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RE: Dead FM Broadcasting Technologies

Garrett Wollman wrote:

>The Dolby B system for audiotape worked like this:
>1) Boost the high-frequency range (where tape hiss is most noticeable)
>of the original source by a few dB.
>2) On playback, attenuate the same range by the same amount.  The
>noise, which (being a playback artifact) was never amplified, drops
>down to inaudible levels.

It was a little more complex than that in where it was also level 
sensitive.  Audio above a certain threshold (~-10 dB iirc) was not 
pre-emphasized/de-emphasized.  The theory being that with signal level 
above a certain point the hiss would be low enough to be inaudible anyway, 
and doing this helped prevent high frequency tape saturation.

>That was the theory, anyhow.  In practice, it didn't always work so
>well.  Later came Dolby C and Dolby S noise-reduction mechanisms,
>neither of which gained much popularity.  I think that the patent on
>``B'' expired just in time for digital recording to take over and
>introduce other sources of noise that are (by design) less

Not to mention improvements in tape formulation and quieter electronics.