Analog radio broadcasts are pretty simple, right? Tune into a given frequency on the AM or FM bands, and what you hear is what you get. Or at least, that used to be the way, before smart engineers started figuring out all kinds of sneaky ways for extra signals to hop on to mainstream broadcasts.
Subcarrier radio once felt like the secret backchannel of the airwaves. Long before Wi-Fi, streaming, and digital multiplexing, these hidden signals beamed anything from elevator music and stock tickers to specialized content for medical professionals. Tuning into your favorite FM stations, you’d never notice them—unless you had the right hardware and a bit of know-how.
Sub-what now?
Subcarrier radio was approved by the FCC under the Subsidiary Communications Authorization. This allowed both AM and FM radio stations to deliver additional content through subchannel broadcasting on their existing designated frequency. Practicalities mean that only FM stations could reasonably use this technique to broadcast additional audio content; AM radio stations were too limited in bandwidth to do so. In the latter case, only low-bitrate data could be sent on a subcarrier. 1983 saw the deregulation of subcarrier broadcasts, with existing broadcasters able to use them largely as they wished.
To understand how this let FM radios broadcast extra programming, we need to know how subcarriers work. Basically, in this context, a subcarrier is a high-frequency signal outside the range of human hearing—usually something like a sine wave at a frequency of 20 KHz to 100 KHz or so. This signal is then amplitude modulated with the desired secondary audio program for broadcast. As this signal is beyond the range of human hearing, it can be mixed with the regular station’s main audio feed without perceptibly altering it to any great degree. The mixed signal is then frequency modulated on to the radio station’s main carrier signal (usually in the range of 88-108 MHz) and sent up the tower for broadcast over radio.
For subchannel broadcasting, FM stations typically used subcarriers at 67 kHz or 92 kHz to carry additional low-fidelity mono audio feeds. These carrier frequencies were chosen to avoid the existing subcarrier signal in FM stereo broadcasts, which carried a left-right channel difference signal at 38 kHz.
Subcarriers were a neat little lifehack that let a single frequency do double or triple duty. A single FM station could deliver its main program, plus a bonus low-fidelity mono channel for various purposes. This facility was used for all kinds of obscure uses. Some broadcasters delivered background music for piping into department stores and the like, while others created special channels reserved for reading-for-the-blind organizations.
The Physician’s Radio Network was also a notable user, which broadcast information of specific relevance to medical professionals. However, the limited audience made it a difficult prospect to keep running from a commercial standpoint, even though it saved money by merely rebroadcasting one hour of programming around the clock on any given day. It eventually went off the air in 1981.
Tuning into these broadcasts wasn’t possible on a regular FM radio. Instead, you needed a device specifically built to pull the subcarrier signal out of the radio broadcast and then demodulate it back into listenable audio. By and large, organizations broadcasting on subchannels would distribute special radios that were tuned to only decode their sub-carrier station. The hardware involved wasn’t complex—it just involved demodulating the FM broadcast signal, then filtering out the subcarrier signal and demodulating that back into audio.
FM subcarriers weren’t just for audio, either. Microsoft famously used 67.7 kHz subcarriers on FM radio stations for its now-defunct DirectBand datacast network. It could deliver data at 12 kbit/second, or over 100 MB a day. The technology was used to deliver things like weather reports and stock prices to early smartwatches and coffee makers in the days before WiFi and celluar internet were cheap and everywhere.
From a hardware hacker’s perspective, these channels were a fun challenge to hunt down. With the right radio receiver and a bit of circuit hacking to tap off the baseband signal, you could decode the subcarrier and reveal the hidden broadcast. Some hobbyists rigged up surplus SCA receivers—often stuff found at flea markets or hamfests—to get free background music, weather reports, or any niche audio that happened to be riding along. Alternatively, decoding the subcarrier was entirely possible by building your own gear. It was kind of a neat analog puzzle—filter out the main audio, isolate the frequency where the secret channel lived, and then demodulate it. The hardware you’d use looked suspiciously like the guts of a standard FM radio, just with a few added filters and demodulation stages stuck in. These days, software defined radio techniques make doing the same thing comparatively easy.
