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Electronics => Beginners => Topic started by: keystroke on December 15, 2012, 07:14:50 am

Title: "Side channel" information on laptop audio jack?
Post by: keystroke on December 15, 2012, 07:14:50 am

I'm playing around with an oscilloscope and my laptop's headphone jack. I get the tip/ring/ground and measure between them. I also measure between them and an external reference ground. I notice I can see when the battery is charging. That is, if the battery is not in the laptop but the AC is in, then I insert the battery, it runs through a few seconds of a test, and then if the battery is not full, it goes to a certain waveform – if it is full, it goes to another "quieter" waveform. This is best done when the laptop is turned off otherwise the waveform is quite noisy.

I'd like to be able to understand more about what is going on. Might there be any other data present in the signal on the headphone jack if I am not putting out an audio waveform? ie. by virtue of the circuit passing through some of the data processing elements of the motherboard, when the computer is powered on? There is a lot of noise in the signal but maybe it is possible to clean it up some.

I thought this was interesting as it is a similar idea: http://cs.tau.ac.il/~tromer/acoustic/ (http://cs.tau.ac.il/~tromer/acoustic/) (Here the noise comes from a bank of 1500µF capacitors).

Thanks everyone!

Title: Re: "Side channel" information on laptop audio jack?
Post by: Rerouter on December 15, 2012, 07:51:11 am

its either radiated noise off the switchmode charging circuit or electrical noise on the supplies and final signals of your laptops amp / dac, its not too uncommon, just depends on what its amplitude is compared to say a quiet piece of music, if its 1/10th or less, you probably wont consciously notice it, (expecially if a constant tone)

Title: Re: "Side channel" information on laptop audio jack?
Post by: T4P on December 15, 2012, 08:36:04 am

Is your laptop PSU isolated? Because if it is then PSU noise will couple into the laptop
OR if it's non-isolated but it doesn't have a third-prong and usually that third prong is used to couple some noise onto the earth pin
Or it could be that your PSU is non-isolated has a third prong but there's a ground loop usually with UK plugs i defeat ground loops by reversing the plug ( if you could, some PSUs might blow up)