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Since radio-waves are additive, I would expect that overlapping stations (eg. two different signals broadcasting on 95.7 within range) to both play over my radio at the same time.

But that's not what happens. Instead, I hear only one station at a time, with the radio switching back and forth between the two stations as I drive, and some static inbetween. Why does this happen? Why don't I ever hear both stations at once?

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  • \$\begingroup\$ Please let me know if this question would be a better fit for physics.SE instead \$\endgroup\$ – BlueRaja - Danny Pflughoeft Sep 4 '12 at 14:01
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    \$\begingroup\$ I can't give you an answer, but this has to do with demodulation. Using AM, you can actually hear multiple stations at once, but that doesn't work on FM, where only the most powerful station can be heard. \$\endgroup\$ – AndrejaKo Sep 4 '12 at 14:04
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    \$\begingroup\$ This is a great question, and the answer probably does have to do with demodulation based on a specific frequency instead of the entire spectrum. But also, I want to note that I have heard many times where a second broadcast does overlap with the current broadcast, thus both playing at once on the radio. \$\endgroup\$ – boardbite Sep 4 '12 at 14:07
  • \$\begingroup\$ @BlueRaja: Also accoustic waves exhibit superposition ("are additive") and still you don't hear "three" if one person says "one" and another person says "two" at the same time. \$\endgroup\$ – Curd Mar 28 '17 at 9:57
  • \$\begingroup\$ @Curd: ...No, you hear both "one" and "two" at the same time, which is exactly what I was expecting with the radio. \$\endgroup\$ – BlueRaja - Danny Pflughoeft Mar 28 '17 at 11:17
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Your mention of the frequency (97.5 MHz) tells us this is an FM receiver. (AM will behave differently, as will other modulation schemes).

Because FM is encoded by modulating the signal frequency, anything to do with AM is undesirable. To deal with this, most receivers over-amplify the signal until it becomes larger than the later stages can pass. The signal then "clips" to the voltage of that amplifier. This stage is called a "limiter"--it limits the amplitude to some fixed value. In theory, any signal weaker than that drops out and just becomes noise, and any signal stronger than that has a very nice fixed level that the FM detector can handle without having to worry about amplitude variations.

The amplifier-limiter stages create a phenomenon called "capture", where the strong signal tends to eliminate the weaker one. This is why you hear only one station.

If the signals were very close in strength, you would indeed hear them "mixing together", but that only happens for a fraction of a second as the signal levels rapidly change (presumably, you are in a vehicle), so you normally don't hear that.

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    \$\begingroup\$ Note that they would have to be very close in strength. Typical capture ratios are under 2dB. \$\endgroup\$ – user207421 Jun 3 '15 at 18:54
  • \$\begingroup\$ No. FM signals are not additive. Adding two FM signals does not make one "mixed" FM signal. Not even if the two carrier signals are synchronous. (And if the receiver didn't "capture" a signal, it would not be able to demodulate it, since ordinary FM receivers use the "captured" signal for demodulation). \$\endgroup\$ – david Mar 28 '17 at 7:43
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Here's my take on it:

While the carrier signals may have the same frequency, they have different phases. As the PLL in the FM decoder drifts, it locks first onto one phase and then onto the other, leading to alternating broadcasts being decoded as time goes on.

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    \$\begingroup\$ Why do you assume they have different phases? \$\endgroup\$ – Szymon Bęczkowski Sep 5 '12 at 6:27
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    \$\begingroup\$ It wouldn't be unusual for them to have different phases. And if we take boardbite's comment into account, they could have the same or similar phases, leading to a different phenomenon with the same basic explanation. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 5 '12 at 13:00
  • \$\begingroup\$ Why do you assume every FM detector is a PLL? \$\endgroup\$ – user207421 Jun 3 '15 at 18:54
  • \$\begingroup\$ @EJP: It is true that PLLs are not the only method of detecting FM. They are, however, one of the least expensive and most compact detectors available, leading to their popular use. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 3 '15 at 19:04
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FM radio signals are not Additive. If you add two FM signals together, you do not get one "mixed" FM signal. You just get noise.

Fortunately, FM receivers are good at picking an FM signal out of noise, so your radio is able to separate one - or the other - of the two signals out of the noise. It does this by locking onto the phase of one carrier signal, which allows it to treat the out-of-phase signal as amplitude noise, which it always rejects.

The problem is that, as the two signals drift in and out of phase (something that happens because of the music, even if the two transmitters are perfectly phase locked, which they aren't), the locked signal gets captured by one - or the other - of the two transmitters. Which signal gets followed depends not on the amplitude (unless the amplitude difference is big enough), but on (1) the music (which is doing transient drifts to the carrier phase) and (2) the transmitter phase drift, and (3) the receiver (which will have a drift preference at any moment).

It is possible to make FM receivers which are not phase-locked. You can, for example, convert the frequency into amplitude before doing conversion. In that case, your mixed FM signal wouldn't work at all, because the noise wouldn't have a recoverable carrier frequency.

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