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TUCoPS :: Radio :: muzakrul.txt

The Rules governing SCA and Muzak

From: (Harold Hallikainen)
Newsgroups: comp.dcom.telecom
Subject: Re: Information Wanted on History of Muzak
Organization: California Polytechnic State University, San Luis Obispo

In his article (Mathew Zank) writes:

> Does anyone know about the history of Muzak, the service that brings
> office music?  All I know is that it is on a sub-channel of a local
> FM radio station.

	From the Broadcasting Yearbook history of broadcasting:

	Subsidiary FM Service - To aid FM broadcasters, the commission
in 1955 enabled them to apply for subsidiary communications
authorizations for supplemental services such as background music.
Sometimes called "functional music", this specialized service is
offered to stores, factories and other business subscribers.

	Originally, subsidiary communications were permitted on a
simplex basis, the station devoting part of the time on its channel to
regular broadcasting and part to this specialized service.  Later,
rules were adopted requiring subsidiary communications to be on a
multiplex basis, that is, using one or more subchannels with the main
channel used for regular broadcasting.  The FCC permits FM
broadcasters to use their subcarriers for a broad variety of new

	Looking back in my 1980 FCC Rules, 73.293 required:

	Programs must be of a broadcast nature which are of interest
to limited segments of the public (ie, are point to multipoint).
Examples cited include background music, storecasting, detailed
weather forecasting, special time signals, special programs for
business, professional, educational, religious, trade, labor,
agricultural or other groups engaged in any lawful activity.  An SCA
could also be used for "operational communications" of the station
(feeding programming to other stations, giving cues to remote program
production units, etc.).  A subcarrier could also be used for the
station's own transmitter telemetry, though this did not require an
SCA (subsidiary communications authorization from the FCC), since it
was not a "point to multipoint" service.

	The rules also made provision for transmission of visual
material.  I believe they had the "electronic newspaper" in mind,
using fax technology of the time.  The SCA application had to include
full details on the visual transmission system proposed.  It's unclear
whether digital data transmission would be considered "visual", since
it is often displayed or printed.

	73.295(b) prohibited the use of subcarrier control signals to
delete main channel material (such as commercials).  In the early
1970's, I worked for an AM/FM combo.  The FM was automated with
"beautiful music".  I modified the automation to provide a separate
program output that consisted of all the music (which came from reel
to reel with 25 Hz EOM (end of message) cues) but did not pick up the
commercials (which came from tape cartridge "carousels").  We fed this
music only service to a few local background music customers over
phone lines.

	73.319 said that subcarriers (whether SCA or operational) must
use frequency modulation.  The instantaneous frequency must be between
20 KHz and 75 KHz for stations not transmitting stereo.  Stereo
stations must keep the instantaneous frequency between 53 and 75 KHz.
Subcarrier "injection" (modulation level of the main carrier by
subcarriers) was limited to 30% for mono stations and 10% for stereo
stations.  The center frequency of the subcarrier was to be within 500
Hz of the authorized frequency (most stations used 67 KHz).

	Now, 73.293 permits FM stations to use subcarriers for a
variety of purposes without specific FCC authorization.

	73.295 permits subsidiary communications to be used for the
purposes listed previously plus radio reading services, specialized
foreign language programs, bilingual television audio (although the
stereo TV SAP channel has probably replaced this application), and
point to point services such as paging, traffic signal control,
utility load management, etc.  They also specifically recognize data
services such as financial market data.

	73.319 now allows any method of modulation of subcarriers.
Previously, FM was required.  Some stations are transmitting multiple
audio programs using several single sideband AM subcarriers.  Others
are using FSK of the subcarrier for data transmission.  It would
appear that any modem technology (including QAM) could be used,
keeping in mind the frequency and noise limitations of the channel.

	Stations transmitting monaural are now allowed to have the
subcarrier carrier and significant sidebands between 20 KHz and 99
KHz.  Stereo stations can have sidebands and subcarriers between 53
KHz and 99 KHz.  The "injection" of subcarriers is still limited to
10% for stereo stations and 30% for monaural stations.  However,
stations transmitting subcarriers are now allowed to "overmodulate"
slightly (go over 75 KHz peak deviation" depending on the number of
subcarriers and their injection levels.

	Now, AM stations and TV stations are also authorized to run
subsidiary communications.  TV's generally run subcarriers on their
aural carrier (which is FM).  This is the basis of the SAP (secondary
audio program) and PRO channels.  The PRO is generally used for
operational communications, including transmitter telemetry.

	AM stations are allowed to run multiplex transmissions
(73.127).  These transmissions were originally limited to subaudible
transmitter telemetry (which used a voltage controlled oscillator
where 20 Hz represented a reading of 0 and 30 Hz was full scale,
although one system used biphase data transmission, real slow).
"Subcarrier injection" was limited to 6%.  Now, stations may transmit
anything they want as long as they stay within the authorized spectrum
(pretty much from carrier out to 10 KHz above and below carrier) and
the signal is not audible on "ordinary consumer receivers".  These
systems have used both amplitude and phase modulation of the AM
carrier.  They have generally been used for utility load management,
though I've heard very little of this use in the past several years.
Both of the currently used AM stereo systems utilize a subaudible
pilot tone, pretty much making the subaudible region unavailable for
subsidiary communications.

	So, there's more than you ever wanted to know about SCA!

Harold Hallikainen    
Hallikainen & Friends, Inc.
141 Suburban Road, Bldg E4      phone 805 541 0200 fax 544 6715
San Luis Obispo, CA 93401-7590  telex 4932775 HFI UI

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