Chapter 1 WHAT IS "SCA"? (Secret Club Activity?) Well, in a way, it's kind of a secret club. At least it is defined by the FCC (Federal Communications Commission) as a transmission system of a broadcast nature not intended for the general public but for a specific audiences reception. (See FCC Rules & Regs. 73.293/73.593) Since SCA transmissions are not "broadcasts" in the public sense, they are protected as to private reception. Only the operator of an SCA may legally purchase SCA receivers and distribute them on a loan or service lease basis. Ownership must remain with the SCA operator, else that receiver might conceivably be used to pick up the SCA of some other station without authorization. Many of us are aware of "bootleg" SCA receivers and/or SCA adapter units that have been offered for sale to the public. Make no mistake - these are strictly illegal and in violation of the Communications Act of l934,Section 605; and punishable by up to $10,000 fine and/or two (2) years in prison. (Sect. 502) Please note that the law, Sect. 605, makes ANY use of such devices unlawful, not just the selling of them, but the possession and even the unintentional reception of the transmitted information. Don't believe those who would try to convince you that "private personal" use is 'OK'. 'Tain't so. Recently a "bootleg" supplier in California has advertised that, since Radio Talking Book programs are "free" (non-commercial), it is OK to receive them. Not so!! The FCC has prosecuted several people for this violation, so there IS precedent. For some examples, several years ago the FCC stopped LaFayette Radio from selling an FM tuner they advertised to include SCA reception as one of it's features. And, more recently, the FCC again denyed a petition for authorization to distribute tuneable, or multi-station selectable, SCA receivers by the "paging" industry.) Why be so "sensitive" to this? Because ARRS (Association of Radio Reading Services) worked very hard for several years to convince Congress that agencies should be exempt from copyright restrictions on materials "broadcast" to handicapped people via SCA. The Congress agreed, since SCA is a private transmission technique, and made a precedent setting change in the copyright laws of this country. There also is the somewhat "touchy" additional concern of "broadcasting" todays books and magazines without editing the language. Of course the material must not be edited -- but you couldn't get away with such "descriptive" wording if it were broadcast to the general public. Thus, we must keep the SCA services as private channels! To allow deterioration of that could terminate all of our services. Well, I guess I've got to 'fess up and be honest with you; SCA doesn't really mean "Secret Club Activity". "SCA" means "Subsidiary Communications Authorization", an FCC term which means just what it says; the FCC has authorized the FM broadcasting station to conduct communications on a secondary (subsidiary) channel to it's main channel, general public broadcast service. Basically how this works is in the fact that every FM broadcast station occupies a channel that is 200KHz (KHz=1000 cycles per second; pronounced "kilohertz") wide. Let it suffice to say that this bandwidth is necessary to the FM broadcast technique. The stations assigned frequency (place on the dial, if you will) lies precisely in the center of the 200KHz channel. That is, the occupied channel of the station is lOOKHz above and below the stations assigned frequency. This assigned "resting" frequency of the station is often termed to be the "carrier". (Actually, this is a terribly inaccurate term, since in actuality, it doesn't really "carry" anything -but we engineers weren't always that smart. But how FM, or AM for that matter, really works is a subject for a different paper!) When you put program information (speech, music, or what have you) onto the stations "carrier", new carriers called "sidebands" are created. These "sidebands" lie above and below the stations main "carrier" in a mirror image fashion. That is, for every sideband that is at some distance above the stations carrier, an identical sideband will appear the same distance below the carrier. Since the "sideband" signals are mirror . images on each side of the "carrier" it is convenient to forget about those "sidebands" below the "carrier", for this discussion, and describe what's happening just with the sidebands above the "carrier". Remember, there is lOOKHz of space available above the "carrier" for sidebands to reside in. Of that space, the F.C.C. currently allows us to use 75KHz maximum, leaving the top 25KHz as a "no mans" area of protection between the station and his "next-door" neighbor stations channel. (More on this later in Chapter 9. In the case of Monophonic FM broadcasts (yes these still exist, but they're getting rare) the program information ("modulation") creates primary sidebands which occupy the space from the "carrier" up to l5KHz. Thus, there is 75KHz-l5KHz=60KHz of "unoccupied" space available for some other uses! In the case of Stereo FM broadcasts, the sum of Left and Right (L+R) audio program information