Whatever happened to those weird noises affecting nearby speakers when we received a call on a cell phone back in the days, as illustrated in the link?



I was appreciating the nice little attention to that detail in gta4, but it got me wondering how come it’s gone nowadays.

In: Engineering

8 Answers

Anonymous 0 Comments

This is pretty tough to ELI5, but I’mma take a shot:

Back when cell phones first went digital, there were two competing “languages” for the cell phones and towers to use. CDMA and GSM. When wireless things talk to each other digitally, usually only one can talk at a time, otherwise the other stuff gets confused as it can be hard to tell what parts of what it is hearing is from one thing versus another. CDMA uses a system where the wireless radios in the devices are pretty much always on. GSM used a system called TDMA which meant each device gets so much time to send a signal, then other devices get turns.

In GSM when it isn’t talking it turns off the transmitter on its radio. It then turns it back on when it talks. Have you ever played with a really old amplifier, like one from the 70’s or so? When you turn them on they can make a pop noise as the power is applied to the speakers. The GSM radio is making a pop noise every time it turns on.

The actual digital signal of the cell phone is so high pitched, that even if your speakers could play it, you wouldn’t hear it. But those pops are timed just right so that they add together to make some awful sounds to our ears. (Non-ELI5, the frequency band of the radio switching is in audible frequencies).

While those pops can be picked up in just about any electronics, on their own they are not strong enough to move a speaker, at least not enough to be heard by humans. However, when they get picked up by the electronics that are then amplified, you hear them over the speakers. This is even more likely to happen if there are microphones around, as microphones put out such a low level of power they get amplified twice. Once to a level that regular electronics can work with them (called Line level, usually done by a pre-amp), then again to be enough to move speakers.

Why don’t we hear it anymore? When GSM moved to 3G, it went to a “language” called UTMS. UTMS uses a new version of CDMA, which wasn’t compatible with the old version of CDMA, called WCDMA. CDMA providers still used regular CDMA for 3G. When all the providers started going to LTE, it was decided to use WCDMA for LTE.

There are other signals and things on cell phones that can cause noises in audio systems, but the very distinctive and memorable one was the GSM noise.

Note below:
US GSM providers: AT&T, T-Mobile (and all the companies that got bought up by them so that we really only have 4 providers today)
US CDMA providers: Verizon, Sprint (and all the companies that got bought up by them so that we really only have 4 providers today)

So if you were in an area with no AT&T or T-Mobile coverage you might not have ran across this.

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