What is a relatively inaudible audio watermarking technique to overlay a broadband audio signal on top of playing music without analysing the music for low-bitrate source identification? Ideally should be resistant to multi-path, reverb and resonance.
I'm not trying to do any kind of DRM, so I don't care about resampling, time compression or frequency shifting - I just want to figure out in real-time which source is playing without adding too much audible noise to the signal.
At the moment I'm broadcasting a low bitrate ultrasonic signal on top of the music, modulated with GMSK around a carrier frequency of 19500Hz, but the high frequencies attenuate fast and the high frequencies are still audible.
I'm considering broadcasting a super wide-band PN-sequence that sounds like pink noise. I only need to sustain a transfer rate of about 3-4 bytes (24-32 bits) per second. But if I stretch out the symbol length, I'm worried about environmental noise.
I could add a delay if I need to pre-analyse the output buffer before, but I hope that I can introduce a very soft full-spectrum "hiss" or hum that can carry my source identification signal.
I don't need to track the source during periods of silence, so there will always be some music to mask my signal.
Update: I've found some prior audio steganography work, but I don't know what the trade-offs are for real-time modulation (esp. on a low-power embedded device) and if this would at all work without look-ahead on the output buffer.