Though it felt like eavesdropping, this wasn’t exactly some top-secret espionage. While technically unauthorized reception was frowned upon by the FCC, it wasn’t heavily policed. Subcarrier channels didn’t exactly have roving gangs of enforcers prowling about the neighborhood. Mostly, these subcarriers delivered paid subscription services, like Muzak, or nonprofit programming authorized under the station’s broadcast license. Their decline coincided with the rise of digital technologies and more flexible content-delivery methods. By the late 20th century, satellite feeds, internet streaming, and multicast digital channels rendered analog subcarriers quaint and unnecessary.
Still, SCA subcarrier signals remain a fascinating piece of broadcasting history. A few still linger today, but it’s now a more obscure medium than ever, lost as mainstream technology has moved on. It’s a reminder that even in the old days of broadcast radio, clever engineers found ways to pack more data into the same old bandwidth—long before we started streaming everything in sight.
Featured image by [windytan]. (Also, check out her work on RDS demodulation.)
IIUC, both HD Radio and local traffic data are normal parts of broadcast FM in the US today. But HD is packed into sidebands instead? And I have no idea how traffic data is transmitted. I’d love to learn more about both.
RDS (RBDS is the US name I think for the near-identical standard) allows TPEG travel data to be broadcast – that’s widespread in Europe
HD is in sidebands. Those sidebands can cause trouble for regular FM receivers that don’t filter them out.
http://www.ham-radio.com/k6sti/hdrsn.htm
Nominally, HD Radio just… clobbers through adjacent spectrum allocations entirely, relying on the fact that the FCC’s spectral mask falls off slowly. So I’m not sure if “sideband” is the right term for something that ends up square in the middle of someone else’s band. I always found it funny that it’s like “oh we transmit digital data in a band that stretches to +/- 0.2 MHz around carrier” and it’s like… (looks at radio dial) uhwhat. In areas where you’ve got nice clear propagation, you can easily get interference between HD stations and distant analog stations.
Basically a form of single sideband
Typically you have two side bands, one above and one below the carrier frequency
Ssb is used more for Hf and Lf bands, early form of multiplexing, sideband have less bandwidth, compared to the main am or fm carrier, but still usable.
SSB.. SSB without a carrier was a stupid idea.
I grew up in a ham radio household and know how nasty SSB is in practice.
You never know were exactly the center frequency is, which is problematic for Pactor communication.
If radio amateurs were wise rather than greedy (“farer with less power”) then the had developed SSB with a lowered carrier rather than a surpressed carrier.
Gratefully, aviation was smarter. Air band communication uses AM, which is non-destructive (like SSB) and has a carrier frequency a radio can “lock on” to.
Receivers in planes also feature synchronous AM detection, which enhances audio quality.
I’m really disappointed how limited the view in terms of SSB is.
Variations like DSB (AM without carrier), if allowed, could provide simultanous transmit of voice and video/data transmission.
A ham operator could talk on USB, while simultanously sending SSTV or NBTV signals on LSB.
Unfortunately, FCC and many hams have no sense for experimental modes anymore.
But even normal SSB could be improved (eSSB), which is needed.
The voice bandwith of about 3 KHz is a joke. It’s barely enough for human speech. Or data communication.
Voices of kids and females are not being taken into account.
They operate at higher frequencies not covered by normal SSB.
If only the hams in the 1960s were smart and had kept the total bandwith of AM and used it for a single side band.
It would have improved audio quality, readability quite a bit.
Unfortunately, history went different.
Hams are so obsessed with “saving” bandwith, power and time that they are cheap on quality.
The contest and fast morse obsession is the embodiment of this, I think.
Focus is too much on the technology and the human/social factor is being neglceted.
