you'd get just from the earth's magnetic field and how well the factory calibration was done. TVs were generally set up with ~5-10% of the image being drawn outside of the edges of the screen anyway which would effectively crop the edges to be a rectangle no matter how far off the scanning pattern was.






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          • "[...] edges of the screen [...] crop the edges to be a rectangle [...]" - not exactly, the CRT screens I remember weren't rectangular, but had quite rounded edges and corners, even adding to the validity of your argument.

            – Ralf Kleberhoff
            3 hours ago














          12












          12








          12







          Canonically in NTSC standards the drawn lines are tilted slightly so that the start of the odd field starting at the left is perfectly level with the top of the even field starting in the middle horizontally. The actual angle though is so tiny, ~0.09 degrees, that it's negligible compared to the rotation error you'd get just from the earth's magnetic field and how well the factory calibration was done. TVs were generally set up with ~5-10% of the image being drawn outside of the edges of the screen anyway which would effectively crop the edges to be a rectangle no matter how far off the scanning pattern was.






          share|improve this answer








          New contributor




          Jamvanderloeff is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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          Canonically in NTSC standards the drawn lines are tilted slightly so that the start of the odd field starting at the left is perfectly level with the top of the even field starting in the middle horizontally. The actual angle though is so tiny, ~0.09 degrees, that it's negligible compared to the rotation error you'd get just from the earth's magnetic field and how well the factory calibration was done. TVs were generally set up with ~5-10% of the image being drawn outside of the edges of the screen anyway which would effectively crop the edges to be a rectangle no matter how far off the scanning pattern was.







          share|improve this answer








          New contributor




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          answered 9 hours ago









          JamvanderloeffJamvanderloeff

          1212




          1212




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          • "[...] edges of the screen [...] crop the edges to be a rectangle [...]" - not exactly, the CRT screens I remember weren't rectangular, but had quite rounded edges and corners, even adding to the validity of your argument.

            – Ralf Kleberhoff
            3 hours ago



















          • "[...] edges of the screen [...] crop the edges to be a rectangle [...]" - not exactly, the CRT screens I remember weren't rectangular, but had quite rounded edges and corners, even adding to the validity of your argument.

            – Ralf Kleberhoff
            3 hours ago

















          "[...] edges of the screen [...] crop the edges to be a rectangle [...]" - not exactly, the CRT screens I remember weren't rectangular, but had quite rounded edges and corners, even adding to the validity of your argument.

          – Ralf Kleberhoff
          3 hours ago





          "[...] edges of the screen [...] crop the edges to be a rectangle [...]" - not exactly, the CRT screens I remember weren't rectangular, but had quite rounded edges and corners, even adding to the validity of your argument.

          – Ralf Kleberhoff
          3 hours ago











          7














          Yes there are a lot of compensations in a CRT like:




          • magnets counteracting background magnetic fields

          • circuits counteracting curvature of CRT screen surface

          • circuits counteracting different length of the beam (edges/center)

          • "linearizations" of brightness (GAMA correction)


          and probably much more I can not think of right now...



          But back to your question the stuff you are describing is applicable only on monochromatic CRTs with analog sweeping. As the color ones got a luminofor masks presenting form of a "pixelated" or "lineated" grids preventing of any skew. But still the compensations must be done (by rotating the sweeping slightly) otherwise the beam could jump a line ...



          However in digital era more recent CRTs are usually driven by a CPU and the sweeping is controlled digitally by DAC so no more skew as the vertical coil is no more changing during the vertical sweep ...






          share|improve this answer




























            7














            Yes there are a lot of compensations in a CRT like:




            • magnets counteracting background magnetic fields

            • circuits counteracting curvature of CRT screen surface

            • circuits counteracting different length of the beam (edges/center)

            • "linearizations" of brightness (GAMA correction)


            and probably much more I can not think of right now...



            But back to your question the stuff you are describing is applicable only on monochromatic CRTs with analog sweeping. As the color ones got a luminofor masks presenting form of a "pixelated" or "lineated" grids preventing of any skew. But still the compensations must be done (by rotating the sweeping slightly) otherwise the beam could jump a line ...



