Chapter 3 SCA AND STEREO COMPATIBILITY (Birdies Should Be Feathered) My good friend, -- well, I think he's still my good friend, and highly respected colleague Mr. Leonard Hedlund, former SCA Research and Design Engineer of McMartin Industries, Inc., wrote an outstanding paper on this subject in about 1970. Although I will perhaps cover most of the same territory here, I would recommend his paper as additional information on this which is an often misunderstood subject. I'm certain his paper is probably still available through McMartin Industries, Inc. Just give Mr. Jay McMartin a call and tell him "Bob" sent 'ya. (This may not get you a paper, but at least we'll find out if we're still friends!) In Leonard's paper he reached the conclusion that, yes, SCA and stereo were completely compatible (He was right!) and then he proceeded to describe in a few pages, how the FM station engineer, or operations, could really "screw it up" if they tried hard enough. Unfortunately, there are too many FM stations already "screwed up". And no one has really tried to fix them eitner! OK, let's start at the beginning. Perhaps you didn't know that a great many FM station owners, and even engineers dislike SCA's. They, quite sincerely, believe that adding SCA service to their station will cause interference with their stereo broadcasts. How could such a ridiculous idea ever get started? Well, it wasn't so ridiculous - or so it seemed for quite a long time. Back in the "good ole days" when FM broadcasting was only monophonic, and many commercial FM stations were kind of starving for lack of adequate listening audiences, - (Remember?......"FM? What's that?" --- "Oh, it's a new kinda' radio that doesn't have any static.") - the FM broadcasters had a great idea. They asked the FCC for permission to transmit another special service along with their normalbroadcasts. The FCC approved this and named it "SCA"! Well, it worked out great. The broadcasters sold music background services to stores, etc., and survived to become todays big stereo stations. Oh yes, the public eventually "got with it" as radio technology improved and more and more people began to listen to FM radio all the time. But, a funny thing happened. When the FM stations with SCA added stereo information to their broadcasts, the station engineer, with the corners of his mouth turned downward and blood in his eyes (the bloodshot eyes were because he had been up all night working on the transmitter!) informed the station manager that the stereo system worked "great - But!" That "but" was that, try as he might, there was a whistle, in@fact a warbling kind of high pitched whistle, in the background on the stereo programs! Never lacking in imagination when it comes to inventing terms to call things, we engineers named such sounds to be "birdies". When our station manager inquired if he meant that there was a "birdy" present, the exhausted and quite frustrated engineer replied.."Birdy, Hell! It sounds like a damn Eagle!" When confronted with difficult technical problems, we engineers always proceed with developing solutions by using the scientific method. That is, we turn things off! Low and behold.....whenever you turned off either the stereo or the SCA the "birdy" disappeared. Since nothing could be found wrong with the stereo, and since nothing could be found wrong with the SCA (after all, the SCA had been working well for several years!) there must be some God ordained incompatibility between these two systems! That being the case, and not having God-like powers ourselves, (oh, some of us often do think we have, or course), then there was little left to do but decide which system had to go! This wasn't too difficult of a decision for the station manager. Being a shrewd businessman, he realized that shuttling off the SCA would cost him revenue; but being the newest, biggest, best STEREO service in town (or so his newspaper ads would claim) would attract more listeners. With more listeners, his advertising time rates could greatly increase and easily more than exceed the lost revenue from the SCA. In this situation he observed, he really could win from losing! (the SCA). So -- SCA's started to be shut off all over town and across the country as stations converted to stereo. The alarm was quickly spread..."Get rid of SCA if you go to stereo or your signal will sound crummy". Now, this problem with the SCA wasn't imagined. All one had to do was listen to the station monitor to hear that "birdy" screaming at you. And, after all, you had just spent about "five grand" to purchase the monitor so wasn't it the best "radio" that money could buy? Of course it was! Not only that, but hundreds of irritated (and often times hostile) listeners were calling the station manager to tell him, if he didn't do something about "that noise", they would never ever listen to his station again. (And they would tell their friends not to listen also!) Believe me, this was more than enough to convince even the most determined of station managers and stubborn engineers. Of course many of us engineers, not wanting to admit that we didn't really have God-like abilities, continued to try to find out what was causing this problem. It took a lot of years in which we explored some really "far out" theories. Actually, it wasn't all that complex... at least it always seems that way once the answers are found. And the answers were found. Through the use of test equipment which can draw