What's with GSM cell phones and ticking sound into radios, speakers etc?

[NeoPTLD]

I have a Verizon phone and I can't get signal in locations I see other people walking around and yakking away and found out they're using T-Mobile so I ordered T-Mobile prepaid to try out.

I noticed when I call my T-Mobile phone, a GSM handset I hear morse code like tick tick tick sound from my computer speaker, radio or even my landline phone if it's nearby.

I've seen this happen before with other people's phone and it wasn't until now I realized it happens with my GSM phone, but not with my CDMA phone.

GSM and CDMA both operate in 850/800 or 1900 MHz range. Why does GSM phone add audible interference into speakers and radios while CDMA doesn't?

 

[herm0016]

my cdma phone does it to crappy computer speakers, but does not effect my larger regular speakers. they also do it to our wireless communication (ClearCom) system in the theater if it is near the belt pack.

 

[djhuber82]

GSM uses TDM: time domain modulation. Basically, A GSM phone is allocated many small time slices when it is allowed to transmit. To save battery life, the phone turns off the power amp when it's not transmitting. The switching transients from this are what you hear through your computer speakers. I think the sounds also has something to do with the particular GSM packet-rate when the phone is getting status information from the base station. You'll notice that the sound always happens right before your phone rings, but not when you're in the middle of a call. CDMA is spread-spectrum, so the power amp stays transmitting continuously and you don't see this phenomenom.

 

[RadiclDreamer]

Actually TDM stands for Time-Division Multiplexing and uses time domain slots to transmit

http://en.wikipedia.org/wiki/Time-division_multiplexing


As for the reason the noise is made. It is a form of RFI, and could be eliminated by use of additional shielding and/or bypass capacitors in these audio devices. However, the increased cost of doing so is difficult for a designer to justify.

It is a common occurrence for a nearby GSM handset to induce a "dit, dit di-dit, dit di-dit, dit di-dit" output on speakers When these audio devices are in the near field of the GSM handset, the radio signal is strong enough that the solid state amplifiers in the audio chain function act as a detector. The clicking noise itself represents the power bursts that carry the TDMA signal. These signals have been known to interfere with other electronic devices, such as car stereos and portable audio players.

 

[bpatters69]

Originally posted by: djhuber82
GSM uses TDM: time domain modulation. Basically, A GSM phone is allocated many small time slices when it is allowed to transmit. To save battery life, the phone turns off the power amp when it's not transmitting. The switching transients from this are what you hear through your computer speakers. I think the sounds also has something to do with the particular GSM packet-rate when the phone is getting status information from the base station. You'll notice that the sound always happens right before your phone rings, but not when you're in the middle of a call. CDMA is spread-spectrum, so the power amp stays transmitting continuously and you don't see this phenomenom.

TDM = Time Division Mulitplexing....

Telco guy here.

 

[RadiclDreamer]

Thought i just said that...