Noise Reduction   

There are various noise reduction systems for audio.  

Most are basically companders (combination expander, compressors.) 

When recording, the compressor, by reducing the differences between the most quiet and loudest sound permits the optimum recording level for the signal.  On playback, the signal is exapnded back to its original dynamic range with the quiet passages (in principle) having less background noise.  If the noise is not masked by loud passages then one may hear the compander “pumping” or “breathing”. 

There are several systems: the more sophisticated divide the signal into multiple bands so that dynamic changes in one part of the spectrum does not aversely affect other parts. 

Dolby A was the first (four band system) followed by Dolby B then the superior Dolby C for home entertainment systems - mostly 1/8” cassette. 

These both deal mostly with the high frequency hiss endemic to cassette systems. 

Dolby SR, the current professional standard, is extremely effective - it enables analogue tape systems to have virtually digital noise specifications.  There is a home entertainment version of SR called Dolby S.

Another system, dbx works in a similar fashion to Dolby with the exception of the dbx system being a linear compander which amplifies high frequencies when encoding of recording the signal and filtering the high frequencies when decoding or playing back the signal.

Tape hiss is undesireable noise present at low levels on each track of recorded material.  

In multitrack systems tape hiss is cumulative. With each doubling of tracks the associated noise due to tape hiss is increased by 3 db. So the noise of a two track recording is 3 db higher than that of a mono recording, while the inherent noise of four tracks is 3 db higher than two, and so on. As track density increases the importance of effective noise reduction techniques becomes increasingly important. Luckily there are very effective noise reduction techniques available for use with analog tape which rival the signal to noise specifications of digital recordings. 

Dolby A, B, C, S, SR 
Compander (input compression, output expansion)