The Weird World Of FM Subcarrier Broadcasting 

I remember making a few SCA converters for myself (which was illegal, but if you weren't selling them, nobody really cared) and the shop I worked for had a contract to set up and repair these radios for two local subscription companies. The radios would typically come from the factory without being tuned for any specified frequency. We would set up the (very tight) notch pass filters to allow only one station be be received while in SCA mode. (Otherwise, you could receive other services, that you weren't paying for, as well.) If I remember correctly, most of the billing was done on the honor system (the company would call subscriber and get the chronometer reading) and was only physically checked once or twice per year. One of the services eventually abandoned the chronometer system and just went 'flat rate' to be more competitive against the other service. The whole system started falling apart in the 80's when offices, beauty salons, doctor's offices, barber shops, etc. realized that their customers really didn't mind hearing a few commercials. As a last ditch effort to save this business model, there was a push in the mid 80's and early 90's to fine businesses who played standard radio for their customers. It was considered "using the radio content to provide entertainment". (It's like the warning on VHS tapes which said that you could only use them in your home and not for audiences.) The law behind it was very loosely based on royalty laws in which bands and mobile DJ's were supposed to keep track of and pay for the songs they used to entertain audiences. (Which very few, if any, did.) The fines levied against businesses were based in a very grey area, to say the least. Let's say that you were a barber and had a radio at your barber station. That was OK. If you placed the radio out in the open and turned it up for your customers to hear, you would get fined. If you connected a remote speaker to the radio to cover a larger area of the barber shop, you would get fined. If you provided music to your customers, even just to make them more comfortable waiting for their car to be repaired or waiting to see a doctor, you would be fined. It was a complete disaster. When the subscription companies finally conceded that they just couldn't force people to pay for radio (and were getting pressure from judges for clogging up court rooms over people fighting the fines), they finally gave up. SCA still exists in some areas, but is mostly used for specialized services and most of the services are free. Some companies like professional offices and hospitals still use music subscription services. Very few are SCA. Now, most are satellite programming. That's about all I remember. I'm sure somebody else can help fill in the blanks or correct anything if I recalled something wrong. Hope that helps. Feel free to ask any other questions. It might jog my memory a bit.

I am a music programmer (aka "DJ") for our NPR station here in Humboldt County, California - KHSU 90.5. We still use our sub-carrier signal to broadcast to blind and sight-impaired individuals. A local non-profit (Volunteer Services of the Redwoods) has people who read the local newspaper and other periodicals - over the air - on a daily basis. No subscriptions - the radios are handed out to those who need them. I suspect there are similar operations scattered throughout the country.

"Fran" is obviously her mild-mannered alter-ego. There's no way this woman isn't out fighting crime at night. Undoing dastardly deeds by nefarious characters and saving the world at the last second.

Fran, back in the 70’s & 80’s I was involved in operating 6 FM stations. We leased our subcarriers out, not to PRN, but to several other companies that provided subscription services. One was an agricultural info channel sold to area farmers. One company provided a one-way “digital” data service that fed data receivers that provided real time commodity pricing and market data. Of course the signal was analog, but a tone signal was broadcast and received as data. The audio sounded much like an old 128 baud modem. Anyway, it wasn’t always the station that sold the subscription service. All we did was lease out the subcarriers. We called it “easy money”!

I used to work for a company named American Quotation Systems originally in Rochester MN and later moved to Champaign Il.. I would install McMartin receivers in grain elevators and farmers homes to receive the Chicago Board of Trade quotes for corn and bean prices. We used the SCÀ on numerous radio stations across the midwest. We had our signal originate on KROC in Rochester then install a radio receiver in Waterloo IA at KWWL that picked up KROC. We would then feed the audio onto the SCA of KWWL. The next hop was to either Carol IA going west or to Moline IL eastbound. Eventually we covered from Nebraska to Indiana and Minnesota to Kansas. My job entailed installing antenas on the radio station towers to connect our networks and on the legs of grain bins or on top of a grain silo and run the coax from it to the sca radio in the office. I usually would have it located near the scales so farmers knew what they would get for their corn or soy beans as they were being weigh or unloading. The radios could be tuned to any station by changing the crystal to each station using a formula of the frequency minus 10.7 devided by 2. So 96.7 was minus 10.7 equaled 86 divided by 2 so I would install a 43 mhź crystal. The tr66 was the most popular unit installed. There was a switch so you could listen to main radio station or the SCA. Unfortunatly many folks didnt care for the station we used ie Ròck or country Music so the left them om the SCA. I had installed 100s of the units at almost every grain company in every town in Iowa and illinois. Eventually I left to work for a large computer company and Reuters bought the lock stock and barrel of AQS. Computer type terminals were coming out and replaced the radio, with the data then sent via the same SCA signal and demodulated to print. on a screen. Progress in the mid 70's

SCA originally stood for Subsidiary Communications Authority, much in the same way that JPG stands for Joint Photographic Group. Over the years it began to be known as Sub-Carrier Audio.

In the early '70's, my wife worked in an realty office that had this same style of radio with the chronometer on the back. Every 3 months someone would come into the office and take the reading off the chronometer, then a week or so later, they'd receive a bill in the mail for the amount of time the "service" was used. It was metered like electricity.......

