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Author Topic: How do text messages get transmitted to my FM car radio?  (Read 4334 times)
KC9KEP
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Posts: 208


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« on: December 02, 2010, 12:46:41 PM »

Hello all,

I bought a "new" (well, new to me) car recently.  (I think it's a 2006).

The FM radio displays short text messages indicating the call letters
of the station that your tuned to, such as WKLH.

It also displays the title and artist of the song currently being played.

It's not subscription or satellite radio (at least, I'm not subscribed to
any such thing :-)

Does anyone know how this is being done?  Sub-sonic carrier?
Parallel digital transmission?  Invisible carrier pigeons? :-)

Thanks!

--Tom Nickel  KC9KEP
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KE3WD
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Posts: 5694




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« Reply #1 on: December 02, 2010, 02:04:36 PM »

RDS = Radio Data System

http://en.wikipedia.org/wiki/Radio_Data_System

Read all about it.


73
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AA4HA
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Posts: 1377




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« Reply #2 on: December 02, 2010, 09:43:48 PM »

You see how it is a data stream on the 57 KHz sub-carrier? If you have the means to do some tinkering with PLL circuits you can demodulate many of the subcarriers on an FM broadcast signal.

19 Khz is FM "Pilot"  unmodulated, indicates that the signal is in stereo
57 Khz is RDS      1200 bits/second
67 Khz is SCA      (muzak, stock reports, reading for the blind)
92 Khz also SCA

Here is one example of a kit;
http://www.ramseyelectronics.com/downloads/manuals/SCA1.pdf

With all of the sub-carriers a commercial FM broadcast can occupy up to 100 Khz of bandwidth. If you use a FM receiver capable of narrow bandwidth you can sometimes swing across an FM signal and pick up the subcarriers. Look at a commercial FM broadcast on a spectrum analyzer and you can see what stations use subcarriers (some do not).

Tisha Hayes
AA4HA
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KC9KEP
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Posts: 208


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« Reply #3 on: December 03, 2010, 12:24:19 PM »

Thank gentlemen .. very inneresting  Smiley
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KU4UV
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Posts: 375




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« Reply #4 on: December 05, 2010, 07:01:10 PM »

Most of the newer GM cars have radios with RDS on them.  My 2001 Impala has RDS.  It is a really neat system to have, and I don't know why more manufacturers don't utilize RDS.  It will tell you the artist and song title, as well as any other information that the station chooses to transmit. 

73,
KU4UV
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G8HQP
Member

Posts: 120




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« Reply #5 on: December 07, 2010, 11:05:19 AM »

RDS has been fairly normal in European car radios since the mid 1990's. I didn't realise it is now an international standard and implemented in the US.

In the UK it has two really useful facilities, but I don't know how these would work elsewhere. If I leave my home tuned to, say, BBC Radio 3 then as I drive along the radio will automatically retune to alternative frequencies carrying the same programme once my home transmitter is too far away.

Optionally, if I am listening to a national station the radio can automatically and temporarily retune to a local station while a traffic flash is being sent. This means that I can hear local information, yet still choose to listen mainly to a national station. The only snag is that the local DJ can forget to push the switch to return me to my preferred station, so I get a burst of (loud) music/chatter after the traffic flash.
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KC0ZOS
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« Reply #6 on: December 07, 2010, 04:55:23 PM »

Isnt this also called Hi Definition Radio?
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K7KBN
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Posts: 2756




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« Reply #7 on: December 07, 2010, 09:04:29 PM »

Isnt this also called Hi Definition Radio?

No - HD radio has only been around a couple years, I believe.  The RDS has been around for around 12 years, and probably longer.

HD radio is actually a digital transmission, requiring a suitable receiver.  Both regular and HD programming can be sent on the same frequency, apparently.  All I really know about it is that my wife has an HD radio in her Nissan truck, and the FM on it sounds very much better than the "regular" radio in my Subaru sedan.
« Last Edit: December 08, 2010, 04:25:04 PM by Pat Bailey » Logged

73
Pat K7KBN
CWO4 USNR Ret.
N1DVJ
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Posts: 382




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« Reply #8 on: December 14, 2010, 11:37:55 AM »

I had a friend who designed something for an FM commercial data stream years ago.  Don't know if it was RDS or maybe one of the early precursors, but it was in the late 80's, I think 1987... 
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AE5NE
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Posts: 91




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« Reply #9 on: December 20, 2010, 11:57:39 AM »

There is no "high-definition" digital radio.

Referring to In-Band-On-Channel digital FM broadcasts in the USA, HD means "Hybrid Digital", and many contend it has less quality than the normal analog program.

And, again, the USA chose a proprietary patent-protected standard while ignoring the already standardized systems present in the rest of the world (just like they did for satellite TV, cable TV, broadcast TV, cellular service.... )
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