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Path: utzoo!watmath!clyde!cbosgd!ihnp4!mgnetp!mgweed!rjr
From: rjr@mgweed.UUCP (Bob Roehrig)
Newsgroups: net.audio
Subject: ringing in my ears
Message-ID: <20479@mgweed.UUCP>
Date: Wed, 18-Sep-85 13:51:54 EDT
Article-I.D.: mgweed.20479
Posted: Wed Sep 18 13:51:54 1985
Date-Received: Thu, 19-Sep-85 06:03:25 EDT
Organization: AT&T Information Systems - Montgomery Illinois
Lines: 37


Subject: Hearing TV sets (etc):

In a TV set, and most video monitors, The High frequency signal heard
is normally 15734 Hz. (In the old Black & White transmission days it
was 15750 Hz). This is the frequency used for the horizontal deflection
circuits. This signal (basically a sawtooth waveform) is used to drive
the horizontal coil in the deflection yoke. This is usually a high current
circuit. The same signal is used to develop the high voltage used to power
the 2nd anode in the CRT. The high voltage is developed either by a winding
on the flyback transformer (then rectified) or by a transformer and a voltage
multiplier circuit. The reason for using this high frequency signal for the
high voltage source is that it needs little filtering after rectification.
It is also safer and cheaper than using a separate high voltage power supply.
(Many years ago the high voltage supply was separate and people were killed
by its high current capability). In the case of color tv's, this rectified
voltage often runs near 24 kilovolts. 

Any component, typically the transformers involved, can vibrate at this
frequency and be heard for quite a distance. The problem is normally
eliminated in manufacture by proprer potting of the devices that can
vibrate.

(By the way, The horizontal TV oscillator is locked on frequency to the
received signal. The only time it will be off frequency is if the horizontal
hold control is WAY off, or there is a malfunction in the set).

Light dimmers:

Light dimmers use a TRIAC type device to control the lamp intensity. 
They are controlled by a pot. Unlike a variable autotransformer or
a pot, the peak voltage out of the dimmer is always the same. What
is actually being varied is the duty cycle of the waveform (or the
pulse width). When the triac fires, The edge of the waveform is sharp,
not sine shapped. This sudden "kick" is what can cause the lamp
filament to vibrate.