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Because I have several good (but older) sound systems in my apartment I am looking for a way to play audio from an iPhone or computer (anything with a headphone jack) synchronously on all of them. All the receivers are hooked up to Analog Cable Radio (Not over the air antenna). So I thought it might be clever to send the music over the existing wiring on a specific carrier frequency which I would set the receiver to.

Does anybody know how to do this? Is there maybe a little gadget or a not-to-complicated way to build it?

Hope you guys get my point and thanks in advance!

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    \$\begingroup\$ Analogue cable radio (en.wikipedia.org/wiki/Cable_radio) doesn't use FM or AM transmission - it uses data from a coax coming into your home. \$\endgroup\$ – Andy aka Jun 27 '13 at 19:25
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    \$\begingroup\$ Yes, there are gadgets you can buy which will plug into a personal audio player and transmit FM stereo over a short range. There have also been published projects for doing so, usually using a special function chip. Building your own hi-fidelity FM stereo modulator would be a challenging project - possible, but probably to be considered more for the educational value than for the result. (Today you'd also have the option of doing it digitally in an FPGA) \$\endgroup\$ – Chris Stratton Jun 27 '13 at 19:26
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    \$\begingroup\$ google mp3 fm transmitter \$\endgroup\$ – JIm Dearden Jun 27 '13 at 20:53
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    \$\begingroup\$ In what way is this question unclear? Seems pretty clear to me. \$\endgroup\$ – Phil Frost Jun 27 '13 at 23:48
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    \$\begingroup\$ @Dario do you have any information on the analog cable radio your stereos use? Make/Model #s of the stereos so we can cross reference? \$\endgroup\$ – Passerby Jun 28 '13 at 7:03
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OK now I'm confident to answer this: -

Analogue cable radio doesn't use FM or AM transmission - it uses data from a coax coming into your home.

If you do have analogue radios that work the traditional way you can buy small FM transmitters (intended for iPhone to car radio connections) just like these

You can get cheaper ones that just take an audi input jack too.

If you are still intent on making an FM transmitter (mono for simplicity) here's a link to a design site with several on.

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You wouldn't want to interfere with the cable system, so as long as the coax is not connected to the cable system this could be done. Basically instead of using the air as your broadcast medium, you would use the coax network. You would need to find a FM transmitter of the appropriate power with has a modular or re-moveable antenna, simply replace the antenna with the coax connection.

If there is an impedance mismatch, you will get a reflection/attenuation/loss proportional to the mismatch; a transformer balun can solve an impeadance mismatch.

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