a picture of the signals, (very much like what was illustrated back at the end of Chapter 1), it was readily proven that these "birdy" sounds were NOT being broadcast by the FM station. That is, they weren't being broadcast if the station was designed, adjusted and operated correctly. The marvelous piece of test equipment which allowed us to prove this, and thus confirm what our theories had already speculated as being the problem, is called a "Spectrum Analyzer". What the spectrum analyzer showed us was that there were no "birdy" signals leaving the FM broadcast transmitter, but lo, when you looked at the stations signals inside the monitor "radio" as they came out of the stereo decoder circuits, there it was! The little devils were being created inside the stereo radio! The same was found to be true in our listeners (and our own) stereo radios. As Pogo once observed..."We've met the enemy and he is us!" We engineers had designed circuits that were excellent -- (well, almost excellent) -- for decoding the stereo information. These circuits really would get the right channel information into the right hand speaker and left into left. But, they were also capable of reacting to the SCA information, which was none of their business. In fact, as it turns out, the "better" the stereo radio was, then the worse was this SCA "birdy" problem. This certainly wasn't good news, since those listeners who were the most serious about getting high quality "Fine Arts" stereo music and were the most critical listeners, spent the most money to get the "best" equipment and ended up with the biggest problem. How do you gently break the news to an irrate listener that has just spent $500.00 or more for an FM tuner, that his new radio is the source of his problem? Well, again, the prudent escape route always was to get rid of the offending SCA instead. Another approach was taken in the attempt to have stereo and SCA coexist. Not a bad idea to try to do this. After all, why not keep the "bucks" rolling in from the SCA service too if you can. (For the less cynical of us, there also was a motivation to try to also provide a needed public service -- Radio Talking Book.) This other approach that was taken was simply to install a "filter trap" inside the FM radio that would prevent the SCA information from reaching the stereo decoder circuits. Well, this really worked pretty well, except no one could design an effective filter-trap that could be manufactured and fit into the radio that didn't cost three to four times the cost of some of the most expensive FM radios, and that wouldn't, to some extent, degrade the stereo performance of the radio. Certainly, it appeared to be time for another engineering compromise. (We engineers do this all the time when we can't think of a solution that will make everyone happy - or solve a problem without spending money.) So the generally accepted solution was for the manufacturers to install an inexpensive "filter-trap" in the radios and ignore the fact that it somewhat reduced the amount of left to right separation of the stereo channels. This made the most people pretty happy. Careful design, with improved circuits and attention to adjustments, just like at the stations transmitter, resulted in most FM stereo radios having little if any "birdy" problems. A few of the very high priced radio manufacturers refused to use the filters so a small number of listeners might still be not too delighted, but what else could we do? By gosh - we did do something! At least the I.C. (Integrated Circuit) manufacturers did! About eight years ago a new I.C. was developed to decode stereo. This circuit used a long known method of "demodulation" called a "Phase Locked Loop" (P.L.L.). This is a system which "locks" onto the broadcast stations signals and tracks what they are doing. Previous to the miniaturization of the I.C.'s, a P.L.L. circuit was large, complex and expensive. But now, with it available as a small plug-in circuit chip that only cost a "few" dollars in mass production, it was availahle to replace our stereo decoders which had been creating SCA "birdies". In addition, the P.L.L. was a much superior stereo decoder, having far better stereo separation performance than other circuits and, for all practical purposes, no SCA "birdies". Since it was actually less costly to use the P.L.L. than the previous discrete circuitry, the radio manufacturers immediately began building their radios using it. Now even the "cheapest" of FM stereo radios have excellent stereo performance (for some other reasons they might not) and be free of any SCA "birdies". Now, isn't that great? We all enjoy stories with a happy ending! Well, not quite. It could be a happy ending -- except.....