Total posts : 45366
You’re sort-of correct, but not quite. FM uses a system of subcarriers embedded on the main carrier to encode the pilot tone, L-R information, RDS and two SCA signals as high audio-frequency carriers above the baseband audio, so the whole shebang gets modulated as a single frequency-modulated carrier.
This is how an FM sigtnal’s baseband looks:
AM stereo, on the other hand, also uses two signals, but they’re superimposed co-carriers, only that the L-R signal is 90 degrees out of phase from the main broadcast signal. A better way to think of it would be that AM stereo works like polarised lenses. Like FM, though, it’s sum and difference, rather than discrete left and right.
A form of AM stereo, called “inndependent sideband” (ISB) operates much the way C-QUAM AM stereo does, except the audio polarity of the L-R difference signal is reversed, allowing phase cancellation to produce discrete left and right channels on the lower and upper sidebands. The same phase cancellation exists in C-QUAM as well, but because the polarity is wrong, it doesn’t produce the correct lef & right channels that way.
You can expect that there are about 10% of all car radios in operation to have AM stereo, namely the C-QUAM (Compatible Quadrature AM) standard.
You asked about how much a transmitter costs- I think Chris Cufdf still sells them, and they run between US$125-$225. Performance is good to 12kHz, and all current TXes are made to have 100mW output. (I have an older, 10mW TX from him.)
There is no signal loss or gain from using AM stereo- unlike FM stereo, where the more energy distributed across the baseband spectrum, the less gets out for each subcarrier- That is, if you just have the L-R difference and pilot, that takes half your power that would go to the baseband audio, and cut your range appropriately.) For AM stereo, this isn’t the case, since the energy is not spread out. That’s one of the biggest myths surrounding AM stereo.