From: gary@ke4zv.uucp (Gary Coffman)
Subject: Re: FM broadcast station sidebands
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Destructive Testing Systems
Date: Thu, 28 Jan 1993 06:00:35 GMT

In article <1993Jan26.122411.10075@hemlock.cray.com> dadams@cray.com writes:
>Ok this may not be the best newsgroup for this, but then hams 
>would know the answer to this better than any other group I can
>think of.
>
> years ago I learned that broadcast stations in the FM comercial 
>broadcast band (88-108 MHz) often do not need to use all of the 
>spectrum alloted to them for broadcasting what they broadcast, and
>hence they some times broadcast other things on sidebands like
>elevator music etc.  You can't pick this up on normal recievers.  They
>sell special recievers to businesses and other organizations so that
>they can play this elevator music or background music.  I think that
>it is usually mono (not sterio).  When I lived in the Chicago area
>many years ago, we had this light and easy station (I don't remember
>the name) that I believe is owned by Boneville International.  They
>would broadcast proceeding of church conferences for the Mormon Church
>on one of these sidebands.  My family bought this radio that would 
>recieve these broadcasts.  This radio was fixed to recieve this one
>side band only.  You couldn't tune it to receive any other sidebands or
>stations.  (On occasion you could hear the main station bleeding over
>I think.)
>
>So my question is, what would one have to do to modify a reciever, or build
>a reciever to tune in such broadcasts?  Would it be hard?  Why don't 
>commercial manufactures supply such a function, at least for the more
>pricy recievers?

The word you are looking for is "subcarrier", and the equipment you
want is called a SCA or SubCarrier Adapter. All stereo FM stations,
and BTSC stereo TV stations transmit subcarriers. For FM stations,
there is a suppressed carrier at 19 kHz carrying the L-R information
of the broadcast while the main channel transmits L+R, and a 38 kHz 
pilot tone to phase lock the quadrature demodulator for the stereo 
reconstruction. These subcarriers are applied as baseband modulation
to the RF carrier. 

Above the stereo subcarriers, other program material may be encoded on 
other subcarriers. One common frequency is 62 kHz. This subcarrier may 
be NBFM modulated with secondary programing such as "elevator music" or 
foreign language translation or reading service for the blind or stock 
ticker information or _Sky Page_ pager information and so on. These 
programs have common carrier status and are not intended for reception 
by the general public. That's why no commercial broadcast receiver has 
a built in SCA. "Elevator music", or Muzak, is a commercial service
that *charges* for it's programing, as do the _Sky Page_ and stock
ticker services. Reading for the Blind is a free service, but only
makes SCA equipment available to blind people.

However, SCA equipment is often sold in the back pages of hobbiest
magazines such as _Popular Electronics_. Basically how they work is
they are connected directly to the demodulator of the broadcast radio
before the low pass filtering that removes any signals above the
audible range. They use a PLL circuit to lock onto and demodulate
the 57, 62, 67, or 72 kHz subcarrier signal.

Gary
-- 
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | uunet!rsiatl!ke4zv!gary
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Newsgroups: rec.radio.amateur.misc