Abstract: A signaling system for FM transmission systems is disclosed that utilizes one or more so-called quadrature subcarriers for transmitting at least one data signal, including signaling tones, digital data, program material, or the like. The signal is modulated on outer edges of the quadrature channel so as to minimize crosstalk interference with the conventional FM stereo difference channel subcarrier and to place the signals in regions of low hearing acuity. Use of a quadrature subchannel also enables a signal to be modulated onto the quadrature subcarrier to provide the capability of modulating a noise compressed difference signal in a manner so as to minimize degrading crosstalk with the adjacent difference channel subcarrier. A receiving apparatus is disclosed for decoding the quadrature subcarrier data independently of standard FM stereo demodulation.

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.