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Fundamentals of Noise Reduction

Here we are in the year 2000 and the following question made it occur to me that many young engineers and musician’s will never know the wonders and idiosyncrasies of analog recording. “I’m using a Fostex 1/4 inch 8-track machine to record a demo for my band. If I record with noise reduction on, does it affect the sound when it is recorded to the tape or does it only affect playback?” Many different forms of noise reduction have been developed and used over the years. Most use an encode/decode process, where the signal is altered as it is recorded to tape, and the process is subsequently reversed when the tape is played back. Dolby and dbx have been the most prominent brands of noise reduction, and with only a couple of exceptions their schemes have all been of the encode/decode variety. The Fostex 1/4 8-track machines have used either Dolby B or Dolby C (depending upon which vintage), both of which do alter the signal going to tape. Dolby does this by applying what is sometimes known as pre-emphasis to the signal before it is recorded. In simplistic terms this is basically a high frequency boost. When the tape is played back the signal is de-emphasized (high frequencies cut), which removes a lot of the noise we hear.

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