CA Technical Details and History

Owl Radio, like most radio reading services, is transmitted on FM SCA channels.  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 sub-audible 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.  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 allocation.  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 89.1 mHz, as in the case of KSTX).

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 also was a major background music supplier, with their background music a part of a security package that could be installed in offices and shopping malls.

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 MULTIPLEX 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 reading service 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.

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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.

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Executive Director: Lisa Miele

Owl Radio Coordinator:

Jeff Chandler,

Low Vision Club Program Coordinator:
Cynthia Garcia

1250 NE Loop 410, Suite 630 San Antonio, TX  78209-1536             (210) 829-4223
Thanks to Texas Public Radio (TPR) for donating our broadcasting sub channel.