            However in digital era more recent CRTs are usually driven by a CPU and the sweeping is controlled digitally by DAC so no more skew as the vertical coil is no more changing during the vertical sweep ...






            share|improve this answer


























              7












              7








              7







              Yes there are a lot of compensations in a CRT like:




              • magnets counteracting background magnetic fields

              • circuits counteracting curvature of CRT screen surface

              • circuits counteracting different length of the beam (edges/center)

              • "linearizations" of brightness (GAMA correction)


              and probably much more I can not think of right now...



              But back to your question the stuff you are describing is applicable only on monochromatic CRTs with analog sweeping. As the color ones got a luminofor masks presenting form of a "pixelated" or "lineated" grids preventing of any skew. But still the compensations must be done (by rotating the sweeping slightly) otherwise the beam could jump a line ...



              However in digital era more recent CRTs are usually driven by a CPU and the sweeping is controlled digitally by DAC so no more skew as the vertical coil is no more changing during the vertical sweep ...






              share|improve this answer













              Yes there are a lot of compensations in a CRT like:




              • magnets counteracting background magnetic fields

              • circuits counteracting curvature of CRT screen surface

              • circuits counteracting different length of the beam (edges/center)

              • "linearizations" of brightness (GAMA correction)


              and probably much more I can not think of right now...



              But back to your question the stuff you are describing is applicable only on monochromatic CRTs with analog sweeping. As the color ones got a luminofor masks presenting form of a "pixelated" or "lineated" grids preventing of any skew. But still the compensations must be done (by rotating the sweeping slightly) otherwise the beam could jump a line ...



              However in digital era more recent CRTs are usually driven by a CPU and the sweeping is controlled digitally by DAC so no more skew as the vertical coil is no more changing during the vertical sweep ...







              share|improve this answer












              share|improve this answer



              share|improve this answer










              answered 15 hours ago









              SpektreSpektre

              3,078617




              3,078617























                  5














                  To expand further, it actually wasn't especially feasible — there is no easy solution that doesn't eliminate the interlacing.



                  Interlacing works because the timing of the vertical retrace varies. On odd fields it is triggered so that scanning resumes at the beginning of a line. On even fields it is triggered so that scanning resumes in the middle. Because of the diagonal scan, that sets one field 0.5 lines higher than the other. If the scan weren't diagonal then the two fields would not enmesh in that manner — they would instead sit exactly on top of each other, just starting in different places.



                  On a classic TV it's undesirable to make the diagonal scan anything other than diagonal because the flying spot during the capture process was diagonal. So you wouldn't be unskewing the image, you'd be skewing it.



                  On a monitor life is slightly different, and true horizontals are likely accurately to reflect the image. But it's also generally the case that monitors have smaller scan lines in order to output a higher resolution, so the effect is less visible anyway — on a 14" 800x600 monitor you're already talking about the right hand side being less than 0.3mm lower than the left, but being almost 28.5cm to the right. With a multi-sync monitor, how far down the right is compared to the left is a variable function of the resolution.






                  share|improve this answer






























                    5














                    To expand further, it actually wasn't especially feasible — there is no easy solution that doesn't eliminate the interlacing.



                    Interlacing works because the timing of the vertical retrace varies. On odd fields it is triggered so that scanning resumes at the beginning of a line. On even fields it is triggered so that scanning resumes in the middle. Because of the diagonal scan, that sets one field 0.5 lines higher than the other. If the scan weren't diagonal then the two fields would not enmesh in that manner — they would instead sit exactly on top of each other, just starting in different places.



                    On a classic TV it's undesirable to make the diagonal scan anything other than diagonal because the flying spot during the capture process was diagonal. So you wouldn't be unskewing the image, you'd be skewing it.



                    On a monitor life is slightly different, and true horizontals are likely accurately to reflect the image. But it's also generally the case that monitors have smaller scan lines in order to output a higher resolution, so the effect is less visible anyway — on a 14" 800x600 monitor you're already talking about the right hand side being less than 0.3mm lower than the left, but being almost 28.5cm to the right. With a multi-sync monitor, how far down the right is compared to the left is a variable function of the resolution.






                    share|improve this answer




























                      5












                      5








                      5







                      To expand further, it actually wasn't especially feasible — there is no easy solution that doesn't eliminate the interlacing.



