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
The picture would occasionally move upwards or left to right and needs to be moved so it is stationary in the middle of the screen.
Yes I remember seeing it, but why did it happen.
It suddenly seemed to be cured and TVs didn’t have these controls anymore?
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.
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.
Great , got it now, thanks guys.
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.
Latest Answers