(Darn. Everytime I say "except" I find my friends beginning to lose faith in me.) You see, there still lurks in the bowels of nature, effects which could also create the troublesome SCA "birdy". That, plus the fact that so many station managers and engineers still believe the "birdy" problem is real and hasn't been solved. (Some people sure are hard to convince!) What are these "natural" effects which can create an SCA "birdy"? Mostly "multipath". Hey, now here's a technical term that means just what it says; multi = more than one, and path = route by which something travels. Multipath signals will always be arriving at the radios location, and vary in their number and strength depending upon the environment. This is because radio signals will bounce off of walls, buildings, airplanes, etc. on their way to you. Very simply expressed,if the signals broadcast by the FM station (or any radio station for that matter) reach the listener by more than one path, they will combine inside the radio in such a manner that will "confuse" the circuits as to "who's on first". The result is the creation of new signals in the radio that were not ever broadcast, and therefore, what comes out of the radio will be various forms of new and different sounds than what you expected. The most visual illustration of the effects of multipath is on a TV set. (And you thought this was about SCA and FM radio? Well, FM radio and VHF TV are both broadcast side by side in the world of "Radio-land" and so exhibit similar effects!) If you use a so-called "rabbit-ears" antenna on top of your TV set, and move them to various positions, or even move yourself invvarious locations, you will observe that the picture will go from "great" to various forms of smearing and "ghosts". You have just experienced the effects of multipath. In your FM radio the effects are not quite so visual. Of course not, you say - they can only be audible, and you're right! Multipath effects can "wipe out" (and frequently does) stereo separation performance, it can produce SCA "birdies" which are usually not "too" bad, but it can happen. Likewise, multipath can (and very frequently does!) cause the main channel stereo broadcasts to be audible into the the SCA program reception (We call this main to SCA "crosstalk"). Can anything be done about this? Yes! There are some electronic circuit methods which are better than others at tolerating multipath reception, and bless those designers and manufacturers who use them. However, the most direct and simplest technique of all is to just use a decent antenna which is also correctly "aimed" at the broadcasting station. Remember how your T.V. picture "cleaned up" and became stable once you installed a better antenna? The same is true for FM stereo and/or SCA. If it isn't possible, or practical, to install an outdoor antenna then make the most of what you've got or can do. For example, try using a simple "folded dipole" antenna made from 300 ohm TV lead-in wire on your FM tuner, instead of just nothing, or a random length of wire. Also, don't forget to try adjusting it's positions. Incidently, a good clue that you have FM stereo multipath troubles on your FM tuners is by watching the signal strength meter and "stereo" light. If eitherone appears to flicker or wiggle along with the program sounds you probably have multipath which needs tending to. (Either that or your radio, or their transmitter, are "sick".) We have now reached that happy ending to our story. SCA "birdies" do not have to exist. If they do then someone is creating them. Snoop around a little bit and then very soon you too will be able to say....."We met the enemy and he is us!" My purpose here is not to try to make FM station engineers of you, nor is it to try to teach other engineers what they probably already know. However, I would feel my comments on this very important subject were not complete without at least mentioning some of the places where "birdies" could be created if care and attention to detail is not pursued. 1. First of all, station monitors are "type approved" devices, and as such they continue to be good "birdy" generators. Don't believe what you hear from them! Install a decent P.L.L. stereo decoder outboard of the monitor and hear how really good your stereo broadcast sounds. (You can't take F.C.C.'s "legal" readings from it - but we all know it's probably more accurate.) 2. Examine the stereo pilot carrier for harmonics and clean them up. (Especially the 3rd harmonic.) They very often are not clean. 3. Assure that the 38KHz stereo subcarrier is kept suppressed to @50Db. Keep an eye on it, they often change over fairly short times. 4. Beware of any instability in any of the R.F. amplifiers. Re- generation tendencies are an often overlooked source of problems. Also, verify that all neutralization adjustments are correct. 5. Assure that lMHz bandwidth is maintained throughout all R.F. stages. 6. Maintain the SCA injection at 9% to 10%. 7. Assure SCA AF bandwidth limit is 5KHz. 8. Maintain the SCA deviation at 5KHz to 6KHz peak. 9. Assure that good quality, sharp cutoff, l5KHz filters are used on left and right audio channels. 10. Control main channel modulation levels religioulsy. If peak limiters are not used, keep the modulation levels down to provide the necessary headroom. 11. Keep VSWR between exiter, driver, final, and antenna a 1:1 . 12. Sweep test antenna system to assure that it is correctly tuned to your transmit frequency and that it's bandwidth (reactance) is symmetrical on each side of resonance. If you assure that these items are attended to, you not only will be rewarded with an SCA operation that will provide very good coverage and performance, but you will also receive a "birdy free" stereo signal that you most likely can correctly brag about as truely being the best one in town! We've been saying that about MPR here in Minnesota for more than a dozen years now. (Sorry, I can't help you much with the programming format selection. That's another department.) So we close this chapter proclaiming (as did Mr. Hedlund around 1970, or so) that, yes, SCA and stereo are completely compatible. May all your birdies be the feathered kind.