[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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
> >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.

 SteveOrdinetz replied
> 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.

As I understand it, high frequencies below the -10 db threshhold were
were boosted such that the lower the level the more the boost i.e. an
expander circuit.  Crucial to Dolby B is calibration.  Dolby B will play
back correctly IF and only IF the record drive is properly set i.e.
record -10db tone and playback produces a -10 db tone.  Some consumer
decks aren't properly calibrated and those that are,  only record and
reproduce correctly for the particular oxide formulation of the tape
used to calibrate the unit.  Let's say a user records on a tape that
plays back 2 db lower than the level recorded.  All high frequencies
will be 2 db lower going into the Dolby decoder.  Since Dolby B
attenuates high frequencies inversely to the recording curve, recorded
high frequency content around -10 db will play back at -12 db.  It will
be reduced in volume per the Dolby B curve.  Result: muddy playback.  I
have noted owner's manuals for many Japanese cassette decks specify
Maxell (Japanese) tape was used to calibrate the deck.  BASF tape is
true Chromium Dioxide (Cr02), whereas Maxell is Cobalt-modified Ferric
Oxide.  While they share identical record bias and playback frequency
curve characteristics, Cr02 requires several more db of record drive
(than Cobal Doped Ferric Oxide) to produce the same output.  Therefore,
BASF, when recorded upon in a deck calibrated for Maxell Cr02, produces
a much lower output on playback and the Dolby circuit will severely
muffle the highs.  Record Drive is an internal setting of the deck's
electronics, not simply setting the external record level higher.

Roger Kirk