Fran I enjoyed that being a retired dentist I had one of those in the early eighties. However, the reason I posted was about the shot of the album cover from Steely Dan. If you look closely at that photo you will see a disembodied hand on the console. That is unless the guy with his boots on the console has 2 left hands. Let's just say that a group of us discovered this in an altered state way back in the day and it has puzzled us to this very day.. I even mumbled "No static at all" when you said "FM".......forgive my musings

Naming it PRN (Physician's Radio Network) was a mildly clever little play on words. So, "prn" is a medical abbreviation (or more precisely an initialism). It's latin for pro re nata. By convention, doctors still write prescriptions in Latin abbreviations. "Prn" is a very common one and it's probably one of the abbreviations most well known by people who aren't in the medical field. It means "as needed" which is the most commonly used english translation and is what you'll see on your prescription label if it was written prn. Maybe a more literal translation is "as the circumstance arises." If you take a pill twice a day, for example, and you're meant to take it continuously without skipping any, your doctor will write the Sig (which means directions) as: i po BID (the i means 1 and it's actually written like an upper case T with a dot on top but small like the lower case letters, there's just no character for it that I can type as far as I know). BUT, if your doctor only wants you to take it only as needed, like xanax or a pain pill or something where if there's no pain or no anxiety at that time, then don't take it, then your doctor will write: i po BID prn (which means 1 by mouth twice daily as needed). I know I'm rambling on but I just thought I'd share. It was mildly amusing to me lol.

Hi Fran. I don’t remember PRN the way that you described it. Rather than a commercial free music broadcast for doctor’s waiting rooms, it was actually educational programming geared toward physicians and included commercial advertising for pharmaceuticals. I do not recall the counter on the back of the radio but my guess was that it was used to tally the amount of time spent listening to the continuing education content in order to claim credit toward a yearly certificate of completion.

When I was a kid, nobody believed me when I told them that AM broadcasts bounce off the ionosphere, and was the reason some stations weren't available a few hours after sunset. It used to drive me nuts when Radio 1 in the UK was only on AM, john peel's show would only be half way through before it faded out.

I worked at an FM station that leased SCA channels. One was leased to the power company. They deployed addressable receivers that controlled AC thermostats and water heaters in peoples homes. They could manage groups of users to reduce peak demand on summer days. The other SCA was leased to a business that rented radios to people that wanted to listen to programming they provided in Greek.

FM subcarriers are still in use, at least in big cities with large ethnic populations, as most of the subcarriers in use today are providing foreign language programming. Most subcarrier receivers are fixed to one frequency; I've never seen a design like this before which provides a tuning dial but uses microswitches to limit the subcarrier tuning range. I guess when doctors were paying for the subcarrier service by the minute, they could afford to make such a complex design!

I have 2 books by Zenith discussing their proposed FM stereo system (which was adopted by the FCC). One of the texts suggests that SCA subscription services were already in use for some time prior to the introduction of FM stereo and that a feature of their stereo multiplex system was that it could coexist with the existing subscription services stations had at the time. It also suggested that at that time many FM stations needed the subscription revenues to survive. I have a 70's vintage SCA adapter kit (though it is missing the IC so I can't build it) the instructions explain that it is designed to connect to the output of the FM detector in a normal FM radio and it will decode the SCA transmissions from that. Meaning that like an FM stereo MPX adapter the SCA adapter once aligned did not require any tuning (tuning was strictly the job of the normal FM dial of the radio). Odds are by adjusting those slugs on the SCA add on board in that Sony you have messed up the SCA adapter alignment likely making it incapable of tuning any SCA transmissions until it is realigned correctly. If my understanding of what I have read is correct both FM MPX stereo and SCA were ultrasonic audio carriers modulated by normal audio. MPX being a 19KHz pilot tone and 32KHz carrier AM modulated by the difference between L and R channels (that would be added and subtracted from the L+R mono FM audio to create stereo). Where SCA was a 67KHz and or 100KHz (what I've read wasn't very clear on freq) carrier that was FM modulated.

I was a volunteer at a local FM station in 1968. The station had huge audio tapes that supplied the sub-carrier with Musak. Never figured out how they sold it.

There’s something enchanting about the static crackles, fading and distant feel of AM, particularly SW. Some of us started in electronics because of it. Oh, don’t forget the oscillator whine in superhet versions of AM. In this particular case, why not build a TV broadcast to FM band downconverter. You will defined be able to detect the voice on your PRN.

