Audible Noise and Interference
AC-powered appliances can give off a characteristic hum, often called "mains hum", at the multiples of the frequencies of AC power that they use (see Magnetostriction). It is usually produced by motor and transformer core laminations vibrating in time with the magnetic field. This hum can also be evident in poorly-constructed audio amplifiers, where the power supply is inadequately filtered.
Most countries chose their television vertical synchronization rate to approximate the local mains supply frequency. This helped to prevent power line hum and magnetic interference from causing visible beat frequencies in the displayed picture of analogue receivers, but is of diminishing importance in modern digital display systems.
Read more about this topic: Utility Frequency
Famous quotes containing the words noise and/or interference:
“I throw myself down in my chamber, and I call in, and invite God, and his Angels thither, and when they are there, I neglect God and his Angels, for the noise of a fly, for the rattling of a coach, for the whining of a door.”
—John Donne (c. 1572–1631)
“The truth is, the whole administration under Roosevelt was demoralized by the system of dealing directly with subordinates. It was obviated in the State Department and the War Department under [Secretary of State Elihu] Root and me [Taft was the Secretary of War], because we simply ignored the interference and went on as we chose.... The subordinates gained nothing by his assumption of authority, but it was not so in the other departments.”
—William Howard Taft (1857–1930)