Audio via RC TX/RX

[Hell-e-Guy]

Hi Guys,

I need to transmit audio to multiple receiver. To make life easier I thought I can use an RC TX module and several receivers and tap into the audio out pin of the FM demodulator IC.

I am pretty sure that the RX can do the job, my problem is the TX.
Do you think that the TX module has an analog input pin?

Any help will be appreciated.

Hell-e

[Rodney]

It is very doubtful that you could make this work. First off, while called FM, most if not all our transmitters for RC are phase shift keying, not true FM, and; as such, are not very linear. As a result, if you could get some semblence of FM modulation , distortion would be a huge problem. Even the old AM units were not very linear as they were usually just pulsed RF, not really a true AM.

[neurofun]

i think this might work, should not be too difficult and be quite cheap:

convert the audio to pwm with the help of a microcontroller with a built-in adc.
feed the pwm signal to the transmitter via the trainer connector.
extract the pwm signal from the receiver and feed it to a low-pass filter.

some info about pwm to analog conversion:
http://ww1.microchip.com/downloads/en/AppNotes/00538c.pdf

[Miami Mike]

Although the coordinates you give in your profile (Latitude: 29.61, Longitude: 33.68) put you in the middle of Egypt's Sinai peninsula, it's more likely that you're somewhere here in the USA. If so, that means that the modification you're contemplating is illegal.From http://a257.g.akamaitech.net/7/257/2422/05dec20031700/edocket.access.gpo.gov/cfr_2003/octqtr/47cfr95.212.htm

You must not use an R/C station--
...
(b) To transmit any message other than for operation of devices at remote locations (no voice, telegraphy, etc.);For other rules governing r/c channel use in the USA, see http://www.access.gpo.gov/nara/cfr/waisidx_03/47cfr95_03.html, sections 95.201 through 95.225.

If you really are in Egypt, never mind. :D

[Comatose]

Um, crazy question I'm sure, but wouldn't it be cheaper and easier to use one of the hundreds of FM trasmitter kits availiable on the internet than to hack an FM transmitter to do the same thing?

There are plenty of dubious legality "for export only" if you're dying to have multiple watts of output power.

[vintage1]

Pretty sure that a varicap modulated oscillator is as near true FM as you want..and the recivers are true FM..Ok we USE them in FSK mode but that is simply to get a very low bitrate digital signal across.

The linearity is usually a function of how many volts the varicap is experiencing. They are very non linear at low voltages, (below about 1v) and pretty linear above it. You need to get to the varicap somehow and inject audio there...and adjust the DC on it to get the tramnsitter spot in the middle of the band.

[Hell-e-Guy]

Thanks for all the answers.
I will try and use an RC receiver and am MP3 car transmitter (just need to find a way to take the frequency down to the 72MHz band).
Mike, Yes I am currently in Egypt's Sinai peninsula. The heat and the extra time on my hand makes me think about stupid hacks :) :)

Hell-e