Why should we turn off mobile phones when we travel in an airplane, Part 1

This video is funny. But some of the challenges from the passengers are quite true.

• "90-Million -dollar aircraft can't ignore the signals from my 40-dollar iPod Shuffle?"
• "Can I hold this plane hostage with my (Nintendo) 3DS?"
• "How come the plane is not interfering with my phone?"
• "Why don't other phones interfere my phone?"
• "I just always leave my phone on and nothing happened?"

I think all the fears of "interferences to navigation" was because of some sound like this one.

I am curious and I want to understand why. Let me explain why the buzz sound in this post.


Only GSM is making the buzz noise

I have noticed that after I upgraded my phone to 3G, the disturbing buzz sound was almost gone, except for some finger-countable occasions. On the other hand, when I was using GSM phones, every time when a call is coming in the disturbing buzz sound will surely take place.

I know for 3G phones, it will fall back its radio operations to GSM when 3G signal quality is not good.

Combining all my observations, I think the buzz sound must be related to GSM.


GSM's TDMA is the problem

After some research on Google, I found this post.

Oona Räisänen, The GSM buzz

Up to 8 phones can fit on a single frequency. They take turns during what's called a TDMA frame. One phone may only transmit once every frame, in its own allocated time slot. One frame lasts 0.004615 seconds, a time slot thus being one eighth of this. .....

It looks like voltage is being switched on for a short moment and then cut off for a longer period. You may know from high-school physics that a radio transmitter produces an oscillation in the electromagnetic field, which in turn creates a current (and voltage) in a conductor. So the voltage spikes are probably the periods during which the phone is transmitting. .....

In other words, GSM generate interfering pulse every 0.004615 seconds. That's the source of buzz noises.

I also could not understand the reasoning immediately reading her post. I started to think of similar examples.


Analogy: Tones made by rotating fan

I had a fan set to cool down my house. I remember it had 3 rotating speeds from low, middle, to high. The higher the speed is the stronger the wind blown out.

I also observed when the fan is rotating at different speeds, the "sound of air" is also different, just like tones of pianos. The higher the rotation speed, the higher pitch the sound was. This video is similar to my memory.

Why the fan is making different pitches of sound? I believe it is because fan blades are flapping the air in different constant rates. When the frequency of flapping is high enough, the tiny flapping sound itself generates a new audible tone.

This is just an estimation. Suppose one of the fan rotating speed is 4000 RPM (Rotation per Minute). The flapping sound frequency is running at  4000/60 ~= 66.7 Hertz. If an audio sound is running 66.7 Hertz, how does it sound like?

I found this web page to generate 66.7 Hertz audio sound samples. I experimented a while. When the frequency was set to 66.7 Hz, and the wave form was set to "Triangle", the sound was quite similar to the fan blade sound.

http://onlinetonegenerator.com/

Therefore, the interference itself is not the source of audible sound. The regular frequency of making interferences was the real cause.

Tone of 216.7 Hz

Let's go back to original question. In her post, the GSM pulse of transmission is happening every 0.004615 seconds. That is, 1 Second/0.004615 seconds ~= 216.7 Hz. Again, using the same sound generator with waveform of "Sawtooth", the sound was really close to that of GSM buzz noise.

Plum flowers in Nantou County, Taiwan.
This photo was taken near this place.

Conclusion

Interference is not making the buzz noise. Frequency of interference is making the buzz noise. Because GSM is Time-Division Multiple Acces (TDMA), it only send out radio in very short pulse, and the frequency of tiny short interference sound was high enough to be audible.

Wiki: TDMA

A disadvantage of TDMA systems is that they create interference at a frequency which is directly connected to the time slot length. This is the buzz which can sometimes be heard if a TDMA phone is left next

One more thing...

Then, why 3G phones (or even 4G LTE) are not making such noise most of the time? This is because 3G radio is not Time-Division technology. It is CDMA-based radio. There are no constant and regular pulses in CDMA.

3G phones only make buzz noises when it falls back to GSM radio. This would relate to my coming post: Why should we turn off mobile phones when we travel in an airplane, Part 2.

To be continued.