Good morning guys. I just posted this over in the Shanling M3s thread. But I thought I would post it hear for y'all as well. This is about the clicking and static sound you get if your cell phone is too close to your DAP/radio/speaker/whatever.
I know we talked about it briefly before, but since I have always wanted to do a quick write-up about it, I thought I would share it here for my friends.
Discontinuous Transmit
I am a Radio Frequency Engineer since 1994, and have a BS in Electrical Engineering. The static or clicking sound that is heard on DAPs, radios, speakers, etc. can be attributed to a phenomena called Discontinuous Transmit. Mobile phones will turn their transmitters on and off during the sending of data to and from the tower. It will usually have a repeating pattern like this. Transmit – Receive – idle – repeat. That is why it is called discontinuous transmit. Because the phone transmitter turns on and off very quickly; it doesn’t transmit continuously.
Modulation schemes that are time based like Time Division Multiplexing (TDM) or Quadrature Amplitude Modulation (QAM 64, QAM 128) the phone will send short bursts of Radio Frequency energy in an on/off format for digital ones and zeros. Those bursts of discontinuous transmit can cause a third order harmonic that is in the audible range. That third order harmonic has such power and amplitude that the magnetic field induces the frequency into the electronics if they are close enough to the source. That is the reason that one can hear clicking and static-like artifacts in your music while being close to a phone.
If the phone is just sitting idle, it still needs to communicate to the towers periodically to update location information and other hand-shaking information. If the phone is in an active data session or phone call, the noise should be more consistent.
To reduce the effects of discontinuous transmit, you can separate your mobile phone from your DAP. Simply moving it to different pockets if you are carrying them together. Or move them 6 to 8 inches apart should be enough if sitting on a desk.
Regarding bluetooth and wifi: I am positive that this is an issue with cellular phones, but I'm not positive about the effects of wi-fi and bluetooth. The harmonics that are generated are a product of all the frequencies involved and the frequency of the on-off transmissions. To figure out if it is an issue, one has to take all of the transmitting frequencies, receive frequencies, on-off transmission frequency and complete Fourier transforms on them. Then identify the 3rd order harmonic products. If those 3rd order harmonic products are in the audible range, then it can be an issue.
Since it has been 20 years or more since I've done Fourier transforms, I will leave this evaluation to some younger people who are closer to their college math than I am.