Check out the interesting patent US6470214 on interestingoldpatents.com. It’s quite fascinating, yet much simpler than it appears. You can encode an audio file and transmit it on any frequency, basically just using a a ham radio, although I haven’t tested this myself. If anyone is interested in testing it, I can share the code for encoding the audio. Simply adjust the carrier frequency in Hz and transmit using SSB modulation on any frequency at a power level that permeates the body, such as 15W on the 70cm ham radio band at 1 meter away.
Quote from US6470214:
“Consequently, a message can be recognized as speech when a listener is preadvised that speech has been sent. However, if the listener does not know the content of the message, the audio signal is unintelligible.”
Hint: The demodulator is biological.
import numpy as np
from scipy.signal import hilbert
from scipy.io import wavfile
#%%
sample_rate, audio_signal = wavfile.read('/Users/felix/Downloads/audiowav.wav')
if audio_signal.ndim == 2:
# Convert to mono by averaging the two channels
audio_signal = np.mean(audio_signal, axis=1)
#%%
carrier_frequency = 144430000
analytic_signal = hilbert(audio_signal)
t = np.arange(len(audio_signal)) / sample_rate
ssb_signal = np.real(analytic_signal * np.exp(2j * np.pi * carrier_frequency * t))
ssb_signal_normalized = ssb_signal / np.max(np.abs(ssb_signal))
ssb_signal_pcm = np.int16(ssb_signal_normalized * 32767)
wavfile.write('ssb_output.wav', sample_rate, ssb_signal_pcm)
print("SSB signal has been written to 'ssb_output.wav'")