Very cool video about CRTs, their lines number and the relation with their frequency. A bit of maths involved but not so much. The presentation is very clever too, with a real CRT as a display instead of video overlays, it's pretty neat.
https://www.youtube.com/watch?v=3GJUM6pCpew
And the making of
https://www.youtube.com/watch?v=PYZJ3csb_rg
There's only one thing I disagree with, because I have to: the "the framerate was chosen so that the TV is in sync with the current its powered with", which I believe is a myth. ...
Well, it's not. The first TVs where indeed synced by the mains. This sort of
syncronization was known as "direct sync" and was quickly abandoned due all the
instability issues it suffered from, yet was pretty cheap to realize. However,
the problem was twofold. Getting rid of the mains was fine, but is only half
the story, because a pretty huge problem for TVs in the earlier days was signal
strength vs. picture quality. With a weak video signal you will get noise etc.,
but the main issue was that the TVs got out-of-sync before the picture quality
turned unacceptable! Hence, huge emphasis was put on building some very
sophisticated sync circuits to keep the image stable even for very weak video
signals. For, the TVs now produced their own sync signals which got syncronized
(due to the syncing circuit) with the sync pulses from the video signal. Now
even with some of the sync pulses missing, the pictures remains stable for a
couple of lines. This has lead to these smooth wavy patterns seen when
receiving a weak video signal, but with the picture's content still perceptible
(up to a given point, when the pulses are missing for too long). Without such
circuits, the image would desync immediately, which was a pain for people
living in the mountains. The knowledge put into these sync circuits is
tremendous esp. considering those build out of vacuum tube.