I found this article about it. DOCTOR'S RADIO NETWORK TO GO OFF THE AIR MAY 31 Published: May 21, 1981 The Physicians Radio Network, a round-the-clock service sponsored by several giant pharmaceutical companies, will discontinue broadcasts on May 31. In 1974, Visual Information Systems, a division of the Republic Corporation, initiated the station exclusively for doctors - it currently reaches 80,000 physicians in 69 cities. Jay E. Raeben, president of Visual Information, said that his company had ''failed to persuade enough of the industry that the radio was a medium important to use.'' Transmitting on an FM sideband frequency, or subchannel, which could not be picked up on a standard radio dial, the network permitted doctors to communicate among themselves, more freely perhaps than might be possible before a listening lay public. This feature, however, contributed to the station's downfall. The necessary special receivers were distributed to doctors upon request and without charge. Mr. Raeben said that the cost of manufacturing and mailing such equipment had diminished revenues substantially. Used by Advertisers Physicians rank as the profession most vigorously sought by adverstisers, because their prescriptions largely determine the profits of billion-dollar drug companies. ''Surveys show that P.R.N. affected sales very positively, especially as it impacted new products,'' said Robert E. Devinna, director of advertising for Roche Laboratories, one of the sponsoring companies. Eight minutes of every hour on the air are devoted to advertising. Programming focused on scientific breakthroughs and significant operations, such as the recent surgery on the Pope. ''P.R.N. rarely announces new drugs,'' said Mr. Raeben, who also acts as managing director of the station. ''We made a very considerable effort to insure that programming was not in the interest of the advertisers.'' If a new drug were announced, it would have to be newsworthy in itself, he asserted. May Turn to Journals Mr. Devinna of Roche Laboratories thinks that some of the major drug companies that advertised on the network will probably turn more to medical journals now. He views this as ''shortsightedness of the marketing industry,'' and added, ''Traditional advertising channels are cluttered - it's a shame to see P.R.N. die.'' The end of Physicians Radio Network will apparently not work any great hardship on its originator. ''Republic Corporation does not expect to lose any money due to the termination of P.R.N.,'' Rembrandt P. Lane, an executive vice president, said He said that Republic had tried unsuccessfully to sell the station, which had, in fact, been profitable for the last three years. ''P.R.N. is a very small investment by a division of Republic Corporation,'' Mr. Lane said, ''having a limited marketplace - and the 80,000 subscribing doctors paid nothing for the service or the radio.'' Mr. Lane declined to specify the operating expenses of the network, or Republic's initial investment.

Thank you Fran for opening Pandora’s box(literally your SONY PRN radio), to allow your viewers a peak into the mystery world of subcarriers. In addition to the primary L-R stereo subcarrier, each secondary subcarrier such as Muzak or PRN had a limited injection level of between 5 to 10%. This low a level affected the signal-to-noise ratio, making subcarriers more susceptible to noise, inter-mod, and interference from the primary L-R stereo channel. Broadcast engineers had to carefully monitor the injection levels, or risk complaints from the subcarrier customers who could be very demanding at times. Besides private music services, subcarriers were used for a poor stations TSL(transmitter to studio link), to return telemetry for remote transmitter meter readings, open door alarms, tower light current (beacon and side-lights), temperature, etc. Later when pagers caught on before cellphones, paging data was sent over a subcarrier, usually on one of the dominant high-power FM stations in a market. Nationwide paging was big for 10-15 years starting in the late 1970s. FM stations were required to obtain a SCA (subcarrier authorization) before industry deregulation. Standardized frequencies of 67kHz and 92kHz are used.

A Sigmund Freud Action Figure.... there has to be a joke there somewhere- or is it just in my subconscious?!

As a broadcast engineer, the SCAs are useful for transmitting satellite signals (sports,news,etc) to a main studio without using a bi-directional STL. (Just as one person mentioned sending transmitter data). This is very useful when you cannot put a C-band satellite dish (remember the BIG old dishes--that's what I'm talking about) at the studio location, but have ample room at the transmitter site. Thanks FRAN for documenting the history of subscription radio! Kinda missed the Fran's Lab intro, though. Keep up your excellent work!

"Empire Of The Air" is an Amazing series to watch. I'm a big fan of Lee DeForest. A few months ago, I was extremely fortunate to come across a box full of brand new Lee DeForest Audion tubes in their original packaging. SCORE!!! LOL Yesterday, while antique shopping, I purchased a 1913 single tube regenerative amateur radio receiver which was complete and in great condition. It required the usual cleaning and such, but the glass envelope of the tube was separated from it's base and the wires weren't long enough to reattach. Lee DeForest to the rescue!!! I used one of the Audions to complete the restoration and it's playing beside me right now.

Subsidiary Communications Authority. Remember "Muzak"? From the 50s through the 90s this was the killer app for a commercial FM station's SCA. Odd fact, you could piggyback TWO SCA channels onto a MONO FM signal. Pretty sure the bandwidth taken up by the stereo subcarrier and signal shrunk this down to just the single 67 kHz SCA carrier. Had a SCA radio station for the blind here in the 80s. Used crystal-controlled receivers, no tuning knob nonsense. Grant ran out, and the receivers scattered to the four winds. Got one here, somewhere. Cool vid, Fran!

Messing with those slugs was a poor idea...even if you DO manage to find a station, you still may not get reception...those coil adjustments can be critical. Also, SCA needs a strong signal, so the antenna needs to be extended(unless the station is REALLY strong). Disabling that switch is necessary...then find a list of SCA enabled stations in your area...then tune one of them in on the normal FM mode, then switch the SCA on...then you might have some luck tuning the coils. Most non-data SCA signals use a 67 Khz Subcarrier, but there are others also.

I was an engineer at WPKM, which was a small FM station in Tampa. They ran "Southern Melody" on their subcarrier. There was a pair of Reel to reel machines that alternated back and forth with huge reels on it.. Typically 24 hours of playback from 2 rolls of tape. They charged for the background music some amount that depended on the client. (One was a dying shopping center that had about 50-100 speakers on the outside where the Southern Melody played.. We had to yank all that equipment when the bull dozers were ordered for the shopping center. They had been paying a lot of money to keep the folk walking around entertained. The station got sold, Souther Melody vanished from Tampa.. and it started with a new (and MUCH more powerful transmitter, now on a bank downtown) format... The Q morning zoo started there.. then one of the folk moved to NY to start it up there....

