The term R.F.I. is commonly used to describe a form of interference in audio circuitry caused by radio frequency signals. It can be abbreviated as “RFI.”
To prevent slew-rate limitations in audio electronics, amplifiers with bandwidth extending significantly higher than audio frequencies are commonly employed. In high gain applications, amplifiers are used with bandwidth extending into the Megahertz range. This is well into the frequency range of “radio” transmission.
As a result, the audio amplifier may operate in a non-ideal manner and some form of distortion or noise may result. This side-effect of the Radio Frequency (RF) signal is referred to as RFI. In some cases the signal may be partially de-modulated in a manner which makes it possible to hear a distorted version of the audio being transmitted by the RF. In other cases, the audible side-effect may be increased distort of the desired audio signal or wide band noise.
This is also the reason output filters are used on DA converters to prevent signals with frequency content higher than audio to pass to audio amplifiers downstream.
For more information on the problem with feeding signals containing frequencies higher than audio to audio amplifiers, see The Optimal Sample Rate for Quality Audio
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