Several decades ago, the only information most car radios displayed was the frequency of which station you’re tuning into, possibly the volume of the stereo, and perhaps the equalizer, balance, and fader settings. That’s it; you were at the mercy of your local DJs and whatever music identifications skills you could muster to keep a handle on a new tracks. However, towards the start of the new millennium, something unusual happened: Car radios started to display song titles. So, how does you car know what song is playing on your local FM station? If you own something from this era, it’s largely thanks to a bit of tech called Radio Data System (RDS), or Radio Broadcast Data System (RBDS) in America.
Before we dig into RDS, let’s talk about FM radio. We’ll let Lewin get into that:
How FM Works
[Lewin Day/Editor’s note: Let’s first learn about how FM radio works to broadcast stereo audio, and then we can learn how radio stations sneak in an additional data stream along with that. FM stands for Frequency Modulation, which basically means that FM radio works by taking a standard signal called a carrier signal, and modifying it with the audio signal you want to play (specifically the audio signal modifies the carrier’s frequency), and then sending that modified carrier signal to a receiver that decodes it to get the audio signal back.
The following diagram from Michel Bakni is a nice visual description of frequency modulation in its simplest form. We have a “data signal”—i.e. some audio; let’s say, your favorite Elton John song—in the form of a sine wave, and we have the “carrier signal” which is our FM radio signal. As the amplitude of the Elton John audio signal rises, the carrier’s (the red wave) frequency is modulated higher; as the amplitude of the data signal decreases, the carrier signal’s frequency is modulated lower. The result is shown in green on the right; this varying-frequency carrier is broadcast to be picked up by receivers. In the radio receiver, the frequency changes in the carrier are detected and processed back into the amplitude variations corresponding to the original audio. They’re then fed to the speaker where we hear them as sound.
Here’s a video showing how a carrier signal (which looks steady like the red one above) changes when merged with an audio signal, thereby creating a frequency modulated audio signal (the green one above) that has the audio data embedded in it (meaning that audio can then be “decoded” out by the FM receiver and played over a speaker):
Of course, that’s just the simplest case. FM radio stations typically broadcast in stereo with two separate audio channels, so how does that work? Well, it’s bloody complicated but I promised David I’d write a good explainer so here we go.
At the radio station, the Left audio channel and Right audio channel are added together to create a combined mono audio signal, with frequencies from 30 Hz to 15 kHz. This is the (L+R) signal. A difference signal is also created by subtracting the right audio channel from the left audio channel, which we call (L-R). The (L-R) signal is amplitude modulated onto a 38 KHz carrier. Basically, what that means is the 38 KHz signal gets “louder” or “quieter” based on the “volume” – or amplitude – of the (L-R) signal. The 38 KHz carrier itself is “suppressed” but for our understanding, we don’t really need to worry about that detail. There’s also a pilot tone, which is just a 19 KHz sine wave. These signals are all summed together and frequency modulated onto a radio signal at many megahertz, such as 92.7 MHz for Fresh FM, or 101.5 MHz for my old station, Radio Adelaide.
(By the way, when we talk about “summing” or “adding” signals, it’s the same thing. It’s where you take the amplitude of each signal at a given point in time and simply add them together.)
On the receiver end, a simple mono FM receiver takes in the signal and demodulates it. All the content above 15 KHz or so is simply filtered out and you just get the (L+R) signal playing out of a single speaker. This is why stereo FM stations are backwards compatible with mono receivers.
Stereo receivers, though, work differently. Of course, they still demodulate the FM signal and take out the base (L+R) audio signal, but they don’t ignore the higher-frequency content of the signal. Instead, they are able to pick up the 19 KHz pilot tone from the signal, which indicates to them that the signal is actually a stereo one. The 19 KHz signal is internally doubled to generate a carrier signal at 38 KHz. This is mixed with the demodulated signal and is used to recover the (L-R) audio signal in time – or “in phase” – with the (L+R) audio signal. By adding the (L+R) and (L-R) audio signals together, you get a 2L signal, which is sent to the left speaker. Meanwhile, subtracting the (L-R) signal from the (L+R) signal creates a 2R signal which is sent to the right speaker.
OK Let’s Get Into The RDS That Let’s Your Radio Know The Song
Okay, so that’s all the audio figured out, but how is the RDS data sent? Well, a further 57 KHz tone is added on along with the (L+R), pilot tone, and (L-R) signals, before the combination of all four is frequency modulated and sent out as radio waves from the transmitter. That 57 KHz signal is amplitude modulated with the data in a scheme beyond the scope of our discussion here. In any case, the 57 KHz signal was chosen to sit safely above the 38 KHz difference signal, which has “sideband” content that extends +/- 15 KHz. With the encoding system used in RDS, it’s able to transmit data at 1187.5 bits per second, or just under 1.2 kBit/second. If you ever used a 56 kBit/s modem in the 90s, you’ll know that 1.2 kBit/s is not very fast at all. However, for sending a bit of text with song titles or station info, it’s more than enough.
