Basics of Audio Communication??

Ok guys, I have been posting for several day to ask for help related to Simulink and USRP devices but got no response. let me give another simple and basic try...

I would like to know how does RF or radio transmission works, I mean what theoratical steps are needed to transmit a voice from microfone over the air e.g FM or AM etc...

Answer 1

[Iain]

Soundwaves are converted to electrical signals by a microphone.

In basic AM transmission, those electrical signals are added to a carrier wave, which is then fed to a transmitter, which transmits EM radiation as per that mixed signal. That signal propogates through the intervening medium to the receiving antenna.

An LCR circuit is then used to reject all of the EM spectrum, except for a small band around the carrier frequency. Then you amplify it and feed it to a speaker of some description.

In FM transmission, the microphone signals are used to modify the phase of the carrier wave. That signal is then fed to a transmitter, etc.

An LCR circuit is then used to reject all of the EM spectrum, except for a small band around the carrier frequency. Then you use a phase-lock-loop to relate the instantaneous frequency back to an analogue signal, and then feed it to a speaker.

Similar techniques are used for stereo transmissions, but you need to transmit two signals (L+R and L-R is typical as it still works with mono receivers)

[Usman Ali]

Thanks lain for your nice comments. what if I would like to model this whole scenario. please correct me if i write these steps as follow.

first the audio signal is sampled/qunatized, passed over a filter to the A/D converter and then transmited. at Rx side the opposite is applied...

[Iain]

Depends how accurate you want to be. The audio signal will be quantized at the electron level before it gets to an A/D. The A/D then quantizes it further. You then need to mix the digital signal onto the carrier wave. If you use the simplest type of encoding (serial), you'll never get a reciever to reliably understand the signal. You need to add information to the signal to let the reciever go "thats the start/end of a word"

The receiver then needs to demodulate the transmitted signal to get to something it can get it's data from.

Digital, in this case, is more complex than analogue.