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FM DIGITAL data Broadcast Project .i need suggestions

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Advanced Member level 4
Feb 22, 2002
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i have this design and i need to broadcast digital data somehow encoded in standart COMERCIAL FM transmision .88 -108 Mhz
Could you suggest a coding scheme to use ,may be using a subcarrier ??

any links ??

i think you know this book already . but if not - there is a link :

You may be referring to the SCA subchannels. They are modulated subcarriers that are above the frequency range carrying the audio. Government regulations specify the subcarrier frequencies and bandwidths.

Hi Friends .
Maybe i was not clear enough with what i want to do .I don't want to send MUSIC.but binaryl data .BUT using the regular FM broadcasting equiment .!
and FM receivers .Some sort of MODEM that will require encoer and decoder
one simple way is to broadcast dial up tunes .but is not very efficient !
i need to send 9600 baudrate or better

This link is obsolete but may be worth to look :
**broken link removed**

There are links related to its modifications

there is a source code to its software implemention in PC (using sound card) :
**broken link removed**
9600 Baud FSK (G3RUH)

If you want to transmit a stereo program band limited to 15KHz, you could use a standard like APTX as your AD/Compression scheme. Using a 12 bit dual AD and a compression 4:1 you will end up with a 256KBPS. This bit rate would fit in a standard FM channel which has an occupied BW equal to 300KHz even if you used a digital modulation scheme like PCMFM, what would allow you to use a regular broadcasting FM transmitter.
In the receiver end everything would be the same as a regular receiver up to the demodulator output, where you should have a baseband filter, a hard clipper and an APTX decoder to rebuilt the analog L and R channels.


Added after 1 minutes:

Thanks nandopg Your solution starts looking like something i could use .Though now i have to do a lot of research on those coding schemes .like APTX
though i may not need it because i don't want to recover analog data but digital ONLY!!!!!!!
do you have any links to info and projects related to it ?

thanks ..a lot for your insight!

Since the audio path is AC coupled you need a scheme with no DC content. Try Manchester encoding. This is also easy to recover the bit clock on the receiving end.

I am afraid that you will need to have the APTX decoder at the receiver end even if you don't want to recover the analog signal. This because you will have to de-compress the 256KBPS bit stream to generate the original 1MBPS.

I don't know if I got very well your application, but the use of APTX algorithm necessarily imply that the original information to be transmitted is audio. If this is not the case then the idea to use PCM-FM as the digital modulation scheme remains valid but in a standard FM channel you will be limited to 256KBPS. Also if this is the case a APTX processing is not required.
In this case you could still use a regular FM receiver to demodulate the carrier, being enough to design a baseband filter and a hard clipper to provide a clean-up (guarantying ISI free filtering) in the digital baseband and to shape the data output waveform respectively . These two blocks would be connected to the FM demodulator output.


i m working on the same project but i want to send data over 1.5 km of area. Somebody tell me how can i do this.

search "Implementation of FM Demodulator
Algorithms on a High Performance Digital
Signal Processor" topic on the net. there may be useful things ))):

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