I remember riding to my friends place, he alway knew I was coming because the old Kawasaki triple would mess up his kitchen radio.

Love your videos, please keep up the good work.

Love the Sigmund Freud Action Figure in the back - hilarious!

Wow. This is the first time in my life I ever heard of PRN. I never heard anyone at Sony speak of this, but I wasn’t in the Consumer Division. Thanks Fran for teaching me new stuff. I’ve heard of Musak… wondering if it’s still around. I’m in the Poconos so if you can’t receive any signals in Philly I’d never get anything here in the boonies. PS… when I lived in Jersey I wasn’t far from the Armstrong tower in Alpine, NJ

Just a current datapoint: using an RTL-SDR and Redhawk, managed to spin an SCA receiver to play the "Sight into Sound" service for the blind on KUT in Austin (90.5MHz, subcarrier at 92KHz). As an aside, also got a crude RBDS decoder working with the same setup. It was lots of fun. Felt like a kid making a secret decrypter ring!

The SCA service (Subsidiary Carrier Authority) is still out there but it's been unregulated by the FCC since about 1983. A lot of stations that still use it broadcast reading services for the blind or programming in other languages or even data services. The chip you couldn't identify was probably a PLL like a LM565. A broadcast station is allowed to have 2 subcarriers -- one at 67kHz and another at 92kHz. Basically, all the SCA receiver does is take the 67kHz output from the FM demodulator, BPF it at 67kHz, uses a PLL to demodulate it, LP filters it at 6kHz and outputs the audio. An easy way to search for signals is with a shortwave receiver that can tune below 100kHz and demodulate FM. You feed the AC coupled baseband audio into the SSB receiver's antenna (some HP filtering is helpful too). Tune the FM radio to a local station and the SSB receiver to 67 or 92 kHz.

I remember seeing plans for these in Radio Electronics magazine.

There are some modern radios that also utilize this system. On the radio in my buddy's car you can still pick up stations like normal, but if you turn the dial it says "HD2" and there is a totally different program even though it is on the same frequency. He said he didn't have to pay any subscription that his radio picks up the additional "HD" station for free.

Back in the day, studio DJs would talk to the transmitter site tech. on the sub carrier. WFCR

"Tatters of Jive and Boogie Woogie"? I think I've found my next album title.

So I went and found a copy of Empire of the Air: The Men Who Invented Radio, as you suggested. It was most fascinating! Although I had heard of Lee DeForest, Howard Armstrong, and David Sarnoff before, I was completely unaware of the intertwinement of their lives. It’s interesting how much David Sarnoff, 100 years ago, was like Steve Jobs, 40 years ago - taking other people’s inventions and combining/marketing/corporatizing them in a way that no one else had thought of before, and thus eventually creating a company that (at the time) rivaled no other. And he was kind of an ass, too. I guess that personality trait is necessary to achieve the level of success they both achieved. Anyway, it was most definitely an interesting and informative documentary, and thank you for recommending it.

hey fran and the others here , i have not watched a lot of the segments on here but i do like the way it is done . several years ago i was removing the old sca gear from transmitters ... as opposed to installing or upgrading sca equipment . i do not know the state of things today as i have been out of the game for some time . yes , the telemetry was a good implementation of the sca when the need for sca services declined and in some cases , was an economical alternative to other methods . i think it was "popular electronics" or "electronics illustrated" that had a construction article on building an sca receiver . i am not sure but "whites radio log" at one time listed the sca(s) associated with certain FM stations . i checked the FCC site and could not find a listing/section for sca ... maybe it is buried way back in there ... or it went the way of "AM stereo" .

Fran is that one carecter still missing in big bang theory ^^

I have an old 70s radio with a sub carrier band FM system AND it has a PRE PAY coin loading system where you pay to use the item, actually built into the full solid metal unit. 👀

I'M IN LOVE with your channel! I learned about the 'storecaster' sub-band as a child, from my Dad who designed radios for GE in Utica, NY.

I actually have an old GE super radio, that has a little kit in it. And there’s all kinds of interesting stuff still on the sub carriers. And that thing is sensitive enough that you can hear some pretty interesting stuff

Wow, I have been finding these radios stuffed away in the back of old banks wiring rooms.

I love tech videos! Especially about older tech like this! I also love that you are a woman who is making all these tech videos! You are inspiring to me, thank you! I hope you are doing well, thank you again for the videos!

With a commercial FM station, SCA can be used to monitor the transmitter. ie data is sent back to the studio about the transmitters condition via SCA. This is mentioned as " Frequency and modulation monitor system for stereo and SCA. SCA required for telemetry monitoring." costing 10k as line item 8 from the bottom on this spreadsheet: www.ntia.doc.gov/legacy/otiahome/ptfp/application/equipcost_Radio.html

The University of Toronto radio station still has a subcarrier that plays Indian music and news. Most muzak stations switched to satellite in the 90s. If you had a cracked reciever you could hear them.

