Quote:
Originally Posted by Kiwick
I think a rectangular screen's surface is "spherical" just like a round screen's one... the only real difference is the shape... ..
|
True in part, but there are a whole bunch of considerations -
Corner convergence;
Bulb strength;
Corner purity - which is related to brightness gradation:
To make obtaining purity easier, the shadow mask holes (or equivalently, the black matrix holes) were made smaller towards the edges. This was done so strongly on some rectangular designs that people complained that there was a bright area in the center of the picture (this was not a sharp-edged area, just the gradual variation from center to edge). Due to the lack of low-frequency response in the human visual system, you can get away with some of this, especially when the picture is viewed up close - but from far away it's more noticeable.
Later tubes were designed with flatter faces, making all the problems more difficult.
By that time, stripe screens had replaced dot screens, allowing self-converging yoke designs. Since vertical beam angle does not affect purity on a stripe screen, that degree of freedom can be used to help get convergence, and the convergence circuits can be eliminated.