do you remember that high-pitched whining sound that old school tube TVs used to make? (š§µ) 
and yes you don't hear it anymore because 1) LCD TVs don't work the same way and 2) you're super old now and you can't hear frequencies that high. speaking of, what frequency was it anyway?
the answer is that it depends on where you live!
in the USA, Canada, and some parts of South America that used the NTSC standard, the sound was about 15.734KHz.
but if you lived in most of the rest of the world which used the PAL standard, the sound was about 15.625KHz. (also used by SECAM)
but if you lived in most of the rest of the world which used the PAL standard, the sound was about 15.625KHz. (also used by SECAM)
what actually makes the sound is not the TV tube itself, but the special flyback transformer that generates the high voltages (30,000V+) required to drive the tube.
which is a little odd, actually, because that frequency is the horizontal retrace rate--the rate at which each line is painted on the screen. so what does that have to do with high voltage?
it turns out you can drive the high voltage transformer at different frequencies, but if it's not exactly locked to the horizontal retrace rate, you will see visible interference on the resulting picture.
same deal with the vertical retrace rates. those were picked to match up with the AC power line frequencies so that you wouldn't see interference patterns. i.e. NTSC=60Hz (actually 59.94Hz for obscure reasons) and PAL=50Hz.
some people say it has to do with the crappy power supplies in cheap commodity TVs, but that's only part of it. electrons can be deflected by magnetic fields, which is how the deflection yoke in the TV works. but it also applies to ambient magnetic fields.
and you better believe that your room is full of magnetic fields generated by the AC wiring in the walls! so syncing those up with the TV frequencies is a really good idea.
so why was i thinking about the whining sound of old school tube TVs? well, NTSC, PAL, and SECAM are not the only TV standards.
from 1936 to 1985, the UK had a 405-line TV standard that had a horizontal retrace rate of 10.125KHz. that must have sounded *really* annoying! i'm curious to hear from anyone in the UK who remembers these sets.
the BBC operated 405-line broadcasts in parallel with PAL for quite a few years, because loads of people had old B&W 405-line sets.
from 1949 to 1984, france was enjoying their 819-line (more than twice as good as the UK system, right? š) TV system with a horizontal retrace rate of 20.475KHz, which would have been inaudible to all but the most sensitive individuals.
in the 1980s, computer monitors typically followed the TV standard frequencies, but by the 1990s the resolutions had increased to the point where typical horizontal retrace rates were >30KHz which is well above the audible range.
