Sounds weird, but it is possible: you can obtain an infrared signal starting from an audio signal, and viceversa: http://jumpjack.wordpress.com/2008/05/20/worlds-cheapest-remote-control-replicator-just-1/ But I miss something: the old Griffin patent states that to produce a 38000 Hz signal using a 20000Hz audio output, you need to connect two IR leds to two audio channels, in such a way their frequencies sum up. Ok, it works; but I want to understand if same task can be accomplished with one single audio channel rather than two. Somebody says it is impossibile; but I did this study: Excel spreadsheet Image (BIG!) It looks like it should work. Actually, it DOES work, if applied to the PC; but it does not if applied to my phone: in that case, only ONE led emits IR light, and I can't understand why. Besides, I have a phone which is said to have "symetrical output", which is quite different from standard left/right channels output: Nokia 6680 pinout I'd like to include in my Excel sheet explanation about this phone, but I can't: any help? Which waveform results at L+, L-, R+ and R- pin upon playing a 38000 Hz sinusoidal signal?