Abstract: A monitor/hands-free intercom system includes a child unit and a parent unit which are each transceivers. The parent unit operates primarily as a receiver, unless and until a talk switch is actuated on the parent unit, enabling the parent unit to transition to a transmitter. The child unit acts primarily as a transmitter unless and until it detects a carrier signal at the appropriate frequency during one of its short sniff periods. If a carrier signal is detected, the child unit extends the sniff period to look for a non-audible hidden tone encoded into the transmission from the parent unit. If the hidden tone is detected, the child unit transitions to a receiver mode and is able to play back the sound waves detected by the parent unit so that the parent can reassure or provide instructions to the child.

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.

Abstract: A wireless audio monitoring system includes a reassurance tone generator for providing the user with assurance that a receiving unit is within range of the monitoring unit's transmitter and that both are operating properly. The reassurance signal is a brief audible tone transmitted at regular intervals by the monitoring unit to the receiving unit. The reassurance tone is either injected as sound into the monitoring unit's microphone, as an electronic signal into the microphone's amplifier or directly into the transmitter. A typical reassurance tone is a quarter-second long audible tone that occurs every ten seconds. At the receiving unit, the user has the option of either hearing or not hearing the reassurance tone by either eliminating the tone itself or by muting the loudspeaker during the time the reassurance tone is being received. Alternatively, the reassurance tone is turned on or off at the monitoring unit by means of a radio control signal sent from the receiving unit.