In the above spectrogram posted to Reddit, we can see a breakdown of an FM stereo signal that has been demodulated/”decoded”. From 0-15 KHz, we see the (L+R) audio. At 19KHz, we see a spike for the pilot tone. Peaking at 38 KHz, we see the center of the (L-R) signal which spreads out roughly 15 KHz either side. And, centered on 57 KHz, we see the RDS signal.
So, we’ve established that one can send a little bit of data with our FM radio signal thanks to the magic of RDS. But what actual information does RDS send? Well, each RDS broadcast normally starts with a four-character hexadecimal number identifying the radio station, which the receiver can then compare with a list of receivable station ID codes. If the radio station being listened to is regional, the receiver may find another signal with that station identifier and switch to it once the originally tuned-into signal grows feeble from distance.
Beyond that, RDS data includes more information on a station’s style of programming. Have you ever seen a button marked “PTY” on a head unit and wondered what it does? It stands for “Program TYpe.” (But music can be a party too). Selecting PTY lets you sort radio stations by genre. Intriguingly, there’s very little overlap between global RDS program types and American RBDS program types. While PTY codes zero, one, and 31 are shared, the rest are all different as per the National Radio Systems Committee. Probably because formats like Top 40 and R&B weren’t massive concerns in Europe around the time of RDS’ first implementation.
Heck, when RDS was first implemented in Europe, category numbers 16 through 30 were blank, as seen in the table above. Obviously, this has since been rectified, but as you can see, European program types have never quite lined up with American program types.
Let’s move on from the PTY stuff.
Once we get past all the important PTY data blocks, we get to data blocks 2B, 3A and 4A in the table above (PTY is in the PT4 block). Now, 2B points to what blocks 3 and 4 consist of, and that could be up to 32 bits of text. Unless your local DJ is spinning, say, “I Slept With Someone In Fall Out Boy And All I Got Was This Stupid Song Written About Me,” that should be enough for many song titles and artists in one shot. If not, the string of text can be cycled out to display all the proper song information while providing a neat scrolling effect. Older radios with multi-segment LCD displays usually cycle-out text as they’re confined to a particularly short length of characters.
Mind you, expecting to receive song titles and artists as text doesn’t always work out. Some small and cash-strapped independent broadcasters just didn’t buy into RDS technology, and some radio stations simply can’t be bothered to cycle out their RDS text, instead just displaying the name and the callsign of the radio station. Then there’s the matter of things not translating across continents, because the global RDS is just different enough from the U.S. RBDS in implementation that RBDS data displayed through an RDS receiver might not look quite right. As told in a European Broadcast Union paper:
If urgent changes to the existing software are nevertheless to be made, the first thing to do would be to add recognition of offset word E and, if PTY is additionally implemented, the new table of US PTY codes will have to be used.
Ah yep, for several years, RBDS had several program types that just didn’t exist in Europe, and that “offset word E” business is about multiplexing with the Modified Mobile Search System that is basically irrelevant today. However, those are inconsequential problems when you consider how RDS reduces the chance you’ll miss the name of a new song.
These days, there’s a second way your car might know what song’s playing on the radio: it might be because the broadcast isn’t an analog FM signal at all. Welcome to HD Radio, a way of broadcasting digital and analog radio signals through the same bandwidth at the same time. Multiplexing FTW! What does this mean? If your receiver is purely analog, you get an old-school FM or AM signal. If you have a receiver capable of getting HD Radio, you get a stream of digital audio signal flowing at up to 128 kBit/s in some FM implementations. Oh, and broadcasters aren’t limited to just one HD channel for each station. As per HD Radio:
Let’s say your favorite local radio station is on 96.9FM. With HD Radio technology, that same station is being broadcast in digital sound on 96.9 HD1. Plus you can access all new content on up to three additional stations: 96.9 HD2, HD3, and HD4.
In addition to simulcasts and extra HD channels, HD Radio lets receivers pick up a whole lot more data, including tiny images sent out on the airwaves through HD Radio’s “Artist Experience” function. When I say tiny, I mean microscopic compared to current trends, as Xperi, the company that owns HD Radio, claims that “The images nominally have a resolution of 200 pixels by 200 pixels and a maximum file size of 24 kilobytes.” Still, what a leap forward.