“Focus is too much on the technology and the human/social factor is being neglected.”
LOL. The aspects of SSB that you decry are there precisely because “human factors” prefer them. Bandwidth, power, and time are indeed important to the majority of hams. Just because you aren’t one of them doesn’t make SSB a technical deficiency.
And including a carrier, even low level, would make an absolute mess of the band when it was busy for a contest or DXpedition. If you had any understanding of signal mixing you’d know that.
I don’t agree, please deposit your LOL somewhere were the sun will never shine.
There were a lot AM ham operators back in the day that saw the disadvantages of SSB and adressed then.
Unfortunately, they had been silenced. It went so far that AM use had been frowned in the ham band plan of many countries.
“If you had any understanding of signal mixing you’d know that.”
AM signals with a carrier wave can mix just fine. Weak stations in background can be heard.
73s
“LOL. The aspects of SSB that you decry are there precisely because “human factors” prefer them. Bandwidth, power, and time are indeed important to the majority of hams.”
FT8. 🙄
“LOL. The aspects of SSB that you decry are there precisely because “human factors” prefer them. Bandwidth, power, and time are indeed important to the majority of hams. Just because you aren’t one of them doesn’t make SSB a technical deficiency.”
The real reason why SSB made into ham radio was range.
Certain hams wanted to have similar range for voice as with CW.
Any why? Contest, DXpeditions, diplomas. And diplomas on the wall=greed, ego, self-esteem.
It wasn’t about international friendship, rag chewing or any higher ideals. Nope, it was about sports.
SSB was their steroid, a way to increase performance.
Audio quality or improved bandwith was no consideration.
It was all about higher, faster, farer.
Ham radio is point-to-point communication; it is enshrined in the law that ham radio is not a broadcast service. To the end goal of point-to-point communication hams use minimum power and minimum bandwidth so as not to disturb other operators. It’s the same reason polite people don’t yell at their friend sitting next to them in a crowded restaurant. Hifi is totally irrelevant to point-to-point communication. Wider bandwidth is not a good thing when it does not serve an appropriate service.
This is not greed, this is behaving as a civilized person.
Well… also cheaply. And easily. And independently. There are a lot of “ethics” or “ethos” or I dunno, “standards” around ham stuff. I can’t think of the right word.
Like, if you really, really wanted to be push range/power/throughput, there are other ways to do it. Voice is a joke; it’s kHz of bandwidth. But the crazy ways to do it require a lot more capability and cost, and that’s kind of “anti-ham.” It’s always been more “look what I can do with this stuff.” (I mean, we’ve got a worldwide time standard you can sync to nanoseconds, sub-100 K LNAs off the shelf, and sub-$200 in low qty GHz ADCs now. Just a different world.)
That’s not meant to be a criticism, mind you: but in my mind it’s what leads to the infighting: because the goals and ethos are in conflict.
Why would you use AM for long distance communication ? Selective fading totally wipes out an AM transmission. And as mentioned earlier, one AM station hogs the bandwith of two SSB operators. The only redeeming feature about AM is the sound quality, if the bandwith is wider than 3,5 kHz. All things being equal, you wont reach out as far with AM as with SSB. But, AM is not dead, the CB bands are alive and well with a plethora of AM, FM and SSB and whatnot transmissions, with little to no supervision. That should cure your AM itch nicely, especially now, with the bands opening up.
@Chris Maple said: “Ham radio is point-to-point communication; it is enshrined in the law that ham radio is not a broadcast service… This is not greed, this is behaving as a civilized person.”
@Chris Maple, you are spot on. Thank You for stating what @Joshua obviously DOES NOT understand at all!
Best 73s – David
Not SSB as no sidebands are suppressed. The usual graphs, such as used here, show the detector output instead of the RF spectrum.
Basically a form of single sideband
Typically you have two side bands, one above and one below the carrier frequency
Ssb is used more for Hf and Lf bands, early form of multiplexing, sideband have less bandwidth, compared to the main am or fm carrier, but still usable.
