Mryntu

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:

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?

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.

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.