A method of encoding a plurality of data signals is disclosed. The method includes selecting a set of M ultrasonic frequencies, wherein each of the M ultrasonic frequencies differs from an adjacent frequency by at least a first frequency spacing, and wherein M is a positive integer. An encoder receives the plurality of data signals. Each of the plurality of data signals is encoded by a respective set of Q of the M ultrasonic frequencies, wherein Q is a positive integer less than M. A minimum frequency separation between any pair of the Q ultrasonic frequencies of any respective set is greater than the first frequency spacing.