Hi So I understand that in AM radio, amplitude is varied to reflectconvey the so

Question

Hi
So I understand that in AM radio, amplitude is varied to reflectconvey the sound information and in FM radio, frequency isvaried.

1. How does the AM wave vary loudness? (the amplitude is used tovary just the pitch of sound, right?)

2. In FM radio, it is the amplitude that varies the loudness of thesound, correct?

3. Again in FM radio, how is frequency of the wave modulated whenthe wave has to be a certain exact frequency for the radio to tunein correctly?

Thank you very much in advance, I would appreciate any input Iget!!

Explanation / Answer

AM: V(t) = 1 + mS(t) m is called themodulation index. The typical value for m is 1, given thatI've normalized S. So, the variation in the amplitude varieswith the magnitude of S. If it is at a max (=1) the wave goesfrom 2 to 0. Same for S=-1. If the max "volume" of S isonly 1/2, then the peaks and valleys will be 1.5 and .5. Thatis how volume is handled.
FM: V(t) = Sin[(S(t) + ) t]. Theamplitude never changes. The frequency of the channelis , though the signal does move back and forth. Theamount of the deviation is limited by the channel spacing. Inbroadcast FM, the channel spacing is 200 kHz, and the deviationlimit (max loudness) is +/- 75 kHz, for 150 kHz max deviation. Remember that the next channel over is also going back andforth at +/- 75 kHz, so the guard band is 50 kHz, 25 kHz on eachside of each band. The amount of the deviation is theloudness.
If a lightning bolt happens nearby, the AM system detects amajor change in amplitude, which is very noisy. In an FMsystem, the amplitude is typically limited, and only the freq isdemodulated. The amplitude jump due to lightning is limitedjust as the main signal is, and you don't hear it. That's whyFM generally sounds so much better than AM. Plus, thefrequency allowed for S(t) is about 8 kHz in AM and 15 kHz inFM.

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