# How does a radio distinguish between two independant signals broadcast on the same frequency?

If a radio receiver is within range of two signals broadcast on the same frequency, the receiver normally allows the station with the stronger signal to come through over the weaker signal. My question is, if the two signals are broadcast on the same frequency, how is the receiver able to distinguish between the two independent signals and only allow one station to be heard???

It doesn’t. If two signals are on the same frequency, you hear both. If one signal is stronger than the other, it will drown out the weaker, though you may be able to hear the other when the first is silent for a moment. Broadcast stations are set up so that the signals don’t overlap.

RealityChuck is right for analog radio (AM, FM, and variants thereof). Cell phones, on the other hand, can share a frequency quite happily due to CDMA (Code-Division Multiple Access).

(I say cell phones, because they’re by far the biggest users, but any digital radio system could be designed to do this.)

In CDMA, the digital signal the sender sends out, which flips between 1 and -1 instead of 1 and 0, is added to a special code signal which goes between 1 and -1 much more frequently than the voice or data signal does. When I say ‘add’, I mean just that: If the data and the code are both at 1, a signal of intensity 2 is sent out, -1 and -1 get you a -2 over the air, and 1 and -1 gets you 0.

The receiver then adds all possible code signals to what it picks up, and sends each result to a separate processor. The processors that see something other than just zeros know that someone is talking using that code on that frequency. The real magic, and this requires some math to pull off, is that the receiver also knows the data it’s getting isn’t being corrupted by anyone else using that frequency: If a specific code is added to the raw input, the signals from everyone not using that code are made into a long string of zeros. It might be corrupted by something else, which is why error-correction codes are used. That’s another post entirely.

This is easier to understand with diagrams.

Not true for FM signals. The capture effect is probably what is being described in the OP.
AM signals do mix, though.

That’s interesting. GSM phones (the more common type) also use Time Division Multiple Access.

It’s hard to explain the capture effect without math and diagrams, but I’ll try.

FM radio works by converting the current received frequency into an amplitude. That amplitude represents the audio waveform that you hear.

Normally, on a given channel, there will be just one frequency at a time. So for instance, at 100.5 MHz (the center frequency), the receiver might hear a frequency of 100.505 MHz. That’s an offset of 5 kHz from the center, and for FM radio that means an amplitude of 0.33 (the total range is 15 kHz). Easy enough.

So what happens if there is another signal? Well, it depends on exactly how the demodulator works. But in a simple sense, what it’s trying to do is “best fit” a sine wave to the signal. The frequency of that best fit wave is then used to produce the amplitude. What’s the best fitting wave to a signal that has a strong primary wave and a weaker secondary wave? Well, it’s just the primary wave! The secondary wave is totally excluded because you can only fit one wave at a time, and the stronger one always wins.

If the two signals are close in amplitude, the demodulator may have a hard time choosing sides. Your station may randomly flip back and forth in this case.

None of this applies to AM because the amplitudes come directly from the signal strength, and so they just add together. You’ll hear the weak station, just more faintly.

And that’s why aircraft use AM rather than FM. You might have a weak signal calling an emergency that would be unheard if using FM. But it could be heard along with a stronger signal on AM.

Even so, so-called ‘talk over’ was one of the contributing factors to the Tenerife disaster.

CDMA for digital signally has alreay been mentioned but the most elementary explanation (of which CDMA is a particular application) is direct sequence spread spectrum.