Emphasis

In recording (and broadcast) emphasis is a process where a signal is somehow altered prior to recording (or transmission) with a subsequent process at the other end that returns the signal to its normal state. At the recording end this is usually referred to as pre-emphasis, and on the playback (or receiving side in broadcast) this is known as de-emphasis. The purpose of emphasis is to change a signal in such a way that when it is eventually returned to normal, undesired characteristics that may be picked up along the way get minimized. The most common example in audio recording is noise reduction. Analog recording always adds some amount of noise to the signal. This white noise, as it’s called, is heard by humans primarily as a high frequency phenomenon (even though it truly is full band). Many popular forms of noise reduction for recording work by boosting (or pre-emphasizing) high frequencies upon recording and then cutting (de-emphasizing) them on playback. The theoretical result of this is that the desired audio is returned to normal sound, but any other high frequency content (noise) acquired along the way is reduced.