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Sound to infrared conversion

  1. Jul 2, 2008 #1
    Sounds weird, but it is possible: you can obtain an infrared signal starting from an audio signal, and viceversa:

    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?
  2. jcsd
  3. Jul 2, 2008 #2

    Andy Resnick

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    I think the jumpjack article refers to the fact that the amplitude modulation of an IR carrier signal occurs at audible frequencies. That's different from heterodyne downconversion (or upconversion).
  4. Jul 2, 2008 #3
    no, it occurs at frequency far higher than audible: almost double! (38000 Hz rather than 20000).
    So, usual audio devices cannot reproduce them; but they can reproduce half the frequency (19000 Hz), and combining two devices outputs, you can obtain 38000 Hz frequency.
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