Editing Dbx (noise reduction) (section)

===Pre-emphasis===
Note that the tape hiss is limited to higher frequencies. That means a signal that is primarily low-frequency does not necessarily require noise reduction. Instead, one can simply roll off all the higher frequencies in a low-pass filter, and the hiss will largely disappear.<ref name="Nave_2001_4"/>

Consider a signal that contains a high-volume section and then low-volume. During recording, these signals are compressed to be much closer together in level, so that the high-volume section does not saturate the tape and the low-volume section is louder than the tape hiss. On playback, the louder section has little or no muting applied, so the tape hiss is also left alone at its natural volume. When the softer section plays, having been amplified during recording, the expander mutes it down its original level. This also mutes down the tape hiss.<ref name="Nave_2001_4"/>

This causes the volume of tape hiss to change during playback. This is not really noticeable when the original signal contains high frequencies that play over the hiss, but for lower frequencies, this can be easily heard. The rise and fall of the tape hiss was known as "breathing" because it sounded like something breathing into a microphone.<ref name="Nave_2001_4"/>

To address this, dbx uses strong high-frequency "pre-emphasis" of the original signal. This amplifies high-frequency sounds before they are sent into the compressor. This causes the compressor to 'back off' the gain in certain circumstances and reduce the audibility of noise modulation – even with this pre-emphasis, noise modulation can become audible when using very noisy media to begin with, such as the cassette format.