I’ve noticed that when my TV displays large amounts of white text, such as in a commercial (Call this toll free number) or in credits, I hear a 60-cycle hum. When the text goes away, so does the hum. I don’t think it’s coming through the speakers, as this happens even if I mute the sound.
I’ve noticed this on practically every TV set that I’ve owned, and having cable doesn’t make any difference.
What causes the hum?
I am so glad you asked this question!
I hear the same thing and it drives me NUTS! My husband doesn’t seem to hear it, even when I point it out to him.
I used to say there was only one answer to this question, but I got my hand slapped so many times, I’ll amend it to two.
First off, remember that pure white is the most intense video signal.
So, when a station or network overlays a pure white block of text or whatever on top of the picture, it overloads the video signal, and the overload comes through as noise.
Then some quibble that it’s not an overlay. Okay, let’s say the automatic gain control in the video circuit can be overloaded by mixing light and dark, and you still get an audio buzz.
The brighter the screen image, the greater the current demand from the high voltage side of the tv internals.
To get those higher(and the actual value is fairly small) currents, greater stress is placed on things like the switching devices and line output transformers.(LOPT)
In the case of LOPT’s the increased magnetic field may cause the laminated layers of the iron core to vibrate.
Switching devices are used in modern tv sets to provide controlled power and even though they are solid state semi-conductors they can still vibrate.
I’d rather not go much into the whys and wherefores as it can get technical fast.
Suffice to say, the more you demand of something or somebody the more likely it is to make a noise about it.
It may well be that the sound you hear is not 60Hz but rather a derivative of it since that is right at the lower end of the hearing range, many switching devices operate well beyond human hearing but produce much lower harmonics.
Higher current demand from the power supply will increase the ripple (ac component). That ac ripple has a frequency of either 60 or 120 Hz depending on the design of the power supply. It’s more likely that you’re hearing 120Hz, as 60 would be a strain on standard lo-fi TV speakers.
Is it an older set? It’s possible that the filters are going dry in the power supply. In any event, all we can do is speculate. Go get the set looked at if it really annoys you. Even a novice tech can identify excessive ripple in power supplies if he has an oscilloscope.