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?
In: 5
Analog TV signals was just a continuous stream of brightness and color info that got painted onto a screen line by line from top to bottom and when it reached the bottom the it started over from the top.
There was no information that said something like “this is info for the part of the picture that is a third from the top and a quarter to the right.” it was just a continuous stream.
So if the TV started to paint the picture at the wrong part of the stream it would end up looking halfway scrolled up or to the side. If the TV started to pain the picture faster or slower than it was supposed to the picture would roll being across the screen.
You could adjust this manually in older TVs so that the picture stayed put and was centered correctly.
More modern TVs didn’t need that manual adjustment anymore.
There is a small gap in the signal when the stream reaches the bottom right before it starts over in the top left. In this gap the signal doesn’t transmit any new information while the mechanism that paints the picture reorients itself (This gap was used in some countries to transmit videotext.)
Newer TVs could actively notice that gap and know when the picture starts in the top left and adjust if they were too slow or fast. So rolling wasn’t really a problem anymore most of the time.
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