Hi Fran - I'm now retired, but had a very enjoyable and rewarding career as an electronics engineer. I still have a lab at home and work on electronics of every vintage. Everything from tubes to surface mount. Anyway..... a few years ago, my wife started showing some interest in electronics. To say I was happy about it would be a huge understatement! :-) The first project I had her build was the Ice Tube Clock from Adafruit. I'm not sure if you're familiar with that kit, but it's extremely challenging for a beginner. I coached her through it, but made sure that she did ALL of the soldering herself and that she also understood what all the components did. I was very proud of her for completing it. When she finished it, I told her "If you can do that, the rest is easy!" We are now expanding my home lab to make room for her to have her own bench, test equipment and work area. Do you have any encouraging videos for women who want to get interested in electronics or could you make one? I want to keep her interest in electronics going and a few words from you could sure help with that. Either way, I thank you for all of these enjoyable videos. You and I have a lot of similar niche interests in electronics (such as our collections of vintage L.E.D.'s!!!) and it's awesome when I find videos of yours that explore one of these specific topics. Take care and thank you!

I worked for Microsoft Research on the SPOT project. We were leasing SCA bandwidth to broadcast digital signals to our watches. en.wikipedia.org/wiki/Smart_Personal_Objects_Technology to read about it. If I'm not mistaken, SCA is now used for HD Radio broadcasts. If a radio station in your area was using that frequency range, you might hear some sort of digital encryption of their HD2/HD3 broadcasts. Not sure what that might sound like on an analog receiver.

Thanks Fran! I do appreciate AM a lot including the statics in winter time :), kinda nostalgia of a book based simple life, when I was a kid sitting next to my dad tuning SW stations and hearing lots of foreign musics and unknown languages and trying to guess what it was :) yet I was excited when later on, we purchased a boombox with a fm stereo indicator and we were full of excitement when that LED lights up, this didn't kill the passion of SWLing: that was our internet back then

A joy as always, Fran

Never heard of this, listened with interest!

Have you ever seen one of the pre-FM multiplex "stereo" tuners or receivers? They used AM-FM simulcasting to have the left channel on AM and the right on FM (or vise-versa) to get stereo. The AM sections of those tuners were wideband, capable of excellent fidelity since it was trying to match the FM performance. I have two, a Sherwood receiver and a Pilot tuner-preamp that were built for that system.

Hi. When young and still in high school, I learned how to build my own SCA decoder using an NE565 phase locked loop chip, and you can still find them on ebay. A project of how to build it was published in Radio Electronics magazine but it was just a real simple circuit that only kinda worked. It needs a good 67Khz band pass filter in front of the PLL chip or it won't work right. I kept buying grab bags of assorted coils from radio shack to try and build the filter and was finally able to get pretty good results. All my high school friends hated the music though and thought I was pretty weird. There was radio reading for the blind and for a few years was the physicians radio network which was the most interesting. Getting older, learned more about electronics and got better at building SCA decoder circuits and eventually used an FM IF chip like the MC3357 to up-convert the 67Khz to 455Khz and use a 455Khz ceramic filter and a resonator to demodulate the SCA signal. I still have one in an old GE super radio that can switch to either 67Khz or 92Khz. I built my own little FM stereo transmitter which also has an SCA exciter built using an 8038 function generator chip tuned to 67Khz with the audio going through about a 5khz low pass filter and then a 150us pre-emphases network to modulate the 67Khz sine wave oscillator that uses the 8038 chip. The transmitter still works and I have a long playlist of good background music on an MP3 player I can connect to it. The SCA from that transmitter still comes through on that old GE Super Radio. You need one of these to go with that old radio you have.

When i worked for Muzak we had them. Used a local radio station sub carrier to broadcast the muzak music in stores.

The local sandwich shop used to give away sub-carriers if you bought more than one sandwich.

Pretty sure most FM stations that used subcarriers for this sort of thing switched them over to digital "HD Radio" which the broadcast industry has the audacity to charge manufacturers a ridiculously expensive licensing fee for. If you find 2 similar radio models, and one is inexplicably about $50 more expensive than the other, it's probably HD Radio compatible. Radio manufacturers will try to hide the fee by adding something like streaming Bluetooth audio with it, but that's like a $5 - $10 chip retail.

I was about to say "Commercial free broadcasting for a fee? We don't have that in your superior imperial masters..." ...then I remembered, Americans don't need a licence to operate a TV like we do... FM subcarrier broadcasters is something I never heard of. I'd have thought the BBC would have tried it, maybe broadcasting surround sound on Radio 3 or something to be listened alongside the regular FM broadcast. In the UK, we have the same band for FM radio, apparently the USSR and Japan used different bands for FM. In the olden days we referred to it as VHF. The BBC used to have simulcasts betweeen FM and Medium Wave. They stopped that in the 1990s, held on to their FM broadcasts and let commercial stations take their former MW frequencies. Now they simulcast with DAB. Though Radio 4 apparently still has their Long Wave broadcast (which also simulcasts with DAB) Also, backward compatibility wasn't thing with TV in the UK. Black and white TV was in the 405 A standard on the VHF band. Colour TV was PAL in 625 lines on UHF only. Now, nothing's backwards compatible, even DAB has made non backwards compatible changes (DAB to DAB+) if only DRM radio (Digital Radio Mondiale) took off, perhaps they should have picked a better name...

