You probably heard this strange robotic noise from your speakers when a cellphone was nearby and about to receive a call or an SMS. But do you know why this audible noise interference happens and why the sequences of buzz is always exactly the same?
For those who don’t see what we are talking about here, this is the sound:
A little bit of history
When the GSM standard was defined, possible interferences with audio speakers were not checked and unfortunately, nobody noticed this issue until it was too late to stop. This was around 1989 when someone working at the Office of Telecomunications in the UK (Oftel) and wearing a hearing aid, noticed this noise for the first time while manipulating a cordless phone.
The interesting part of the story that you can read here is that when the issue was escalated to the Ministry of Health in the UK (as this noise was impacting most of hearing aid devices, and in particular the ones designed in Europe), nobody really seemed to care, which was probably a good thing for the GSM standard as production of phone chipsets and base station equipment was already ongoing.
Instead, the hearing aid manufacturers had to review their design to strenghten the devices agains GSM interferences.
The technical issue
As a reminder, GSM uses a combination of FDMA and TDMA to establish communications between the base stations (BTS) and the mobile phones (MS). This audible interference issue comes from the TDMA (time division multiple access) part. The visual representation of this noise is shown below:
Visual illustration of GSM noise interference with audio speakers
GSM has been deployed on 900 MHz and 1800 MHz in Europe (850 MHz and 1900 MHz in America). Those frequencies are way out of the audible frequency spectrum band which typically spans between 20 Hz and 20 kHz. However, this is not the case of the TDMA multiplexing technique which is used to allow alternatively transmitters and receivers (mobiles phones and the base station) to exchange bursts of messages in good synchronization.
In GSM, a phone will transmit burst of information during time slots of 15/26 ms = 0.577 ms. One TDMA frame of 4.615ms is composed of 8 burst periods allocated to 8 different phones, and one complete frame sequence, which lasts 120 ms, is the sequence of 26 TDMA frames, including 25 frames during which phones are transmitting, and the 26th which is unused.
The resulting interference noise looks like below:
Time domain form and spectrum form of the GSM interference
In the illustration above you can see on the right side the three bigger bumps that represent the 0.577 ms bursts (1,733 Hz), and the 8 peaks on each big bump which represent the duration of 4.615 ms of one TDMA frame (217 Hz).
The illustration of the noise with the scale provided by this website shows clearly the link between the noise pattern and the duration of the TDMA frame and its bursts: you can see the 4.615ms frame duration and one 0.577ms bursts during which the cellphone is transmitting. During the 7 others the phone remains silent as other slots are reserved for other phones:
Zoom on the GSM audible noise
Problem: those frequencies are in the audible spectrum band, and our GSM mobile phones are transmitting at a high power level when they are trying to establish a communication with the base station (up to 2 Watt).
During this event, both the DC power variation of the circuit which is working to generate the signal, or the power transmitted into the antenna RF can be coupled with the circuits of your favorite audio speaker and generate this uncomfortable noise.
Once the call is connected, the noise usually fades out as the mobile phone negotiates a lower transmission power with the base station to save his battery life.
And now what?
The GSM standard has been used for decades as the standard for 2G (second generation networks). Networks are being upgraded today to 3G (UMTS/HSPA) and 4G (LTE) which are not using TDMA as a muliple access technique. Instead, 3G and 4G use Code Division Multiple Access and Orthogonal Frequency Multiple Access, respectively.
So the annoying buzz you kept hearing when leaving your cellphone near your speakers should become a thing of the past very soon. If you still hear it, sorry my friend, that means you are latched on 2G!