| 摘要 |
The emission device has a module assigned to record a message in a nonvolatile RAM with signal compression. An IR sensor is provided to detect the presence of a person. A unit measures the distance to the IR source, and send a coded signal to a control unit which processes the signal and, if this distance meets chosen criteria, outputs an authorising signal. Upon reception of this signal, an audio circuitry sends the message through an amplifier and a loudspeaker. The distance measurement unit uses infrared, ultrasonic or microwave signals. The echo reception timing uses encoded wave trains to distinguish between confusing targets. The control unit allows or stop announcements when a person is approaching or receding.
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