There's some FM Broadcast history you've missed! Edwin Armstrong pioneered wide-band FM broadcast in the USA. He established a chain of stations starting in 1941. RCA had declined to buy Armstrong's technology and became concerned at the competition. Entirely coincidentally in 1945 the FCC reassigned the FM APEX band to other uses, such as TV. This put the FM radios using Armstrong's patents (about 395,000) out of service. There was no 'reverse compatibility'. Is it me or does something stink, here? RCA went on using Armstrong's patented technology without paying the fees for years. The litigation drove Armstrong to suicide. His widow continued the fight and eventually won. Wikipedia has a good article about Armstrong:

That was fun! I built a bootleg SCA decoder back in the 70's when I was still in High School. It was based on a single 16pin DIP IC that did the heavy lifting with a few other passive components. It was fed by the MPX output of a Sony portable radio. I got it to work ok. BTW, I lived near Chicago, so there were several SCA signals available. Ya, it worked, but the fidelity was communication quality, like maybe 200-3500Hz? The novelty wore off pretty fast. I was more impressed when I assembled a stereo decoder at the same time! That was cool. Ran it off the same ole Sony portable radio! Had fun then.

Used to work for a FM subcarrier paging company a franchise of CUE (TM) paging, boy the memories. Be advised that there is more than one subcarrier frequency or allowed band so those stations you tuned may have a sub carrier but your radios subcarrier receiver is not tuned to them. The FM subcarrier paging carrier actually could coexist with music services like Muzac (TM) but could be somewhat problematic with harmonics. Modern RDS is also a subcarrier protocol that give you the artist and title in your car if implemented. Used to have fun with an op amp trick with the pagers and listen to the station they tuned to and mostly country western BTW. Large national and regional service companies liked the network which was distributed via C-band satellite to FM radio stations across the US for the large and usually not serviced rural footprint. Of course cell phone killed the paging industry in general. THX for your vids and stay well.....

Wow! I never learned this stuff from my dad! I have now learned a whole lot about FM radio!

I think you will find that the meter on the back is called a "Hobbs Meter" Fran... love your channel btw.

Hi Fran! Great video... There's a fascinating history behind FM. It actually existed long before WWII. FM was invented at Columbia University in the 1920's using the 50 MHz band. In 1945 at the behest of David Sarnoff (the CEO of RCA) the FCC reallocated the FM band to the present day 88-108 MHz. This was done intentionally to put Armstrong and the Yankee Network out of business to protect RCA's AM empire. So much for backward compatibility... In the early 1960's, there were competing FM stereo systems. The two under consideration were the Crosby system and the GE/Zenith system. The Crosby system was simpler (i.e. easier to demodulate) and had better sound quality with much lower noise but unfortunately used more bandwidth. The GE/Zenith system was more complicated, and had much more noise but used less bandwidth. Unfortunately the FCC caved in to pressure from GE and Zenith as well as from broadcasters looking to make money renting bandwidth on their carriers and so the GE/Zenith system was selected. Less bandwidth for the stereo broadcast meant that more bandwidth was available for other subcarriers. So, the FM stereo we grew up listening to could have been quite a bit better, especially if the signal was weak had the Crosby system been selected. Oh well... Subcarrier stations are really not used anymore today. Services like Muzak are delivered over the internet now. The history of radio is almost as interesting as the electronics!

it’s crazy to think about all those tv and radio broadcasts out there in space, trickling in and trickling out of existence as they fly by whatever’s out there listening.

Steely Dan! Back in the 80’s as an elementary school student, I made my own crystal set with the germanium diode. I loved the wonder of that set.

Wow! I recorded content for “Muzak” years ago. I used to hear myself in elevators, Walgreens and Jewel foods but never admitted it to people!

Good timing on this. I'm going through my subscriptions and you're talking about FM Subcarrier which is cool enough on it's own. To add to it, All American Five Radio came out with a video right after this that breaks down the composition of what a modulated FM Stereo carrier looks like. It's super neat. What a treat from both of you!

One of the Australian electronics magazines, I think Electronics Australia, published a subcarrier decoder kit, known as SRS or supplementary radio service. The kit was super simple, the main component was a 4046 PLL chip. Here there were three main subcarrier frequencies, though the most used by any given station was two as I recall. The kit included a pot to allow you to tune it to the different subcarrier frequencies. The subcarrier modulation was FM, hence the need for the PLL.

Hi Fran, very interesting video thanks. I remember listening to several sub-carriers in my area back in the 90's using a Ramsey Electronics kit I think it was that I installed into an old FM receiver I had. Also, there is a very easy way to do it if you have both and FM receiver and a good quality SW receiver that will tune down into the below 100 Khz range, can't remember for sure how I did it but I think I took the audio output of the FM headphone jack and ran that through a .01 cap into the antenna jack of the SW radio and tune to the sub-carrier freq of whatever they were, like 67 khz or 90 khz or something, there were a few frequencies being used. It worked great and I could tune them in that way, it was published in a Monitoring Times magazine issue back in the day I think on how to do that. Been a while since I did that but I think that was correct. It was very simple I know. Keep up the great videos, I enjoy very much!

