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Before the smartphones (or maybe before the some new tech) when a cell phone rings, the speakers around it made noises. Hovewer, nowadays they do not. Why? What changed?

WhoCares
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2 Answers2

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You need to understand a bit how cell phone technology works.

  • When your friend next to you at the bus stop wants to make a call at the exact same time as you, chances are you are going to share the same frequency channel. Two transmitters on the same frequency interfere with each other, so how is this possible?

Well there are different methods to overcome this problem. GSM, that's the mobile technology that causes the clicking sound you're talking about, uses a technique called TDMA, which stand for Time Division Multiple Access. That's a complex term for a very simple concept. It basically means that your cellphone only transmits for a very short time, goes in idle mode, and comes to life again after a very short period. The transmitter in your friend's cellphone transmits when yours is idle and visa versa. This all happens so fast that user won't notice these interruptions. GSM has 8 of these time slots available per frequency channel.

How does your cellphone know when to transmit? Well the cell tower knows who's connected and tells every cellphone which time slot to use. Bit like airplanes coming in for landing.

Unfortunately this technology has a side effect. The duration of such slot is about 4.4 milliseconds which which works out to be 217 Hz. And that's audible. So if you're in the proximity of audio equipment that's sensitive enough to pick it up, a car stereo for instance, you can hear these time-slots.

  • So why doesn't your smart phone cause this problem? Well these devices use a different system called CDMA, and that stands for Code Division Multiple Access. There are no time slots in this system, but instead the data from individual cellphones is scrambled with a unique code which allows a number of phones to transmit simultaneously on the same frequency channel. The decoder in the cell tower knows your code and descrambles the entire signal, eliminating all data from other cellphones in the process.
RockPaperLz- Mask it or Casket
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Ambiorix
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The classic audio breakthrough tones produced by some cellphones are generated by a 'bug' in the GSM radio system.

They still occur if GSM is used.
They do not occur for any other system.

Russell McMahon
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