Remember the olden days of TV’s? There were doohickeys for adjusting the picture. I haven’t seen them in many years–nor have I seen a decent vertical or horizontal roll in many years.
Whatever happened to such CRT gymnastics?
I imagine they are done automatically by the TV now, like the tracking on your VCR. Most VCRs do that by themselves now.
There’s usually a set of adjustments in the back, but since they’re so stable now, they often have to be accessed behind a panel or with a skinny skrewdriver in a deep-set hole, so amatuers aren’t tempted to change them.
In order for the picture to be vertically stable (no rolling) and horizontally stable (no tearing), circuits inside the set called oscillators had to be set to a particular frequence depending on the station you were trying to receive. That meant every time you changed the station, the set’s oscillator had to change along with it. This was usually accomplished by “ganging” the oscillator setting together with the channel setting. By rotating the channel sdelector, you were simultaneously selecting the desired station as well as setting the local oscillator to the correct frequency necessary to keep the picture steady.
This is now all done with magic. Chips inside the set know what the correct local oscillator frequency is supposed to be for every channel. They set & monitor the frequency to prevent it from drifting away. Drift was a problem with old analog oscillators- you would set the correct frequency (by adjustments to that wheel that was beneath the channel selector (remember that funky dial you had to simultaneously push in while turning?) and then a moment later (usually when you got back to your chair) the picture would slowly begin to roll/tear again.
The general term for a self-correcting circuit that holds oscillators at a certain frequency is phase locked loop, or PLL for short. Just about anything that can be said to have any kind of tuner in it makes use of a PLL circuit.