On 2023-06-20 15:06, Lennart Sorensen wrote:

My understanding was that on a CRT it needed a bit of blanking time in
order to have time to change the magnetic field so the beam could start
at the begining of the next line.  Whether you used composite sync or
seperate H and V sync didn't matter, it still needed the time.

Yep. Here's what Wikipedia says:
"The VBI was originally needed because of the inductive inertia of the magnetic coils which deflect the electron beam vertically in a CRT; the magnetic field, and hence the position being drawn, cannot change instantly. Additionally, the speed of older circuits was limited. For horizontal deflection, there is also a pause between successive lines, to allow the beam to return from right to left, called the horizontal blanking interval. Modern CRT circuitry does not require such a long blanking interval, and thin panel displays require none, but the standards were established when the delay was needed (and to allow the continued use of older equipment). Blanking of a CRT may not be perfect due to equipment faults or brightness set very high; in this case a white retrace line shows on the screen, often alternating between fairly steep diagonals from right to left and less-steep diagonals back from left to right, starting in the lower right of the display."
https://en.wikipedia.org/wiki/Vertical_blanking_interval#Vertical_blanking_interval_in_digital_video

It was also possible to display video on an oscilloscope, which used electrostatic deflection. The scope would just need a trigger pulse, which didn't have much width.