Excellent! Way back when I was a poor ham operator college student, I built a simple adapter for SCA reception. There were easily 7 or 8 Muzak channels on the Chicago FM dial, plus PRN, NPR reading service for the blind, and stock quote encrypted modem tones for a paid subscription zero- delay stock quote service. The modem tones were Bell 202 modem tones. I used a ham radio packet radio TNC in dumb modem mode to try to decode. No luck. My SCA adapter used the old Signetics NE565 phase locked loop chip and a passive low pass filter to cut out main channel audio bleed through. These chips were very static-sensitive and the oscillator circuit in the chip would frequently blow. I added a tuning pot to mine to switch between the two popular ultrasonic carrier frequencies. You needed to tap the demodulator audio output before the de-emphasis capacitor, which would shunt the SCA carrier from the received audio. Recently, I used two daisy-chained SDR dongles to recover the SCA audio. The secondary channel was also FM, but narrow band with low deviation. Amazingly, there’s still a few stations broadcasting in the Chicago area. The doctor my wife worked for back in the 70s had this identical PRN radio in his office at the time. Half of the PRN programming was drug ads. This was back when it was illegal to advertise prescription drugs directly to consumers.

5:00 Nice Steely Dan Reference Fran!! Great tune.

I know you will get a kick out of this Fran. When I rebuilt my first street bike when I was 16 it was a Kawasaki H1500. It had one of the first production CDI units. (That I know of). Two large boxes mounted under the seat. When I would come home at night, my parents (in the finished basement, watching TV (pre cable) ) knew I was home because the antenna for the TV was in the attic whose wall intersected the garage. The CDI was so "unshielded" that it made the TV picture get all wavy. (I think I got complaints and checked it out myself by idling my bike in the garage and then going down to look at the TV.) Sure enough, it had an obvious effect on the picture. After my discovery, I always came home and shut my bike off as quickly as I could. At least Mom and Dad knew I survived yet another day of riding. That was '79.

LM357 would be the chip. Built a few SCA decoders over the years to pirate the subcarriers.

My Dad had one, locked to a single station. The programming was free, physician continuing education stuff. SWTPC made a kit for SCA, almost wonder if that board inside your Sony is one of theirs.

if you don't want to muck around inside that radio you can use various techniques with an rtlsdr ($10 software defined radio dongle) to receive fm subcarrier broadcasts. see web.archive.org/web/20160304171452/rtl-sdr.better-than.tv/?page_id=193 for a guide on how to do this relatively easily. there is also a bit more involved method using a gnuradio set up for the usrp receiver but it can be easily adapted to use an rtlsdr as a receiver github.com/gnuradio/gnuradio/blob/master/gr-uhd/examples/python/usrp_wfm_rcv_sca.py

PRN also used to do a talk network aimed at information for doctors. My friend used to install SCA converters in radios. We also had a station that carried the ABC Radio Networks on their subcarrier and of course there were background music services like Muzak. Fun times to discover this hidden audio.

Addendum to the comment by Max Zomboni: As a pre-med student in the 1970s I built an SCA adapter and hooked it to my stereo (you had to tap the signal from the detector, prior to the de-emphasis network). I used to listen to PRN, which was completely separate from Muzak. These were the (better) days when it was illegal to advertise prescription drugs to the public. As Max said, PRN radios were given free to any MD who requested one. The purpose was to provide a private channel to MDs that drug companies could use to hawk drugs. They would broadcast medical news and other content of interest to MDs and intersperse prescription drug ads which were just as absurd as the ones we now have on network TV. The demise of PRN was due to the legalization of ads to the public advising people to tell their doctor how to manage their diabetes, hypercoagulation, and cancer chemotherapy.

Looks like PRN was a short-lived venture. Found a NYT article in May 1975 announcing its launch after a year of testing, and another article in May 1981 announcing its termination. Thanks for the video! Fascinating.

Hi Fran, The chronometer resolution isn't necessarily an indication of the billing rate. The chronometer they used might have been the best compromise of form, fit, function, reliability, cost, availability, etc. that PRN could get. Perhaps PRN charged a flat rate for the first 0-100 hours and didn't pay much attention to the first number ring. I imagine the most practical use of the first number ring would likely be for the support person who shows up to take the meter reading. They could quickly and easily confirm that the counter was still operational (not bypassed for cheating the system).

