March 27, 2008

Cheap TVs Buzzing

Stupid Question ™
Feb. 25, 1999
By John Ruch
© 1999

Q: Why do some cheap TVs make a buzzing sound when words are written over the picture?

A: Cheap TVs often have cheap audio circuitry. They’re vulnerable to overloads, which result in that nasty buzzing.

The buzz usually happens when there’s a bright, light-colored image on the screen—and white lettering used by news, sports and advertising spots is one of the brightest possible images.

The overload is a problem with “IF,” or “intermediate frequency,” circuits in the tuner. They’re amplifier circuits—some just for sound, others for both the picture and the sound.

The IF circuits mix the weak incoming signal from the antenna (or cable line) with a stronger internal signal, eventually making it strong enough for us to see and hear on the TV set.

Cheap IF circuits may overamplify the signal, or conversely, not be able to handle the incoming signal’s strength. Either situation can lead to overload.
The previously pure frequency of the audio will be disrupted and cluttered with “electrical noise” (the electromagnetic disturbances caused by heat and leaking electricity), producing the dissonant tones we call “buzz” when it finally comes out the speakers.

If you’re stuck with a buzzer, you can try adjusting the contrast control downward. The contrast control adjusts the gain of the video signals in the IF circuits. Turning that down may weaken the signals enough that they no longer overload the audio. But you may have to lower it so much the picture is a muddy mess.

TVs also have an “automatic gain control” that’s supposed to stabilize the signal level and prevent some types of overload. A professional will have to open the set up to tinker with that.

But the root problem is the sheer cheapness, and it’s hard to find a cure for that. I asked a manager at the local repair shop Thompson Electronics what he’d do about this sort of buzz.

His suggestion: “Live with it.”

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