Of course, terrestrial radio has been going out of fashion ever since conglomeration reared its ugly head and all stations started to fit into specific genres. For decades now, consumers willing to pay a bit extra have popped for iPods or satellite radio subscriptions. However, Apple’s canned the iPod and those coast-to-coast yet somewhat crappy sounding subscription-only satellite radio stations are rapidly falling out of favor due to the rise of streaming and cheap data plans. Yep, I’m one of those streaming people, and while I still buy physical media when I can, the convenience and curation of internet radio stations is second to none.
Mind you, if you’re sending Bluetooth streaming audio to a car’s head unit, it’ll still need a way to pull up track information. That’s where Gracenote comes in, the same brand of track-identifying technology used in iTunes and WinAmp. It’s built into all manner of cars from Toyotas to BMWs, is used for everything from radio to streaming, and it’ll likely continue to be key as some manufacturers ditch phone mirroring. More on Gracenotes in an upcoming article, because I know you’re all excited.
(Photo credits: DoulosBen – Own work, CC BY-SA 4.0, Nissan, Jaguar, EBU, Xperi, Gracenote)
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Am I the only commenter who listens to DAB in their car?
(BBC 6Music, pretty much exclusively)
Excellent write up. I would love to see a follow up on how HD Radio works, and in particular how it compares to DAB. I’m an electrical engineer but only knew how mono FM works, not stereo or the various side bands.
Riveting stuff- When can we expect the CD-TEXT explainer talking about how older cars know what track your CD Player is blazing.
I remember the factory radio in my ’03 Mini had the ability to turn off the PTY. Screen said “NO PTY” when you did. Which the first time I accidentally changed that setting made it look like “NO PITY” and I was thinking cars definitely should have a “no pity” drive mode.
I rarely listen to the radio nowadays in my car, more out of habit/continuity for whatever I was listening to on my phone, but I do have presets saved. I’ll have to scroll through later to see if I’m making it up, but I think when the preset was saved, it also saves some of the RDS in the display from whatever was showing at the time too, so it pulls up the tail end of a song name from long ago or something. Happens more on the stations owned by big corporations.
The RDS can get someone in hot water too. When prepping or photographing cars for the online listings, understandably whoever’s working might tune it to a station of their choice to listen to in the meantime. But the RDS could be scrolling through the wrong thing at the time a pic of the radio gets snapped. An amusing moment when I see it when virtual window shopping, but to the right pearl-clutcher they could find it offensive and complain. Oh no a swear!
The only radio station I listen to is our local Community radio station (KVMR 89.) in Nevada City CA. When I’m on the road to the Tacoma WA area as I did several times last year, I tune in to their internet broadcast. When I’m driving someone else’s car, or on a short jaunt from being there, I have to listen to the local radio stations. Ugh, I hate commercials with a passion, so I search out other community radio stations.
KVMR has Spotify running on their website, so I can check out the artist info, but they haven’t spent their under funded station on this fancy RDS stuff.
Nicely done. Never did radio, but it’s very similar to what we did in analog satellite TV communications. Changed careers about the time of the digital transformation. I remember ABC and CBS had different audio specifications where we would have to adjust the audio carrier wave higher or lower depending on the company. CBS audio would be lower but had more depth of sound, while ABC wanted higher amplitude to, as they said, made the audio pop.
Informative article—but what caught me is the differences between PTY categories in the US and Europe.
We get Information, Religious Talk, Religious Music, Personalities!
They get Education, Drama, Culture, and Science!
gotta admit I’m curious as to the difference between Light Clasical & Serious Classical. Is it the artist’s hair? Do they have to have Epic Hair to be considered Serious?
I noticed on that list that 8 character US PTY for Nostalgia is actually 9 characters.
As expected, the US has country, oldies, classic rock, and religious content, whole Europe has culture.
Damnit: you beat me to it
I have never seen a “PTY” button, but I thought I saw a “PTY CAT” in my old G35 coupe head unit.
Like Turbeaux, this was a “how does that work but i’ll look it up later” thing for me. The college station near us will show song titles intermittently and I always wonder why some and not others. Are the songs themselves encoded? Is it a situation where the more modern wavs or mp3s or whatever format they use has it encoded, and when they play an old record it’s not? Or is there a Gracenote-like service or system that identifies it on its way out to the airwaves, and that data added on? This doesn’t seem likely as any service worth a damn would be able to identify 99.7% of anything a “normal” radio station would play, even college stations. Or is it manual, with some college kid keying in the songs set for that session’s play list, and if a DJ goes off book, there’s nobody doing that keying in real time?
