SCA Technical Details and History
Most radio reading service are transmitted on FM SCAs. SCA stands for Subsidiary
Communications Authorization. In the late '40s and early '50s, the FM band was not a major
profit center for the broadcasters that had invested heavily in transmission equipment, and they
asked the FCC for additional ways to make money. Some FM broadcasters played easy listening
music and preceded all commercial breaks with a subaudible tone. Then, they "sold" the signal to
stores, with receivers that would mute the commercials when the tone was detected. The FCC
outlawed this practice, saying that although it would be legal to build a radio that could detect
commercials and dump them, it would not be legal for broadcasters to generate any kind of
special signal on the air to identify commercials. But, this practice helped create a market for commercial background music.
Because the FM signal contains so much more bandwidth than that required to send one high
fidelity signal, there were options that had not been implemented. (An AM station occupies 10
kiloHertz of bandwidth, and stations are spaced every 10 kHz. This allows a frequency response
almost up to 10 kHz, providing quite good fidelity to a decent AM radio (a rare breed in this day and age). However, an FM station
is allowed 100 kHz of bandwidth, and stations are spaced 200 kHz apart. The frequency response
is extended only up to 15 kHz, so much of the extra space was unused on mono FM. Even a stereo
FM station only uses about 55 kHz of bandwidth.)
To provide extra channels that could be sold, the FCC created SCA. Originally the commission
specified very precisely the SCA frequencies and bandwidth, but now broadcasters are allowed to
place any kind of signal, digital or analog, at any frequency and bandwidth, as long as it doesn't
damage the main channel signal or extend the bandwidth of the FM station (much) beyond the 100 kHz
A typical traditional SCA channel is an FM carrier wave (basically a high-pitched whistle at, say, 41, 67 or 92 kHz) carrying voice
signals (frequency response up to about 5 kHz), being carried on a much higher frequency carrier
wave (at maybe 91.5 mHz, as in the case of KANU).
In the early days, FM stations could put up two SCA channels, each with less fidelity (both
frequency response and signal-to-noise ratio) than AM radio. A trip up and down the SCA band
in a major city during the '50s, '60s and '70s would have shown three or four kinds of easy
background music, a field dominated by Muzak. ADT was a major background music supplier,
with their background music a part of a security package that could be installed in offices and
Then, the FCC added to the capabilities of FM radio by implementing a standard for stereo
broadcasting. Like color TV on black-and-white receivers, it was important to create a new
expanded signal that could be received with no change by all the existing mono receivers. So the
new stereo encoding technique involved the main channel as it always was, adding a carrier at 38
kHz (an octave above the best human hearing) with a channel called L-R (Left minus Right).
When this channel is added to the main channel, you get the left stereo channel. When it is
subtracted (electrically phase-reversed and added) you get the right stereo channel. To help your
radio find this channel and turn on a STEREO light, there is a strong pilot signal (about 10% of
the FM station's signal) at 19 kHz. That frequency is just high enough that you shouldn't hear it,
but your dog might. It is also possible for some of this signal to slip out your radio when you're
recording a cassette, and confuse the Dolby noise reduction circuitry; that's why most good
cassettes have a FILTER setting on the Dolby switch.
The problem with stereo FM was that it took up SCA space. Mono stations could transmit 2
SCAs, but stereo stations could only send one. That is one reason so many broadcasters resisted
"going stereo" until the big FM boom of the '70s, when people began to depend on the Stereo
light as an indication they had found a station. Around 1970, 75% of radio listening was on the
AM band. By the end of the decade, almost 75% of listening was FM.
By that time, the SCA market was exploding, and Radio Reading Services were afraid of losing
their carriers. Most RRS SCAs are provided for free (just like the end product to the listener) or
at very little expense to the RRS, but the same carriers began to be worth at least a couple
thousand dollars a year to commercial interests. In the '70s, many voice and data signals began to
appear on SCAs. Agriports used voice channels (and later data transmission) to bring commodity
prices to their network of grain elevators, brokers, etc. Physician's Radio Network created a
national audio network of advertiser-supported news reports. The Bonneville Corporation
created a high-speed data service with commodity prices. Many large cities became homes of
ethnic stations, with music and news in Chinese, Greek, Korean and other languages. Some
companies tried pagers. (In fact, Seiko is one of the major leasers of SCA channels because of
the success of their pagers around the US.) Power companies considered controlling
"load shedding" programs from SCAs, allowing them to turn off some large air conditioners and
other big-draw equipment during peak periods. Cities wanted stoplight control on SCA.
As a result, the Association of Radio Reading Services sought protection from the FCC. Most
services were based on carriers which were supplied free or cheap, but could be taken away the
moment a lucrative lease opportunity arose. The Commission finally decided, in Docket 82-1,
that a non-commercial FM had an obligation to provide to a radio reading service, if requested,
one SCA channel for a fee that would only cover the station's actual costs, if the station was
selling or leasing an SCA channel for profit. Enforcement is not simplified by any form of
codification, such as what actual costs can be charged, or what penalties could be.
With the advent of the SAP channel on broadcast television, some services adapted the idea, or
even based their entire system on TV SAP. Technically, the SAP (Separate Audio Program)
channel is much like radio's SCA, although the sound is better. However, the real difference is in
the reception. SCAs are usually heard by people using borrowed specialized radios; the actual
radio is owned by the service, which presumably knows where the radios are and who's listening.
Conversely, almost all stereo TVs (most TVs sold today) can receive the SAP channel.
Some services have worked out an agreement with their local PBS TV station to carry the RRS
signal until PBS transmits a TV show with Audio Description of the events. During the show, the
SAP channel is devoted to Audio Description, then back to the radio reading service when
broadcasting shows that don't have SAP channel information. Then again, more and more
programming is being provided with a Spanish soundtrack on the SAP channel; if this trend
is taken up by PBS, more SAP hours will be taken from the "radio" service.
Some RRS audio is available on the Internet in RealAudio and other streaming formats. To find out what
services are netcasting, go to the
International Association of Audio Information Services site.