Experiments in InfraSonics Notebook

02/09/00: First InfraSonic AFK Communications test:

One of my main interests is experimenting with ultra-narrowband communications.  Early in the process of investigating InfraSonic such experimentation was foremost in my mind. The upper frequency range of InfraSonics (5 - 15Hz) lends itself to the use of ultra-narrowband via the standard PC soundcard.   For example, at 15Hz, even a bandwidth of 0.01Hz is still more than 500ppm - well within the capability of a good quality soundcard.  This applies to the "standard" sampling rates of 11025, 22050 and 44100Hz.

The method used here is AFK where the characters are encoded directly in frequency tones.  For InfraSonic tests I used a nominal frequency of 15Hz and difference between the character tones of 0.02Hz.

The transmitter end is just an old low-quality stereo speaker driven directly by the soundcard speaker output.  The receiver is the DCAL401DN ultra low pressure sensor backed up with 1m x 90mm diameter pipe reference volume.   The distance between the transmitter and receiver was about 10m.

A check using Spectrogram found that the signal was not visible even using the narrowest bandwidth setting.

However, using my ISAFK48 software the signal was easily decoded as shown below.

The success of this method would seem to point to further success at longer distances.   Perhaps using resonant cavities at each end of the circuit would substantially increase the range.  Unfortunately, a few simple calculations reveal that in the case of InfraSonic signals there is a fundamental limitation.   The use of channel spacings of 0.021Hz means that the stability of the received signal must be in the order of 500ppm.  While (as mentioned above) this is not an onerous requirement for the electronic side of the circuit, the nature of the transmission path means there is a lower limit on the narrowest bandwidth that can be used.  Calculating the effect a breeze of just 1 km/hour results in a doppler shift of just over 0.01Hz.  Therefore any breeze above 2km/hour is going to be a problem.  If the breeze was steady over the time of the transmission it could be compensated for - but this is extremely unlikely to happen.

The best success for InfraSonic communications would seem to by the standard QRSS mode using operating bandwidths of about 1Hz.   This will probably be the mode I will use in further experiments in InfraSonic comms.

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