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The fundamental line frequency of PAL TVs is 31.25 kHz but, because of interlacing, there is a component at 15.63 kHz, which is audible for the not-so-elderly.
The noise is produced by the line transformer which is also used to generate the 25 kV anode voltage of the CRT, the mechanism being magnetostriction. This phenomenon is because the transformer is wound on a ferrite core which changes size slightly according to the instantaneous magnetic field as the molecules reorient themselves. The core is a rectangle with a typical length of about 8 - 10 cm and the change in length is about only 0.1 micrometre (invisible to the eye) but is sufficient to create air pressure waves. Because of the hysteresis in the B-H curve of its magnetic properties, the effect is non-linear, which means that when maximum power is absorbed (white screen), the noise is disproportionally louder, but the non-linearity will also cause harmonics into the MHz range to be generated and possibly some low-level sub-harmonics, depending on the mechanical resonance of the ferrite. The transformer is always placed, nowadays, in a metal enclosure to comply with electromagnetic radiation regulations (at least in Europe). On the more expensive sets, the enclosure will have a foam lining to damp the acoustic waves by a few dB.
The noise is produced by the line transformer which is also used to generate the 25 kV anode voltage of the CRT, the mechanism being magnetostriction. This phenomenon is because the transformer is wound on a ferrite core which changes size slightly according to the instantaneous magnetic field as the molecules reorient themselves. The core is a rectangle with a typical length of about 8 - 10 cm and the change in length is about only 0.1 micrometre (invisible to the eye) but is sufficient to create air pressure waves. Because of the hysteresis in the B-H curve of its magnetic properties, the effect is non-linear, which means that when maximum power is absorbed (white screen), the noise is disproportionally louder, but the non-linearity will also cause harmonics into the MHz range to be generated and possibly some low-level sub-harmonics, depending on the mechanical resonance of the ferrite. The transformer is always placed, nowadays, in a metal enclosure to comply with electromagnetic radiation regulations (at least in Europe). On the more expensive sets, the enclosure will have a foam lining to damp the acoustic waves by a few dB.
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