[LAU] 'morse code' picked up by the speakers

Jacob jacob01 at gmx.net
Wed Jun 14 08:56:19 UTC 2017


On 06/13/2017 07:04 PM, jonetsu wrote:
> Yes, that's the case.  Just to add to this a tiny bit, cell phone
> waves are way up there in the frequencies, so that cannot make such a
> low pulsating interference. Not sure, but it seems that the cell phone
> waves are modulated in the (very) low frequency range and that
> modulation is what's picked up by the (magnet of the) speakers. That
> modulation creates another frequency.

I'll explain some related GSM basics below, but keep in mind that this
is a quite simplified description.

GSM RF transmission is basically done in bursts with a length of ~ 0.5
ms each. These are organised in frames of 8 timeslots. If the phone
needs to send data (either audio, data, acknowledgements, control
messages), it requests a channel from the network which allocates an
uplink timeslot for the phone. Since each burst may only carry a few
bits (normal burst: 114 data bits brutto minus forward error
correction), you'll need some of them which gives you a sequence of
bursts at a rate of ~ 250Hz (-> audio base frequency). Initially, when
the phone tries to set up a connection it sends a sequence of so-called
RACH bursts on different timeslots with a higher RF power level
(involving some allocation randomness to have a chance to get a free
slot even if another phone tries to do it at the same time). So it
sounds differently at the start of the connection. The transmission
power of your phone can be reduced by the base station later on, so
there can be an audible power reduction step depending on the distance
to the base station. Things get more complicated with GRPS/EDGE (which
can also be used for SMS transmission) because the slot allocation for
packet data can be more dynamic.

If you combine this with a non-linear receiver you just get AM
demodulation of the burst sequence sent by your phone giving you exactly
this kind of sounds.


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