Abstract: A wireless speaker system that transmits and receives both analog and digital audio signals simultaneously over a single channel at frequencies above 900 MHz. The present invention advantageously uses continuously variable slope delta (CVSD) modulation to perform analog-to-digital (A/D) and digital-to-analog (D/A) conversion of audio signals in the transmitter and receiver, respectively, using complementary filters and CVSD modulation circuits in both the transmitter and receiver. Analog signals are modulated in the transmitter with a high-frequency carrier and band-pass filtered before being combined with the digitized audio signal to modulate a high-frequency carrier, i.e. over 900 MHz, which is transmitted to the receiver. Similarly, the receiver band-pass filters and demodulates the received signal to recover the analog signal components.

Abstract: A wireless child monitoring and location device consisting of a device pair having a guardian unit and a child unit, both of which are operable in the 900 MHz frequency band and in the presence of other wireless devices transmitting and receiving within the same frequency band. The guardian unit is able to detect when a child strays beyond a preset distance or when the child is simply out of sight, e.g. around a corner but within the preset distance. The child and guardian units execute a "handshake" sequence during power-up to select a unique digital operating address or channel and establish a time marker used to synchronize transmission between the units. This allows each child/guardian pair to operate in the presence of similarly configured devices without the undesirable possibility of interference among the devices. The child and guardian units are capable of electronically reconnecting if communication between the units is interrupted.

Abstract: A communication device for monitoring audio signals transmitted st high frequencies from a remote location. The device includes a transmitter unit and a receiver unit containing a transmitter circuit and a receiver circuit, respectively. The transmitter and receiver circuits are designed for DC operation at voltages substantially equal to the DC voltages directly applied to the transmitter and receiver units so that the use of step-up converters can be avoided, thereby increasing the duration of time for DC power operation.