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Laptop 3,5mm headset port characteristics

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David_

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Hello.

This question should be solved by me looking at the signals through my oscilloscope but I don't have access to do so for a while, I want to experiment with my laptops soundcard using matlab and a arduino Due.
But I have no idea of what voltage that will come from the 3,5mm plugg, all I have is a multimeter and when trying to output a single sinewave and measure its amplitude the multimeter said it was less then when no signal was being sent.

Does you know what I should expect?
I want to read the laptop output with my Due internal ADC and I am about to build a op-amp level shift circuit using the µC VCC/2 to center the signal around. But I don't know if I have to attenuate the signal or not, its a 3,3V µC.
 

The absolute maximum amplitude is 1V, so calculate your circuit for 2Vpp but don't expect to see this magnitude when playing music in "normal" modus operandi. I suspect you'll have to max all your software audio sliders, and even then there is probably a built in software limit.
 
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    David_

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I have a laptop with 2 headphone ports. One is louder than the other.

I have not examined the output with a scope, but it must be AC with no DC component. Our headphones would not appreciate a DC component.

Computers nowadays can detect where something is plugged in (such as your headphone output). I imagine it detects a voltage drop. That may be the reason your meter gives you some reading when nothing is plugged in.

I believe you need to attach a resistor to the output, to make the computer think headphones are plugged in. Then it will send AC audio to that output. I think a reasonable value is between 10 and 100 ohms.
 

I will try and attach a resistor to the cable from the output to see, I have not jet built the level shifter circuit but I will do both as soon as I have a moment.

I need to get me one of these 3,5mm plugs that has 4 sections to be able to use the microphone input, when reading the specs for soundcards nowadays it sounds as it should be a quite nice measuring device provided some external analog front-end.
 

Most multimeters are made to measure voltages of only 50Hz and 60Hz from the mains accurately. At higher audio frequencies the measurement is low or zero.
A sound card should measure and play up to 20kHz accurately.
 

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