Various types of warning signals, or other audible tone generators are known; some of these tone generators provide a warning tone signal, in which the audible intensity gradually increases. Warning tone generators of this type are particularly important for incorporation in automobile, or other mobile receivers in which ambient noise level may be variable, and in which the operator's attention can be distracted by numerous external influences, for example attention to traffic and the like; nevertheless, the warning tone should not only be heard, but considered by the operator. Changing the intensity of the warning tone attracts attention.
Various automobile receivers, and other mobile receivers are equipped with arrangements to receive special broadcasts, such as emergency warnings or the like. Transmitters which are equipped to provide such special broadcasts radiate a radio station, or region recognition signal, hereinafter RR signal, which indicates that the particular station is equipped to provide the special announcement. As a vehicle travels along a highway away from the particular transmitter to which is tuned and which radiates the RR signal, the operator should be warned that he is leaving the region of a transmitter which provides these special announcements. It is, therefore, desirable to provide an audible indication to the operator of the vehicle that he is leaving the reception range of a transmitter which furnishes the special announcements, so that the operator may return the receiver, or place the receiver into a search mode to automatically select another transmitter, for example geographically closer to the vehicle as it is then positioned, and which will also radiate the appropriate announcements.
In order to provide an audible indication to the operator of the warning signal, it has been proposed to use special warning tone oscillators, warning tone or spiral generators, and the like, coupled to control apparatus to vary the audio intensity. All such apparatus requires substantial circuitry.