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
Alright, imagine you’ve been given a big blank whiteboard and your job is to write stuff on it. I’m going to hand you a series of letters – you’re going to write the letters in a row, left to right, until you reach the end. When you hit the end of the board, you run back to the left, one row down, and start writing again.
1 2 3 4 56 7 8 9…
This is how old CRT televisions (and computer monitors) worked – a beam of electrons is sweeping across the screen, side to side, displaying one row of image at a time before moving on to the next row. When it hits the very end of the bottom corner of the screen, it snaps back to the top and starts over with a new image. It did this *very* fast, so fast (when it worked correctly!) that the human eye couldn’t really tell and just thought we saw a bunch of complete images flash on the screen.
But this leads to a synchronization problem: What if the list of stuff to write isn’t synced up to where the tv writes it (either because we’re ‘writing’ the wrong line, or because we aren’t starting new lines where we should be)? The stuff on our screen would be misaligned!
So, we include special synchronization information in the list of stuff to write. When we see that sync signal, we know that we’re supposed to be starting a new line or new image and we should be able to adjust automatically if we’ve gone out of sync…but just in case we can give controls to a person to manually tell the tv to either directly adjust how it’s synchronized, or to force the tv to wait for the next sync information it gets and go from there. Those are your ‘sync’ or ‘hold’ controls you’ll see on some old hardware.
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