Importance

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Abstract

An in-band signaling system for injecting at least one low frequency audio tone into an audio stereo signal includes a tone generator circuit and a circuit for generation of first and second component signals for each tone, each component signal being of substantially equal amplitude and opposite phase with respect to the other signal, the tone generator circuit being controlled to apply the tone or tones to the component generating circuit, to thereby provide an indication of the present state of auxiliary circuitry in an FM stereo transmission system. A first summing circuit injects the first component signal into the left channel of the stereo signal, while a second summing circuit injects the opposite phase component signal into the right channel of the stereo signal, such that when the left and right stereo channels are FM multiplexed in a conventional manner, the tone or tones appears only on the difference channel of said FM signal and not on the sum channel. A blend circuit eliminates stereo separation information between the left and right stereo channels below a certain cutoff frequency to enhance the detectability of injected tones of a frequency below the cutoff frequency.

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First Claim