I thought that by posting the same information twice you were making a sideband joke. But then I realized that the second copy should have been mirrored.
ɹǝɐlᴉzǝp ʇɥɐʇ ʇɥǝ sǝɔoup ɔodʎ sɥonlp ɥɐʌǝ qǝǝu ɯᴉɹɹoɹǝp˙
I ʇɥonƃɥʇ ʇɥɐʇ qʎ dosʇᴉuƃ ʇɥǝ sɐɯǝ ᴉuɟoɹɯɐʇᴉou ʇʍᴉɔǝ ʎon ʍǝɹǝ ɯɐʞᴉuƃ ɐ sᴉpǝqɐup ɾoʞǝ˙ qnʇ ʇɥǝu I
It’s a glitch with the hackaday website scripts posted once but it gets double posted
Long ago, I picked up a Ramsey SCA decoder at Goodwill ($0.49).
I never hooked it up.
http://sourcefmtransmitter.com/ramsey-electronic-manuals/SCA1-manual.pdf
Thanks, interesting circuit, maybe it will help me build my RDS decoder. Does anyone have a circuit diagram for that?
I spent about 40 minutes looking for it, without luck,
but I found this one, which might be better.
http://www.blackcatsystems.com/radio/sca.html
Not especially helpful, but I remember a project featured in one of the magazines (Pop Elec. perhaps). SCA.
Analog television stations also broadcast subcarriers on their audio carrier. The station I worked for broadcast stock ticker data that way for many years. We even had one of the ticker displays in master control so the operators could insure that the signal was present.
*ensure
Ensure is a high calorie high protein dietary supplement drink:
https://www.amazon.com/Ensure-Nutrition-High-Quality-Replacement-Chocolate/dp/B07CWFJV63/
I recall PLL + FSK demod circuits for receiving SCA. They had audio books for the blind read on some station subs.
Back around 1983 any comprehensive free stock market data had a 15 minute delay. The SCA stock market stream was encoded fairly well so that people not paying for the special device couldn’t decode the stream.
I think there was an issue of Radio-TV Experimenter back in the 60s that had a tutorial on SCA and how to build a receiver. Mainly the tutorial was centered on receiving commercial free audio (elevator music) which was a paid service at that time. If I remember the article was more about what SCA was and how to receive it rather than how to pirate it. There were a lot of fascinating DIY articles over the lifetime of the magazine (like how to make a wrist radio or a two tube FM receiver) and my favorite was White’s Radio Log in the back of the issue as I enjoyed SWL. I still have my QSL cards from now defunct stations worldwide stations. Those were the good ol days of digging out a low power pirate station from background noise, working skip, listening to the number “spy” stations, or WWV/B/H for the time “heartbeat”.
You could listen to SCA’s using a simple FM converter. Simply punch in the modulated carrier of an AM car radio from an FM converter. Then move the receiver off the main channel AM carrier up the dial. You will be able to hear the SCA’s. Mind you, you will be “slope detecting”. It’s not perfect, but it works. I used my Audiovox FM converter and was able to listen to the Muzak services, PRN, telemetry chirps, Talking Information Center, AM simulcasts, weather forecasts being fed to other stations. Back in the early 80’s, you had oodles of SCA feeds here in the Boston area. Muzak was usually on the 67 kHz sub, Muzak’s foreground (AC) music service was on the 92 kHz sub. Today, the 57kHz is strictly for RDS encoding.
i was expecting to find more comments here about RDS-TMC: Traffic message channel. it might be a European thing but we have it here in Australia, My 2016 Renault Koleos has it built in, in Australia the service is provided by SUNO.
Thanks for the read, might have to go SDR’ing over the break :)
I served on the wiring standards committee. Each vendor pushed their own solutions. The ultimate goal though was cost, how to deliver a solution using existing wiring. There were numerous better solutions for speed amd reliability, but nothing close to cost savings.