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I was wondering how the range of the volume is transmitted in AM radio.

With amplitude modulation, the volume is directly linked to the amplitude of the radio signal. But this would mean that the overall volume of the radio receiver would decrease quadratically with the distance to the emitting antenna (or linearly due to atmospheric refraction and earth reflection)

Rémi Baudoux
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    To some extent yes; it depends on the receiver design. AGC reduces the dependence on signal strength but most AGC circuits aren't perfect. –  Apr 06 '21 at 22:44
  • Yes, and depends on the modulation technique (with or without carrier) – Rémi Baudoux Apr 06 '21 at 22:59
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    "AM" is by convention with carrier; without carrier it would be DSBSC (or DSB for short) –  Apr 07 '21 at 14:00

2 Answers2

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AM signal has a good reference - the carrier. AM receivers are generally designed to have an automatic gain controlling circuit (=AGC) which tries to keep the DC voltage that's obtained by rectifying the carrier in the diode detector constant. It's quite possible that a distant station needs 1000x higher total power gain than another which is either strong or placed in the same city as the receiver.

The simplest crystal receivers do not have that gain control and with them the volume varies as you thought.

There's a variation of AM signal - the SSB - which doesn't have a carrier. If someone isn't talking right now, SSB signal is non-existent. Gain control must either be manual or cleverly increase the gain slowly if nothing is detected until either the noise jumps too high or some strong signal appears.

FM radio signal has a carrier. AGC is used to adjust the signal level to be suitable for the FM demodulator circuit.

psmears
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    It's worth noting that the human ear is sensitive across an amazing range of amplitudes, which makes it possible for AM radios to be somewhat practical even without AGC. – Russell Borogove Apr 07 '21 at 16:11
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Radios often have automatic gain control. The output of the IF (intermediate frequency) stage is controlled to have some average amplitude. Thus, when the AF (audio frequency) signal is "detected" from the IF signal, the amplitude of the signal is proportional to the percentage of modulation, (which is proportional to the original modulation signal) rather than to the amplitude of the RF signal. Cheaper radios used to not have automatic gain control, and some may still exist, but I would suspect they are rare.

Math Keeps Me Busy
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