Wireless sound monitoring apparatus with subaudible squelch control

Abstract

In a wireless sound monitoring system, ambient sounds are monitored by a microphone and transmitted over a phase or frequency modulated transmitter to a receiver. A sound actuated switch responsive to ambient sounds louder than a predetermined level turns the transmitter on and causes a hidden tone to be generated. A delay circuit caused the hidden tone to be terminated before the transmitter is turned off when the sound level decreases. In the receiver, a mute circuit responds to the hidden tone to control the output of a loudspeaker, and thus, the hidden tone prevents a squelch tail from being heard. The monitoring unit also includes a pulse generator to cause the hidden tones to be generated at regular intervals, but with short pulse widths. In the receiver, a mute delay circuit prevents the un-muting of the loudspeaker in response to these short pulses, but as long as the hidden tone pulses are detected within a predetermined period of time, a visual indicator, an LED, will light to show the proper operation of the system. Each hidden tone detected will cause the LED to blink momentarily; however, if the hidden tones are not detected, the LED will be extinguished and a distinctive audio tone will be generated to alert the user.