3 ms·

Why did that stop happening? I remember back in the early 2000s you might even come across a live concert where an incoming text message got caught in the PA an

Why did that stop happening? I remember back in the early 2000s you might even come across a live concert where an incoming text message got caught in the PA and broadcast to the entire audience.

davnicwil 4y ago
Wow. I've just for the first time ever realised that you might be able to derive the content of the SMS from the specific pattern of interference you would hear on speakers. Like, it seems plausible that is the case. Is that what you mean here by broadcast - or just the fact there was a message was broadcast?
- chedabob 4y ago
  I thought communication between the tower and the device was encrypted? That's not to say it's not using some archaic weak encryption scheme left in for backwards compatability though.
  - pas 4y ago
    yes and yes indeed https://en.m.wikipedia.org/wiki/A5/1 https://en.m.wikipedia.org/wiki/A5/1
- ajsnigrutin 4y ago
  Not really, speakers just reacted to the bursts of power, because the transmission frequency is way too high for us to hear. GSM uses time division multiplex, which means it shares the 8-timeslot channel with 6 more devices (7 devices + 1 signalling), meaning your phone transmits for 577us, then waits quietly for all the 7 timeslots that other devices use (so, for 4.039 milliseconds - 7x577us), and again transmits for 577us, and again waits. Bursts of power every ~4.6milliseconds make a noise that we can actually hear at a bit over 200Hz.
BluSyn 4y ago
Good question. Best guess: Better EMI shielding on newer products / different cell frequencies on newer phones. Perhaps combination of both?
rkangel 4y ago
Because we moved from GSM and its original set of frequencies to 3G and (usually) a new set of frequency bands. The noise was caused by a beat frequency between the ring channel and the call/data channel. Changing either the coding scheme (so you don't get anything like a tone as the beat) or the channel separation (so the beat isn't in audio range) causes it to go away.
- ajsnigrutin 4y ago
  Nah, the frequencies are stil around there, they're all too high to hear, but GSM used TDM - time division multiplex. To simplify, up to 7 users (+1 timeslot for signalling) share the same channel/frequency, but each of them is allowed to transmit only 1/8 of the time (even less, due to guard intervals). This basically means, that your phone transmitted for a bit over 0.5 milliseconds (577us), then waited quietly for 7 more intervals like that for other devices to transmit, then your phone transmitted again, and the affected speakers "buzzed" because of these 577ms bursts of power every 4.616 milliseconds (this causes a buzz in the frequency range we can actually hear).
washadjeffmad 4y ago
It still happens if you lay your phone directly on audio cables with cell data on. You can also see interference happening on high datarate cables or if placed over certain components in a laptop.

Why did that stop happening? I remember back in the early 2000s you might even come across a live concert where an incoming text message got caught in the PA and broadcast to the entire audience.

davnicwil 4y ago

Wow. I've just for the first time ever realised that you might be able to derive the content of the SMS from the specific pattern of interference you would hear on speakers. Like, it seems plausible that is the case. Is that what you mean here by broadcast - or just the fact there was a message was broadcast?

chedabob 4y ago
I thought communication between the tower and the device was encrypted? That's not to say it's not using some archaic weak encryption scheme left in for backwards compatability though.
- pas 4y ago
  yes and yes indeed https://en.m.wikipedia.org/wiki/A5/1 https://en.m.wikipedia.org/wiki/A5/1
ajsnigrutin 4y ago
Not really, speakers just reacted to the bursts of power, because the transmission frequency is way too high for us to hear. GSM uses time division multiplex, which means it shares the 8-timeslot channel with 6 more devices (7 devices + 1 signalling), meaning your phone transmits for 577us, then waits quietly for all the 7 timeslots that other devices use (so, for 4.039 milliseconds - 7x577us), and again transmits for 577us, and again waits. Bursts of power every ~4.6milliseconds make a noise that we can actually hear at a bit over 200Hz.

BluSyn 4y ago

Good question. Best guess: Better EMI shielding on newer products / different cell frequencies on newer phones. Perhaps combination of both?

rkangel 4y ago

Because we moved from GSM and its original set of frequencies to 3G and (usually) a new set of frequency bands. The noise was caused by a beat frequency between the ring channel and the call/data channel. Changing either the coding scheme (so you don't get anything like a tone as the beat) or the channel separation (so the beat isn't in audio range) causes it to go away.

ajsnigrutin 4y ago
Nah, the frequencies are stil around there, they're all too high to hear, but GSM used TDM - time division multiplex. To simplify, up to 7 users (+1 timeslot for signalling) share the same channel/frequency, but each of them is allowed to transmit only 1/8 of the time (even less, due to guard intervals). This basically means, that your phone transmitted for a bit over 0.5 milliseconds (577us), then waited quietly for 7 more intervals like that for other devices to transmit, then your phone transmitted again, and the affected speakers "buzzed" because of these 577ms bursts of power every 4.616 milliseconds (this causes a buzz in the frequency range we can actually hear).

washadjeffmad 4y ago

It still happens if you lay your phone directly on audio cables with cell data on. You can also see interference happening on high datarate cables or if placed over certain components in a laptop.