                      Interlacing works because the timing of the vertical retrace varies. On odd fields it is triggered so that scanning resumes at the beginning of a line. On even fields it is triggered so that scanning resumes in the middle. Because of the diagonal scan, that sets one field 0.5 lines higher than the other. If the scan weren't diagonal then the two fields would not enmesh in that manner — they would instead sit exactly on top of each other, just starting in different places.



                      On a classic TV it's undesirable to make the diagonal scan anything other than diagonal because the flying spot during the capture process was diagonal. So you wouldn't be unskewing the image, you'd be skewing it.



                      On a monitor life is slightly different, and true horizontals are likely accurately to reflect the image. But it's also generally the case that monitors have smaller scan lines in order to output a higher resolution, so the effect is less visible anyway — on a 14" 800x600 monitor you're already talking about the right hand side being less than 0.3mm lower than the left, but being almost 28.5cm to the right. With a multi-sync monitor, how far down the right is compared to the left is a variable function of the resolution.






                      share|improve this answer















                      To expand further, it actually wasn't especially feasible — there is no easy solution that doesn't eliminate the interlacing.



                      Interlacing works because the timing of the vertical retrace varies. On odd fields it is triggered so that scanning resumes at the beginning of a line. On even fields it is triggered so that scanning resumes in the middle. Because of the diagonal scan, that sets one field 0.5 lines higher than the other. If the scan weren't diagonal then the two fields would not enmesh in that manner — they would instead sit exactly on top of each other, just starting in different places.



                      On a classic TV it's undesirable to make the diagonal scan anything other than diagonal because the flying spot during the capture process was diagonal. So you wouldn't be unskewing the image, you'd be skewing it.



                      On a monitor life is slightly different, and true horizontals are likely accurately to reflect the image. But it's also generally the case that monitors have smaller scan lines in order to output a higher resolution, so the effect is less visible anyway — on a 14" 800x600 monitor you're already talking about the right hand side being less than 0.3mm lower than the left, but being almost 28.5cm to the right. With a multi-sync monitor, how far down the right is compared to the left is a variable function of the resolution.







                      share|improve this answer














                      share|improve this answer



                      share|improve this answer








                      edited 19 hours ago









                      manassehkatz

                      3,032623




                      3,032623










                      answered 20 hours ago









                      TommyTommy

                      15.5k14376




                      15.5k14376























                          3














                          The effect you are describing did not matter, mainly because in a television the effect is so small, and the TV camera also had a CRT tube that converts light to video signal in a matching scanning pattern so the picture is in fact not tilted due to the scanning. Computer systems usually used progressive scanning on CRT monitors so for example VGA has twice faster line rate than NTSC TV so the lines are more horizontal. Even with earlier computer systems such as CGA that was NTSC compatible, most likely other distortions of the CRT were more noticeable than scan lines not being exactly horizontal.






                          share|improve this answer






























                            3














                            The effect you are describing did not matter, mainly because in a television the effect is so small, and the TV camera also had a CRT tube that converts light to video signal in a matching scanning pattern so the picture is in fact not tilted due to the scanning. Computer systems usually used progressive scanning on CRT monitors so for example VGA has twice faster line rate than NTSC TV so the lines are more horizontal. Even with earlier computer systems such as CGA that was NTSC compatible, most likely other distortions of the CRT were more noticeable than scan lines not being exactly horizontal.






                            share|improve this answer




























                              3












                              3








                              3







                              The effect you are describing did not matter, mainly because in a television the effect is so small, and the TV camera also had a CRT tube that converts light to video signal in a matching scanning pattern so the picture is in fact not tilted due to the scanning. Computer systems usually used progressive scanning on CRT monitors so for example VGA has twice faster line rate than NTSC TV so the lines are more horizontal. Even with earlier computer systems such as CGA that was NTSC compatible, most likely other distortions of the CRT were more noticeable than scan lines not being exactly horizontal.






