Many many years ago when TV was in its infant stages TV sets had controls on them called ‘horizontal hold ‘ and ‘ vertical hold ‘.
Why were these necessary and what did they actually do?
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As mentioned, there are synchronization signals included in the TV signal. But even without any signal being received, the TVs were designed to still paint something (static) on the screen. This was to aid people in adjusting the antenna and tuning to the signal.
To paint the screen when no signal was being received, the TV needed it’s own timing signal to trigger the painting of each line on the screen. The timing devices were not highly accurate in early TVs, so the triggering time wasn’t perfect. If the TV signal being received was weak, the TV might not lock-on to the synchronization signal being sent. Adjusting the controls for ‘horizontal hold’ and ‘vertical hold’ would adjust the internal timing in the TV set to match the incoming signal.
When the internal timers got better, this became less and less of a problem. First the ‘horizontal hold’ and ‘vertical hold’ controls moved from the front of the TV to the back. Then they moved them inside the case where generally only technicians would adjust them. I don’t know if they went away completely before TVs became computerized.
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