| CPC H04B 11/00 (2013.01) [H04L 5/0048 (2013.01); H04L 27/2613 (2013.01); H04L 27/26134 (2021.01); H04L 27/2675 (2013.01); H04R 1/22 (2013.01)] | 19 Claims | 

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               1. An apparatus comprising a device configured to: 
            generate a first signal by modulating a transmitter carrier frequency using a sequence of signals; 
                apply a filter to the first signal to limit an effective bandwidth of a second signal including the first signal; 
                transmit the second signal, wherein the second signal is an acoustic signal, and wherein the acoustic signal is in an acoustic band that comprises frequencies beyond an audible range of human hearing. 
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