Hi Fran. Just thought this info would be of use to you.DOCTOR'S RADIO NETWORK TO GO OFF THE AIR MAY 31 Published: May 21, 1981 The Physicians Radio Network, a round-the-clock service sponsored by several giant pharmaceutical companies, will discontinue broadcasts on May 31. In 1974, Visual Information Systems, a division of the Republic Corporation, initiated the station exclusively for doctors - it currently reaches 80,000 physicians in 69 cities. Jay E. Raeben, president of Visual Information, said that his company had ''failed to persuade enough of the industry that the radio was a medium important to use.'' Transmitting on an FM sideband frequency, or subchannel, which could not be picked up on a standard radio dial, the network permitted doctors to communicate among themselves, more freely perhaps than might be possible before a listening lay public. This feature, however, contributed to the station's downfall. The necessary special receivers were distributed to doctors upon request and without charge. Mr. Raeben said that the cost of manufacturing and mailing such equipment had diminished revenues substantially. Used by Advertisers Physicians rank as the profession most vigorously sought by adverstisers, because their prescriptions largely determine the profits of billion-dollar drug companies. ''Surveys show that P.R.N. affected sales very positively, especially as it impacted new products,'' said Robert E. Devinna, director of advertising for Roche Laboratories, one of the sponsoring companies. Eight minutes of every hour on the air are devoted to advertising. Programming focused on scientific breakthroughs and significant operations, such as the recent surgery on the Pope. ''P.R.N. rarely announces new drugs,'' said Mr. Raeben, who also acts as managing director of the station. ''We made a very considerable effort to insure that programming was not in the interest of the advertisers.'' If a new drug were announced, it would have to be newsworthy in itself, he asserted. May Turn to Journals Mr. Devinna of Roche Laboratories thinks that some of the major drug companies that advertised on the network will probably turn more to medical journals now. He views this as ''shortsightedness of the marketing industry,'' and added, ''Traditional advertising channels are cluttered - it's a shame to see P.R.N. die.'' The end of Physicians Radio Network will apparently not work any great hardship on its originator. ''Republic Corporation does not expect to lose any money due to the termination of P.R.N.,'' Rembrandt P. Lane, an executive vice president, said He said that Republic had tried unsuccessfully to sell the station, which had, in fact, been profitable for the last three years. ''P.R.N. is a very small investment by a division of Republic Corporation,'' Mr. Lane said, ''having a limited marketplace - and the 80,000 subscribing doctors paid nothing for the service or the radio.'' Mr. Lane declined to specify the operating expenses of the network, or Republic's initial investment.

it would be cool to hear about the two versions of AM stereo and RDS

Very Interesting Fran - I have recently been playing around with FM baseband demodulation in software, and you've inspired me to add some code to extract the subcarriers. I've noticed that most commercial FM stations in Australia have RDS at +57khz (for text transmission), but I can see (from the waterfall display in my SDR software) that others do indeed have analog stuff up there - I'm interested to find out what it is !

Thanks for bringing back memories, Fran. I was an engineer at WKBQ-FM in St. Louis, and (for a while) we carried PRN on our subcarrier.

Minnesota Public Radio was running SCA back in the 80s to deliver talking books for the blind. It was nice and I think it was delivered as a public service.

We in the Uk still pay a fee for an ad-free station its called the bbc

Back in the 1940s, when Q102 was WFIL-FM, "fax over FM" was experimented with. Philadelphia Inquirer used the subcarrier to transmit faxes to specialized FM fax machines under the WFIL-FX callsign. People that used the service would get the latest news before it hit the newspaper. The equipment was too expensive and the service ended after a couple years. Fun Philly fact :)

You were talking about arcing causing interference on AM radio. I've heard that's how utility company used to find a broken power line was by driving the truck next to the power lines with the AM radio tuned to a weak station.

Here's where I first heard about this. americanradiohistory.com/Archive-Hands-On-Electronics/Hands-On-1989-01.pdf I was curious but as a kid couldn't source the parts. I never did build that device. I believe I remember seeing an SCA module in GNU Radio Companion. All you need is a $5 RTL-SDR (European TV) USB stick.

Fran just marry me, because there is too much bad stock out there.

Part of what took so long for FM to gain popularity was the FCC first allocated one frequency band then then 88-108 band. The inventor of FM was Edwin Armstrong - he got shafted - part of it compliments of RCA and Sarnoff.

That's not a chronometer, it's what's called an hours meter or a Hobbs meter. They're used in aircraft and equipment in general where you need to keep track of hours in service. I've got a few around my shop.

Still broadcasting FM stereo radio in UK…I can remember when living in Swansea UK centre when Kilvey went stereo and I put up a 6 element aerial and my red light lit up on my Korting receiver. Now I have all my LPs some worth a small fortune…integrated into my satellite/ broadband/Blu-ray ATMOS system.

6 minutes in, I'm ready to fall asleep. Sorry, the pace is that of a funeral procession. I'll have to come back to this. And, I know about SCA FM radio. I used to listen to Books for the Blind and digital "Stock Market Ticker."

I'm viewing this 3 years after you posted it, but you would have been really amazed at the early "analog" satellite signals (think pre-1990s), some transponders had many dozen subcarriers on them

Believe it or not, subcarrier broadcasting is still widely used! For stations that do live remotes using a microwave link to the studio, the studio will commonly use one of the subcarrrier channels as a "talk back" channel to communicate with the people doing the live remote! RDS data that allows modern radios to display information about what is currently playing are on the lower subcarrier channel as well...

Commercial FM radio is a dying fad. someday the 88 to 108mhz spectrum will be mostly silent, and then we can re-use it for other things such as wireless microphones, wireless speakers, and pirate radio stations. Many thanks to Fran for memorializing it.

Some stations used this for college football networks. The football network would have stations pick up the game on the dedicated sub channel and simulcast the games on the main FM station. I worked for Muzak. We used subchannels to the location received on a fixed channel non tunable receiver and out to an amp and speakers. The service was sold as a flat monthly fee.

The Late Great Lafayette radio had a normal analog tuner that had SCA available... if you knew to cut a jumper on the motherbard....

When did they stop making Sprague yellow jackets? Don't remember seeing any in things same age as this radio. Wondering if boards were made in house with what ever? Could explain all the extra parts, just a guy screwing around with design?

If you look closely, you will see the hidden switch in the bookcase to open the secret door to the Fran Cave . There, she dons her outfit to fight those electronic criminals in the dark web lol

Thanks again for sharing. This is something I was completely unaware of.

Peace, Love, and No Static at All.