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Comment by alexkus | Hacker News Reader

> High-frequency transmission requires that you have elements capable of transmitting externally at very high frequencies and elements capable of receiving at very high frequencies.

You only need to transmit the carrier at the high (inaudible) frequency, you can receive at any sub harmonic frequency as long as the transmission protocol is designed with this in mind.

So a carrier signal broadcast at 20KHz (within the range of the speakers) but inaudible to most humans can be picked up by sampling at 10KHz (within the range of laptop microphones). The protocol just needs to send each bit twice (or any multiple of two) at 20KHz.

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Your sentence "The protocol just needs to send each bit twice (or any multiple of two) at 20KHz." nearly makes no sense. You should learn about modulation, sampling, aliasing, and so over.

If you want to stay ultrasonic you will need to use a narrow bandwidth around 20k, otherwise people with a good hearing and children will be able to hear it. I don't think you'll be able to receive anything useful by using a cheap PC microphone at 10k in those conditions, especially if you use it in the way it is intended to be used (the hardware will filter out high freq to prevent aliasing)... Maybe there is a convoluted way to receive a specially crafted transmission by sampling in a subharmonic when a special antialias filter (or no filter at all in otherwise quiet environment) is used.

I don't see much point in trying to use such a channel to transmit bios malware related stuff anyway. The original badBios article seems to be just a joke.

I think it'd be much easier to use spread-spectrum techniques. With enough coding gain the signal can even be well below the noise floor and still reliably recoverable. (This is the spread-spectrum equivalent to sending a very narrowband signal and receiving it through a very narrowband filter.) Generating a DSSS waveform is extremely computationally cheap; receiving one is less so, but probably still negligible amount of cpu time on a modern processor.

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