# How does a radio distinguish between two independant signals broadcast on the same frequency? **URL:** \ **Category:** Factual Questions\ **Created:** [January 29, 2014, 11:32pm UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083 "2014-01-29T23:32:34Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![millen85](https://avatars.discourse-cdn.com/v4/letter/m/9fc348/32.png) [@millen85](https://boards.straightdope.com/u/millen85)\ **Post date:** [January 29, 2014, 11:32pm UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/1 "2014-01-29T23:32:34Z")
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??? ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [January 29, 2014, 11:55pm UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/2 "2014-01-29T23:55:51Z")
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. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [January 30, 2014, 12:32am UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/3 "2014-01-30T00:32:14Z")
**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.](https://en.wikipedia.org/wiki/Code_division_multiple_access) ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [January 30, 2014, 12:34am UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/4 "2014-01-30T00:34:20Z")
> [@RealityChuck](#): > > It doesn’t. If two signals are on the same frequency, you hear both. Not true for FM signals. The [capture effect](http://en.wikipedia.org/wiki/Capture_effect) is probably what is being described in the OP. AM signals do mix, though. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [January 30, 2014, 1:19am UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/5 "2014-01-30T01:19:40Z")
> [@Derleth](#): > > **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.](https://en.wikipedia.org/wiki/Code_division_multiple_access) That’s interesting. GSM phones (the more common type) also use Time Division Multiple Access. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [January 30, 2014, 1:27am UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/6 "2014-01-30T01:27:05Z")
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. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [January 30, 2014, 3:30am UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/7 "2014-01-30T03:30:58Z")
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. ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [February 1, 2014, 1:51am UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/8 "2014-02-01T01:51:10Z")
> [@GaryM](#): > > 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.](https://en.wikipedia.org/wiki/Tenerife_airport_disaster) ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [February 1, 2014, 2:12am UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/9 "2014-02-01T02:12:05Z")
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. > **[Direct-sequence spread spectrum](https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum)** > > In telecommunications, direct-sequence spread spectrum (DSSS) is a spread-spectrum modulation technique primarily used to reduce overall signal interference. The direct-sequence modulation makes the transmitted signal wider in bandwidth than the information bandwidth. > After the despreading or removal of the direct-sequence modulation in the receiver, the information bandwidth is restored, while the unintentional and intentional interference is substantially reduced. > The first example of thi... ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [February 1, 2014, 6:33pm UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/10 "2014-02-01T18:33:28Z")
> [@Hail\_Ants](#): > > Even so, so-called ‘talk over’ was one of the contributing factors to the [Tenerife disaster.](https://en.wikipedia.org/wiki/Tenerife_airport_disaster) True, but we’ll never know how many would have occurred if we’d been using FM. No way to tell. Military uses FM for tactical radios I believe. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [February 2, 2014, 6:13pm UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/11 "2014-02-02T18:13:06Z")
> [@Dr.Strangelove](#): > > It’s hard to explain the capture effect without math and diagrams, but I’ll try. And you did a very good job! I don’t think you could have been any clearer. ---
**Author:** ![SpyOne](https://avatars.discourse-cdn.com/v4/letter/s/82dd89/32.png) [@SpyOne](https://boards.straightdope.com/u/SpyOne)\ **Post date:** [February 5, 2014, 8:41am UTC](https://boards.straightdope.com/t/how-does-a-radio-distinguish-between-two-independant-signals-broadcast-on-the-same-frequency/680083/12 "2014-02-05T08:41:15Z")
> [@Hail\_Ants](#): > > Even so, so-called ‘talk over’ was one of the contributing factors to the [Tenerife disaster.](https://en.wikipedia.org/wiki/Tenerife_airport_disaster) I believe the “talk over” at Tenarife worked like this: if two radios close to each other in strength broadcast at the same time, the receivers heard a squeal caused by the signals mixing. The main problem with this was that neither broadcaster would be aware they hadn’t been heard, because keying the mic to transmit cuts the audio of receiving to prevent feedback. Going further aside, this is why most US police departments use radios that use two different frequencies: one for officer-to-dispatch, another for dispatch-to-officer, so the officer can always hear dispatch, even while broadcasting, and vice-versa.