                              share|improve this answer















                              The effect you are describing did not matter, mainly because in a television the effect is so small, and the TV camera also had a CRT tube that converts light to video signal in a matching scanning pattern so the picture is in fact not tilted due to the scanning. Computer systems usually used progressive scanning on CRT monitors so for example VGA has twice faster line rate than NTSC TV so the lines are more horizontal. Even with earlier computer systems such as CGA that was NTSC compatible, most likely other distortions of the CRT were more noticeable than scan lines not being exactly horizontal.







                              share|improve this answer














                              share|improve this answer



                              share|improve this answer








                              edited 17 hours ago

























                              answered 18 hours ago









                              JustmeJustme

                              2342




                              2342























                                  0














                                  If the horizontal deflection circuit had no effect on the vertical positioning of the beam, and if the vertical deflection circuit had no effect on the horizontal positioning, then the shape would indeed be a parallelogram.



                                  In practice, however, it would be very difficult to design a CRT-based display in which the effects were cleanly isolated in that fashion. Horizontal and vertical deflection circuits interact with each other for a variety of reasons, and instead of trying to prevent such interactions, most displays instead try to compensate for them with a mixture of adjustable and non-adjustable compensation circuits that can independently adjust the width of the screen at the top, middle, and bottom. Further, the angle of the yoke assembly is often adjustable. While vertical motion during each scan line would cause a slight skew if horizontal and vertical deflection were independent, consumer-grade equipment is seldom close enough to perfect calibration to make that significant.






                                  share|improve this answer




























                                    0














                                    If the horizontal deflection circuit had no effect on the vertical positioning of the beam, and if the vertical deflection circuit had no effect on the horizontal positioning, then the shape would indeed be a parallelogram.



                                    In practice, however, it would be very difficult to design a CRT-based display in which the effects were cleanly isolated in that fashion. Horizontal and vertical deflection circuits interact with each other for a variety of reasons, and instead of trying to prevent such interactions, most displays instead try to compensate for them with a mixture of adjustable and non-adjustable compensation circuits that can independently adjust the width of the screen at the top, middle, and bottom. Further, the angle of the yoke assembly is often adjustable. While vertical motion during each scan line would cause a slight skew if horizontal and vertical deflection were independent, consumer-grade equipment is seldom close enough to perfect calibration to make that significant.






                                    share|improve this answer


























                                      0












                                      0








                                      0







                                      If the horizontal deflection circuit had no effect on the vertical positioning of the beam, and if the vertical deflection circuit had no effect on the horizontal positioning, then the shape would indeed be a parallelogram.



                                      In practice, however, it would be very difficult to design a CRT-based display in which the effects were cleanly isolated in that fashion. Horizontal and vertical deflection circuits interact with each other for a variety of reasons, and instead of trying to prevent such interactions, most displays instead try to compensate for them with a mixture of adjustable and non-adjustable compensation circuits that can independently adjust the width of the screen at the top, middle, and bottom. Further, the angle of the yoke assembly is often adjustable. While vertical motion during each scan line would cause a slight skew if horizontal and vertical deflection were independent, consumer-grade equipment is seldom close enough to perfect calibration to make that significant.






                                      share|improve this answer













                                      If the horizontal deflection circuit had no effect on the vertical positioning of the beam, and if the vertical deflection circuit had no effect on the horizontal positioning, then the shape would indeed be a parallelogram.



                                      In practice, however, it would be very difficult to design a CRT-based display in which the effects were cleanly isolated in that fashion. Horizontal and vertical deflection circuits interact with each other for a variety of reasons, and instead of trying to prevent such interactions, most displays instead try to compensate for them with a mixture of adjustable and non-adjustable compensation circuits that can independently adjust the width of the screen at the top, middle, and bottom. Further, the angle of the yoke assembly is often adjustable. While vertical motion during each scan line would cause a slight skew if horizontal and vertical deflection were independent, consumer-grade equipment is seldom close enough to perfect calibration to make that significant.







                                      share|improve this answer












                                      share|improve this answer



                                      share|improve this answer










                                      answered 6 hours ago









                                      supercatsupercat

                                      7,352740




                                      7,352740






















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