I have some understanding of what frequency modulation entails – it entails "adding" a new frequency to the frequency spectrum of a signal. But how, exactly, is a modulated signal selected in the receiver? Say you're listening to your car radio and want the radio signal modulated at 80 Hz or something. How does the actual system select it? In general, not in car radios specifically.
Frequency modulation and incoming signal selection
Modulationsignal processing
Related Solutions
Your perfectly single-sideband suppressed-carrier modulated sinusoid certainly has a phase which can be measured. However, what you cannot tell is what the contributions of that measured phase from the audio input and the RF oscillator were.
There is another form of single-sideband modulation, in which not only one sideband but also the carrier component is transmitted. This provides a reference which can be used to synchronize the receive LO to the transmit one - normally done to insure exact tuning, but it would also give you the ability to recover the original audio phase.
It is also quite possible, especially with modern DSP gear, to transmit two separate audio channels, one on each side band. This is commonly called independent sideband modulation (ISB).
Many spread spectrum implementations are DSP based and capable of receiving multiple channels at once - GPS being a good example.
This appears to be on-off keying which can be thought of as amplitude modulation where the modulating signal (SO) has just two levels, 0 (no carrier) and 1 (full carrier).
(source: sitelec.org)
Best Answer
First of all, realize that all forms of modulation add frequencies to that of the carrier. These frequencies appear close to the carrier so that the modulated signal can be selected with a bandpass filter. You do this by tuning the radio to a specific station. Then the signal from the selected station is demodulated. For amplitude modulation, a detector sensitive to amplitude variations is used. For frequency modulation, a detector sensitive to frequency variations is used. This is a greatly simplified explanation. For details, you need to look in any communications textbook or search the internet.