sending digital over FM transmitter

how to send digital information using fm

i have an FM handheld... how can i send digital information of rate about (4.25KHz) over it... my receiver is another identical FM handheld.. i tried by connecting my digital source directly to the mic input but didn't succeed..

can anyone offer help?

The microphone input will be AC coupled and filtered to voice frequencys.

The usuall way is to use audio frequency shift keying. There is a lot of information on the web about packet radio with an amateur radio license. When I played with it a few years ago the standard mode was 1200baud, I vaguely recall it sent a single cycle of 1200HZ or 2400Hz audio tone to represent a 0 or a 1. 9600baud required modifications to the radio as it could not be done using AFSK through the audio path
intended for voice.

The audio passband of an FM transceiver made for 12.5KHz or 25KHz channel spacing is a bit small for 4.25Kbps data using AFSK. It may be possible using a more complex modulation scheme, signal to noise ratio allowing.

frist digital signal is in kbps (as i think).
how did u generate a digital signal. give full info.
is it for transmitting digital signal coz Fm recivers have LPF in end which cause the widing of signal. thus if proper timming is not provided u may get a DC signal at O/P.
so give more info

>how did [you] generate a digital signal. give full info [please].

You need an analog signal to put through the radios not a digital signal.
I used some DOS software called PC Flexnet, a soundcard to generate the
audio tones and a small circuit with a resistor, a transistor and a diode connected to
the serial port to operate the PTT on the transmitter. Might work with VOX instead of PTT switching but you will need to increase the preamble time.

Flexnet is badly documented and out of date. There is newer software such as gmfsk and AGWPE which I have not used myself.

>Fm recivers have LPF in end which cause the widing of signal.
Filters make signals narrower not wider.

>thus if proper timming is not provided u may get a DC signal at O/P.
No. Audio transmitters and receivers remove DC and low frequency components
of the audio signal so digital signals would get distored.

> how can i[you] send digital information of rate about (4.25KHz) over it.

i think u should use a capacitor between o/p of comp and i/p of Fm transmitter.
secondly if ur using windows Xp then the DOS program may not be able to produce the tones because it xp does not allow direct interfacing with hardware of a program. it has to go through API.

wot u can do to test that is XP blocking ur program to access the hardware is , u run a song mp3 and the o/p from the sound card out <vere u put ur speaker pin> to FM transmitter by a capacitor.

that i think may solve ur problem.

and if u want to transmit digital signals u can use serial port , printer port or USB port.

>i think [you] should use a capacitor between o/p of comp and i/p of Fm transmitter.

You can if you like. Every soundcard output and transmitter microphone input I'v ever
seen has already had a capacitor on it so it is unlikley to be necessary.

one more thing LPF is low pass filter it makes a pulse sharp pulse broader. makes it continious. and band pass filter makes a signal to fall under two frequencies.

>LPF is low pass filter it makes a pulse sharp pulse broader.
Yes. I meant that a LPF makes a signal narrower in the frequency domain.

>and band pass filter makes a signal to fall under two frequencies.

A band pass filter attenuates frequency components above one frequency and below another.

if his bit rate it too high this pulsed after passing through LPF stage <after demodulation>of an FM reciver will widen the pulse and he will be unable to receive wot he tranmitted coz of pulse widing.

i was trying same using printer port, but in ASK.

i am not good at electronics but trying to help

i can be wrong. if i am point then do point at it. and correct me

thx a lot guys

Added after 45 seconds:

can anyone offer more help

try to use FX469 in-chip modem IC (ffsk)

Hello

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