A warning radio receiver is adapted to receive not only a recognition signal for emergency broadcast but also two further recognition signals, namely a warning network transmitter recognition signal and a warning region recognition signal. If, for example, the currently tuned transmitter changes its warning region recognition signal, after the expiration of the timing period of a first timer 16, one or more seeking operations for other transmitters with the predetermined warning region recognition signal are started. If a predetermined number of such seeking operations remain unsuccessful, the reception readiness of the transmitter is demonstrated by switchover to a transmitter associated with a different warning region. If such other transmitters also cannot be found, exceeding another predetermined number of fruitless seeking operations results in the generation of a receiver malfunction indication.
Radio receivers which scan through a predetermined band of the radio spectrum, in a station-seeking mode, must independently detect whether the currently tuned frequency is the central frequency of the transmitting station and whether this transmitter provides a signal whose strength is sufficient, i.e. whose strength exceeds a predetermined threshold value. Once these two conditions are satisfied, the radio generates a "stop" signal which terminates the station-seeking mode, i.e. keeps the tuning circuit set to the frequency which provided the sufficiently strong signal. The present invention provides an unusually simple way to determine whether these two conditions are satisfied, by using a Finite-Impulse-Response digital filter (2) to extract a d.c. component of the demodulated signal (MPX), for comparison (6) with a predetermined maximum value, and by filtering (10) a field strength signal (FST), for comparison (11) with a predetermined minimum value. When both conditions are satisfied, a simple AND-gate (8) generates the stop-signal.
A warning system for alerting a person to an emergency situation which includes a transmitter responsive to a visual or acoustic alerting system for transmitting an alarm signal on an RF carrier and a control signal on a sideband of the carrier. A tuneable receiver for receiving commercial broadcasts in a normal operating mode transfers to an emergency mode upon detection of the control signal and converts the alarm signal to an acoustic signal. In one embodiment, the visual or acoustic alerting system is a siren and light system of an emergency vehicle. In this embodiment, a transceiver is preferably included for dedicated communications among emergency vehicles. The range of both the transmitter and transceiver complement and are substantially the same as the effective range of the visual or acoustic alerting system.