Anyway, thank you for this. I know have more questions than I did before, especially with that tease of the “signal is amplitude modulated with the data in a scheme beyond the scope of our discussion here“
I’ve noticed the same for my local college radio station, intermittent or sometimes it’s a delay like they remembered to update a bunch at once and it’s catching up, or just a delay in processing. They also keep a playlist online so there is a log synced up somewhere.
Thanks for the details. Some local stations have RDS and some don’t, for extra fun, if we travel and hit a station with the same frequency that does use RDS the text hangs around for a few days.
In the big city they sometimes have HD Radio, a separate digital standard that provides artist, title and album art. This “feature” was,the cause of the legendary incident in Seattle where NPR sent a bad art file and bricked a bunch of Mazda radios
I could never get over how bad Siruis/XM/SiriusXM sounded. It was so awful, so tinny that most of the music was rendered unlistenable, just this glassy wash with bass stuffed underneath it. Try compressing an mp3 down to 24kbps and you’ll get a pretty good idea. And, at least in an ex’s car, there was no caching at all, so every time you went under an overpass it cut out.
Thanks for this well-explained Explainer! The community radio stations I listen to rarely display song titles, but it’s still good to know how RDS works.
Thank you for this – I always wondered even why it was called “RBDS” much less how it worked!
Annoyingly though, now that FM radio is heavily conglomerated here in the U.S., several of the stations where I live have hacked the signal so it no longer scrolls out the song title, but instead, corporate sponsor names. Sigh.
Yep. I still listen to the radio if I’m running a quick errand or two and don’t feel like plugging my phone in and my local radio station of choice runs ads instead of just showing the artist and song name.
All I have to add to this is on my Dad’s Suburban, the display only shows so many characters, and it scrolls. One time, the band Beartooth came on, and my kids thought it was hilarious that it would be displayed as Beartoot, then Eartooth.
Okay, I thought it was funny, too.
Learned something today that I was always curious about, but not enough so to look up myself. Thanks for the article
In the network world, a multiplexer is (or was) commonly call a “mux”.
And for those who like ‘Total Eclipse of the Heart’ but want something more upbeat, please enjoy Nicki French’s dance version:
https://www.youtube.com/watch?v=7EolLvBcWaA
Try to ignore the backup dancer. I assume Nicki had her there as a favor to someone.
Oooof! Jim Steinman is spinning in his grave!
I’ll listen to “I Need a Hero” if I want a more upbeat Bonnie Tyler.
You mean the song from the Guiness commercial? 🙂
shrek (2 i think)
“Brilliant!”
Any time anyone mentions Total Eclipse of the Heart I will always share the Hurrah Torpedo version (they’re the national band of Norway I think).
https://www.youtube.com/watch?v=ysUjYAi0WcQ
That was definitely a thing that I watched… 😮
I know, their genius is staggering at first but it gets better the more times you watch it 😉
I shall take your word for that. 🙂
It took a little while but by the end I was chuckling. I was worried that the guy playing the stove was going to lose his pants.
It’s definitely worth watching ’till the end 😉
Hurrah Torpedo! There’s a name I haven’t heard in a long, long time. One of the first viral videos I can recall latching on to me.
Fascinating article. But yeah, It’s been a solid 10 years or so since I listened to “the radio”. I was an early adopter of Google Play Music and when Google killed that, as Google is wont to do, I switched to Spotify. Great for finding new music, no obnoxious DJ’s, no obnoxious commercials, and when I find something I really like I buy the uncompressed track/album and add it to Plex. Spotify and this site are two of very, very few subscriptions I don’t mind paying.
The main reason I still listen to terrestrial radio is for the “commercials”. Not necessarily the generic viagra meds, but for the stuff happening around town and traffic updates. Waze is okay, but it’s always one step behind because it doesn’t change your route until there’s already a traffic jam.
Like you, I switched to Spotify, but I still mourn Google Play Music because YT Music just isn’t a replacement for it. In fact YT Music couldn’t suck any harder if Google hired a team of sucking consultants to advise them how to make it suck more because it already sucks that hard. Screw YT Music! Bring back Google Play Music!!!!!!
Sorry, sore subject for me 😉
“I’m so glad Google is killing off Google Podcasts also!”
-no one ever
Sorry, I had to pile on. They’re pushing YT music for podcasts but I’m loathe to change, so I guess I’ll just make do without any podcasts anymore.
And then there are the stations in my town that sell the RDS feed to the highest bidder so you get things like 800-TITLEPAWN and I.M.Scummy BUYS HOUSES!
Holy crap, really? Wouldn’t that be against some FCC something-or-other? Either way it’s icky.
You would hope but we live in a late stage capitalist wasteland / libertarian paradise / free society where everything is for sale.
