RE: Dead FM Broadcasting Technologies

["Hakim Madjid" <HMadjid@mindspring.com>]

Richard Chonak writes:

>Didn't WCRB use Dolby FM?   Man, that was annoying.  Perhaps it
>was useful to the 3 audiophiles in Massachusetts with Dolby FM
>receivers.

>Yes WCRB did use Dolby FM, IIRC - they were the first or one of
>the first to do so in the Boston area.

Theoretically, the change from 75 microsecond (50 in europe) pre-emphasis
to 25 microsecond pre-emphasis when transmitting Dolby - FM was supposed
to partly compensate for the treble boost created by the Dolby decoder when
listening
on non-Dolby equipped receivers.

A couple of things doomed Dolby FM to the proverbial ash-heap, IMHO.
One was the aformentioned pre-emphasis nonsense which meant exisiting
receivers would have to be modified to properly decode Dolby FM.
Switchcraft (which marketed to consumers at the time) sold a special
box that you would insert between the line output of your tuner and
your external Dolby unit. This little box would compensate for different
de-emphasis required. Unfortuneately the box was a passive component, and
it had something like a 12 to 15db insertion loss. That being said,
there were FM tuners and receivers that were brought to market that
had a front panel 'Dolby FM' switch - to change the de-emphasiss in
the tuner. And a few tuners/receivers were brought out on the market with
with built-in Dolby.

The other user unfriendly aspect of Dolby-FM was that to avoid audiable
artifacts, the Dolby decoder has to be calibrated properly. That means
you had to be listening in during the times of the day when the station
would broadcast a Dolby level calibration tone.

One other problem was that in the early to mid 1970s Dolby was a relatively
expensive feature to build into audio components or car radios. The first
single-chip Dolby B decoder (made by Signetics IIRC) did not come out until
1979 or 1980 or so.

Who knows? Had there been an inexpensive Dolby decoder chip in 1975 we might
be all listening to Dolby FM nowadays.

73, de Hakim (N1ZFF)