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Dbx (noise reduction)
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===dbx vs. Dolby=== Both dbx and the [[Dolby noise-reduction system]] use companding to control noise. They differ in the way they address the frequency response of the companding process. dbx uses a single frequency pre-emphasis system, whereas Dolby uses four separate pre-emphasis amplifiers, each for a different frequency band. Since tape hiss is primarily a problem for high-frequency sounds, Dolby uses much stronger pre-emphasis at high frequencies than low. This means that a low-volume, low-frequency signal may see little or no companding, whereas the same volume at high-frequencies will have been strongly pre-emphasized to a higher volume level before compression.<ref name="Nave_2001_5"/> The use of separate pre-emphasizing "encoding curves" allows the overall compression to be much less than it would be on dbx, where it is always 2 to 1. For lower frequency signals, like a conversation, Dolby may apply no compression at all. In contrast, dbx would continue to compand these signals, in which case the tape hiss is also re-expanded on playback, continually varying as the volume changes.<ref name="Roberts_2004"/>
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