Yeah, where I live this happens – no song/artist, just personal injury attorneys and junk hauling services.
I really hate it as damnit, I need to know if that song’s Tone Loc or Young MC as they sound the same and it’s been too long and I’m too old!!
Young MC = Bust a Move
Tone Loc = Funky Cold Medina and Wild Thing
If either of them is heard performing any other song, pull over immediately and donate money to that independent radio station.
You just nearly made me do a spit take all over my monitor. Well played, and…will do! 🙂
yep, I see something like #webuyhouses or 1800carinjury or talcsettlement.com on some of the more rural stations.
Grim.
Stuff like that is why I listen to public radio. No commercials! Shout out to 89.3 The Current in Minneapolis/St. Paul! (these days I stream it through their app because I live in California)
And here I thought the PTY button meant “Party”
You’ll need a 4Runner for that.
So why, if RDS is transmitted at 1.2kbaud (I’m an old), does it not update that data until the song is half over? I know it’s slow, but sheesh. My a 9600 baud modem could download the entire top half of Erica Eleniak’s pixelated head in that amount of time. Sure, that’s 8 times faster, but it’s also like 1000x more data. Does the error correction just suck that bad?
Also, seems like a missed opportunity not using Whitney Houston in that lead image instead of Bonnie Tyler. But I guess I’ll just keep holding out for that hero. I’ll always love you guys anyway.
Flashmod iPod solves most radio problems, and most streaming problems.
I lost my iPod on a flight and I hate myself for it. Then I was like “I should buy a new one” and 2 weeks later Apple killed it, and now between that and DankPods (love that guy), prices have gone up.
Why bother with iPods when used smartphones exist?
Throw in a good sized SD card and put all your music on it in whatever format you like. Given micro SD memory is running $60/TB you can probably get your entire CD collection on your phone in WAV format for less than a latte.
Why wav when FLAC exists?
Use what you like.
I like WAV because its the closest to the original format and because I can. Given how cheap memory is and how fast data transfers are I don’t see the point of FLAC anymore, at least with a master archive. Why risk it?
If phone space is at a premium you can always rip to whatever lossless format you prefer and compress what you put on your phone separately. Unless you have your phone hardwired to one Hell of an audio setup AND you have the sweet, sweet hearing ability of a teenager you probably won’t be able to tell any significant difference between WAV and 128 kbps anyway. Especially over Bluetooth.
Why touchscreen when iPods with a flashmod can run up to a terabyte/40k-ish songs?
Because you probably already own a smartphone and because you only have so much pocket space.
No headphone jack and bluetooth sucks battery it needs to be a phone. At least when your Gameboy died it didn’t cut you off from your Thomas guide
I have a 3.5mm headphone jack. I also have on board B&O tuned quad 32 bit DAC(!), a dedicated 24-bit high fidelity audio recorder “able to record up to 24-bit/192 kHz with manual channel controls for effective noise elimination of up to 50% in audio/video recording compared to other smartphone audio recorders”, a FM receiver and an IR blaster to make my phone into a universal remote. Lots of other stuff too. For fun I picked up a pair of the OEM B&O Play H3 earbuds. They sound OK to me.
I don’t care if Bluetooth sucks battery because I can plug it in to the wall, a car, a battery backup or a solar charger or I can swap the dead battery for one of several full backup ones in about 10 seconds and recharge the dead one offline. Spare OEM batteries run about $10 on Ebay. Ridiculously oversized aftermarket batteries are an option too.
I can also drop it and it is very unlikely to break the screen. It was one of the most shock resistant, durable and resilient consumer smartphones of 2016. Good thing too. Not long after I got it I watched in horror as it it tumbled, unprotected, to the pavement when my bike’s crappy phone mount broke on a bump. The phone was undamaged save for a small corner crack on the plastic trim. I didn’t need to but I replaced it anyway. The trim was $3 on eBay and it took just a couple of minutes. Boom! Good as new! Now I have a rock solid bike phone mount and a silicone skin. The phone still looks new.
If you like what you read you can pick up a good, used LG V20 on EBay for about $40-$70. Or $120 if you want the international dual SIM version like mine. (You don’t lose the micro SD with that). If you don’t care about using it as a cellphone you can probably get a Sprint version for almost nothing as those no longer work on any carrier. WiFi should be fine though.
https://en.m.wikipedia.org/wiki/LG_V20
So my stereo doesn’t just have a wide range of musical knowledge and a pretty good ear? You’re shattering my worldview here.
So, we have a follow-up piece to this that might actually suggest your stereo does have exactly that…
“There’s this little man who lives in the radio who has a good ear for music, and a keyboard…”