running mic and speaker signals together through UTP

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reinoso

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Hello,

In my new project I need to design a peripheral that contains a microphone and speaker (something like a handsfree device). I'm going to interface it with the main board with stantard UTP cable (network twisted pair 100ohm).

One pair for VCC and GND, one to balanced MIC signal, one to speaker signal and the last to a couple control lines.

My question is, could the two audio signals interfere with each other? Both the mic and the speaker signal are amplified inside the peripheral (the speaker signal is sent from the main board in line level).

Thanks in advance.
 

The easy answer is yes!, the question is how much, I would think cross talk in the order of 40 dB, i.e. 1% of the peak level of one channel will actually be the other channel. I believe that ordinary telephone lines are rated for 28 dB cross talk, ~4% ? Keep all your inputs and outputs balanced with differential inputs and outputs, keep the levels about the same, i.e. use a mic pre-amp to raise the level to line (0 dB) and reduce the bandwidth as much as tolerable (3 kHZ ?).
Frank
** Telephone lines do run for miles, perhaps your cabling is shorter.
 

As Frank states regarding both Audio signals being very well balanced.
You may find that using your control signals un-balanced will induce currents into your signal lines. However a 'cure' for this is to reduce switching rise/fall times such that they are below your lower Audio Bandwidth so as to reduce/prevent audible clicks.
Same applies for your VCC and Ground where changes in Current can also cause problems.

Crosstalk, and thus your system stability, will be determined by the Common Mode Rejection performance of your Balanced Line Receivers and Line Outputs.

hope this assists
Mik
 

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