also occupies the first l5KHz of space. Next, a new sideband, termed the stereo pilot carrier (this is what turns on the "stereo" indicator lamp on your FM tuner, besides doing some other important functions inside the tuners circuits),is created at exactly l9KHz. Next, the difference between the Left and Right audio program information (L-R) creates new sidebands which lie from 23KHz up to 53KHz of the space. Thus, an FM station broadcasting in stereo occupies up to 53KHz of the available 75KHz of space. Therefore, 75KHz-53KHz=22KHz of "unoccupied" space still available for some other use. SCA is this other use! If a new "sideband" is created which will fall into the otherwise unoccupied space of the FM broadcast station, and if this new "sideband" can also be modulated by some program information, then a new "broadcast" service can be conducted via the FM station in addition to it's usual broadcast operation. Before stereo was authorized by the F.C.C. (Yes, SCA existed long before stereo, and in fact was used by the early FM stations to keep themselves fiscally healthy at a time when commercial FM broadcasting was viewed by much of the public as a curiousity), SCA was found at different locations, 41KHz being very popular once. Also, with the 60KHz of available space on monophonic FM it was possible to have more than one SCA being carried. After stereo FM broadcasting became a reality, SCA's were usually placed at 67KHz. This put the SCA l4KHz above the L-R stereo information (which extends up to 53KHz) and 8KHz below the 75KHz maximum allowable limit of the FCC. Today SCA at 67KHz is a defacto standard. It should be mentioned that the program information which modulates the SCA is applied as an FM technique also. Therefore, the SCA can be viewed to be a "carrier" which also occupies space. The loudest program sounds will cause the SCA to extend 6KHz above and below the SCA "carrier" frequency of 67KHz, thus the occupied space of the SCA on the FM broadcast station will be from 67KHz-6KHz=61KHz to 67KHz+6KHz=73KHz. (In actuality it will be greater than this, but full explanation requires understanding of Bessell functions, among other things, and as stated earlier, is a subject for another paper on how FM "really" works.) To bring this all back to a level of practical, visual understandability and (hopefully) erase much of the mystique we engineers sometimes seem to enjoy creating (good for the ego, you know!), let me give this illustration of an SCA. If you were to go into the FM stations control room and start whistling when the announcers microphone was active on-the-air, he would hear you and so would all of the listeners to the station! In short, you would not be very popular, immediately. But suppose you went into that stations control room carrying one of those so called "silent" dog whistles, (you know - they are such high pitched sounds our ears cannot hear them but Fido's ears work at higher pitches than ours so he can.), and now you blow that whistle into the stations microphone. The announcer won't hear it, the listeners to the station won't hear it.(If their Hi-Fi systems are really super then their dogs just might!) So now you remained popular as you whistled this time, and if you played a tune on your dog whistle, maybe the Fido's out there in Radioland might enjoy the music. This is very basically what the SCA is. A high pitched, above audible tune added to the FM station's programs. At the receiving locations, special detecting and decoding electronics in the radio convert the information contained on this SCA "whistle" back into understandable sounds of speech (or music). This diagram of the FM stations channel occupancy will (hopefully)clarify the relationships of the transmitted program information by the station: ("Lower Sideband" Signals) 0 ("Upper Sidband" Signals) ......||||||..|||||||||||..|||||||||||..|||||||||||..||||||........ -100 -73-61 -53 -23-15 0 15 23 53 61 73 100 SCA STEREO MAIN CHAN. STEREO SCA 200KHz FCC ASSIGNED FM STATION CHANNEL Definitions: 0KHz = FCC Assigned Station Frequency. (i.e. 91.1MHz) 0 to l5KHz = Main Channel (L + R) Program Information l9KHz = Stereo Pilot Carrier 23 to 53KHz = Stereo SubChannel (L - R) Program Information 61 to 73KHz = Subsidiary Communications Authorization (SCA) 75KHz = Legal Channel Limit. (FCC Rules & Regs.) lOOKHz = End Limit of FM Channel (FCC Rules & Regs.) Remember, the L - R program information and the stereo pilot carrier will only exist during those times that the station is engaged in stereo broadcasting of it's main channel programs. In summary there is nothing very secret nor mysterious about SCA. It is a simple technique by which we add additional information to the existing FM stations channel in a manner that will be inaudible to the regular listeners to the station. For practical operational reasons the use of this SCA service must remain to be a private, non-general public, transmission. With care, correct technique and controls the SCA and the main channel operations will provide excellent quality performance and co-exist in complete harmony.(More on this in Chapter 3, "SCA and Stereo Compatibility".)