The probable answer is that it is the antenna that picks the noise up. Your car antenna is located further away from the cell phone than your desk top radio. There is also a lot of background noise in the car that makes the "morse code" less noticable.
It is also possible that the energy is getting into the low-frequency part of the desk top amplifier with its plastic or wood housing while the car radio usually is housed in a metal casing that keeps HF energy out.
Gunner, I believe he's talking about his desktop PC speakers.
The worst offenders are those that have amplifiers in each speaker (typical setup), which goes to show you how terrible the shielding on them is. The "morse code" you hear is the cell phone answering the tower's request for a positional/readiness update... those happen from time to time automatically, or when a phone call comes in.
macgybers2000 is correct on the sheilding issue. Some audio op-amp front ends will detect (base-emmiter diode effect) the RF from the cell phone. The TDMA burst will be like an impulse. Sets of the TDMA bursts will be like a tone. Grouping of the bust sets will be like Morse code. i.e. for GSM the TDMA rate is 217 Hz. Bad for hearing aids.
When a cell phone wakes up or does a RACCH )Random Access Control Channel) the power will ramp in burst sets until the base station can hear it.
Your cell phone is busy using up expensive operator (your cell phone company) equipment BW and power even hen you are not using your phone. Your phone may be making 8 phone calls (on control channels to different antenna towers) at a time just to keep your phone ready to go with your payload.
So, keep phones away from the conference phone at meetings and your speakers!
Audio equipment as such is not required to comply with FCC emissions rules since there is no generator of a frequency that exceeds 10KHz. The shielding in audio is more directed at the 60Hz hum.
Whatever is not shielded against emitting EMI is also not shielded against receiving EMI. The cell phone buzz is modulated in the 1.9GHz range, with a wavelength in the inch range that can easily be picked up by wiring or pcb traces in a non-protected circuit.
A non-linearity in the circuit will translate into a demodulation of an RF signal. If the demodulated signal is in the audio range, you will hear it. I remember my old turntable sometimes picking up CB transmissions from illegal radios, probably because of a galvanic effect between cables and connectors.
I was out camping in the sticks one time and brought my wife's cellphone. Never had even used one before. Fired up my shortwave radio that night and kept getting periodic bursts of pulse noises all over the SW bands. Thought my SW went on the blink. Happened to be rummaging through my gear about 20 feet away and found the cellphone which was on and roaming (and quite warm by the way). Not like it was going to get a cell connection out there (remote desert in Arizona). Intuition kicked in and I turned off the phone, the SW reception cleared right up. So I was able to enjoy a night of listening to the propoganda spewing out of Radio Cuba. All I can guess is that it was trying to call out to connect to a cell.
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"It's the questions that drive us"
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Angryman,
The amplifiers on my boat and in my car do pick up that "noise". Alerts me to grab my phone.
analogkid2digitalman,
I was just informed that a cell will continuosly look for a tower to com with. This will shorten batt life. I have had my Nextel disturb the display on my PC's CRT as well as the pop pop pop thru the speakers. I was just in Phoenix a couple of weeks ago. It was 112 in the day. Are you sure the cell being warm was not from the climate?
Scott
In a hundred years, it isn't going to matter anyway.
It's a dry heat (that is until the Monsoon season kicks in-just about this time of the year)!
The heat was definetly being generated by the phone. It was consuming more power than normal 'in network' power looking for a cell to hook up with, I presume.
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"It's the questions that drive us"
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