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Comment by pjot | Hacker News Reader

The speakers buzzing just before getting a SMS text is the closest I’ve gotten to predicting the future.

Speaking of that... Why did that go away? Less 3g? Moore speakers with better insulation?

It was a peculiarity of the 2G signal. While the frequency of the signal itself was high enough that it did not interfere with speakers, the channel was shared between phones using TDMA, in which the channel is divided in fixed-size slots, and a phone transmits in only one of these slots while remaining silent for the rest of them. It's the envelope of these transmissions (transmit for a while, stay silent for a while, repeat) which caused the interference with the speakers.

According to Wikipedia (if I understood it correctly), the interval between the start of one transmission and the next one is 4.615 ms, or 9.230 ms for half-rate, which corresponds to 217 Hz or 108 Hz (if I did the calculations right), well within audible range.

Newer cellular standards use CDMA or OFDMA instead of TDMA, so they do not have that same low-frequency on-off behavior.

Well, TDD, which means the base station and the phones take turns transmitting on a single frequency, is still possible in 4G and 5G. How often the switch happens is configurable by the operator and in 4G can range from 1ms for phones and 4ms for base station, to 3 ms for phones and 2 ms for base station. So, just the directional switching shouldn't be the full story (and even though I used to work on base station radio software goes beyond my understanding. Too near the digital processing side, I guess :)

Did it go away?

When we are on Teamspeak/Mumble with friends, we can always tell who will be receiving a text in the coming seconds; I assumed the headset wires were acting as an antenna and catching the SMS radio signal.

When I play electric guitar with a phone in my pocket I can still hear interference. Not as satisfying as in the GSM era, now it is just noise as opposed to a recognizable pattern.

TDMA (Time Division Multiple Access) isn't a thing anymore.

It was such a consistent swing pattern, too.

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