Abstract: A digital time-division duplex radio communication link is established or re-established between one of a plurality of portable units and one or more base units forming a cordless telephone system, so as to avoid potential interference created by the use of an unsynchronized call-up transmission in the presence of synchronized signals being exchanged by the rest of the system. The call-up transmission is pre-synchronized with the rest of the system so that switchovers between transmission bursts and reception windows occur at the same time as between the call-up signal and the rest of the system. This is achieved by deriving synchronization information from a synchronization signal which may, for example, be an existing signal on the system containing synchronization information (i.e., an existing call on the system) or may be a beacon signal which, in some systems, is transmitted by the fixed units for the purpose of range indication.

Abstract: A problem arises in electronic equipment incorporating a display, such as an LCD display, which is not intrinsically illuminated. The present invention provides a controlled backlight for such displays, and is particularly useful in battery-powered equipment where power consumption must be kept to a minimum. The circuitry comprises a display 1 controlled via a conventional display driver 6 from other equipment (not shown) such as a microprocessor. Whenever the display is changed a signal is sent also to a timer circuit 7 which triggers, and passes a control signal to an electronic switch 4 which in turn passes power from a battery 3 to a lamp 2. The lamp 2 is arranged to illuminate the display 1. After a preset period, the timer circuit 7 sends a further control signal to the switch 4 to switch lamp 2 off.

Abstract: A communication procedure suitable for a cordless telephone system involves time division duplex radio communication between a handset 11 and a base station using alternating bursts of transmission over a single radio channel. Once a radio link has been set up, initial transmissions carry a synchronization logical channel S and a signalling logical channel D multiplexed together, but the link may switch to bursts carrying a communications logical channel B for the speech data and the signalling logical channel D. Burst synchronization is achieved by the asynchronous detection of words in a synchronization channel S. These words have bit patterns reducing the probability of incorrect asynchronous detection of them. If one part ceases to receive handset signals from the other, it transmits a special signal, informing the other part. This enables both parts to detect the failure of a link at substantially the same time, so that their actions to re-establish the link are synchronized.

Abstract: A time division duplex transceiver incorporating a dual conversion superheterodyne receiver, and in which the first receiver local oscillator doubles as the frequency source for the transmitter. The oscillator is a voltage controlled oscillator VCO forming part of a mixer-type phase locked loop. Frequency deviation in the loop is achieved by mixing the VCO output with a selected harmonic of the output of the second receiver local oscillator. The reference oscillator is a crystal controlled oscillator whose output is switched as between transmit mode and receive mode at a switch before being passed to the loop phase detector. The transceiver is able to rapidly and repeatedly alternate between transmit and receive modes and is thus well suited to time division multiplex operation.

Abstract: A method of establishing a digital time-division duplex radio communication link between one of a plurality of portable units and a base unit in a cordless telephone system, transmissions from the base unit are allowed only on a ping-pong basis with transmission bursts from all the active transmitters in the base unit being synchronized with one another. This prevents blocking of receiver sections due to co-sited transmitters transmitting at the same time on a different channel. The difficulty of synchronizing the transmission bursts of the active portable units with their respective transceivers within the base unit is overcome by allowing the portable unit to make an initial continuous call comprising multiple identical sequences of data containing units small enough to be completely received within the limited reception windows of the base unit.

Abstract: An amplifier for use in the I.F. stages of an F.M. radio receiver comprises several identical direct-coupled stages 2,4,6 each comprising common emitter transistors 8 and inductive loads 10.

Abstract: A data processing system of the kind in which there is a transmission of a code format having a pattern that provides a plurality of reversals 10101100 etc. from which it is desirable to be able to differentiate between valid data and invalid data. Invalid data means the absence of data, data corrupted by noise or data alien to the system. The system is characterized by means arranged to provide the times between n zero crossings as given by the expression: ##EQU1## where I is the integer part of x, n is an integer representing the number of zero crossings and t.sub.i is the duration of the ith interval between zero crossings measured on a bit base of unity thereby to permit the use of said number (R) when compared with a predetermined tolerance range to provide an indication of the quality of said transmission.

Abstract: An averaging detector having a first transistor having a first resistor connected across the base and emitter, a first capacitor connected in a signal input line and to the base of a second transistor, the collectors of the first and second transistors being connected to a positive voltage line and the base of the first transistor being connected to an output voltage line, together with the emitter of the second transistor via a second resistor, an inhibit switch comprising a third transistor being connected between the base of the second transistor and a zero voltage line, a third resistor and second capacitor being connected in parallel between the zero voltage line and the output voltage line, the arrangement being such that on application of a voltage greater than the threshold voltage of the first transistor successive positive and negative cycles alternately reverse bias and forward bias the first and second transistors to achieve a balance between the half cycle charging through the second resistor and

Abstract: A tracking pulse detector comprising a feedback loop and means for adjustment of the detector bias to compensate for varying input signal levels whereby noise signals present at the input of the detector are maintained below the detector threshold thereby rendering the detector immune to false triggering. An AGC output is taken from a junction between resistors of a voltage divider and a capacitor is connected between the AGC output line and an OV line, the AGC voltage output being related to the total carrier level. The average current in the detector is kept approximately constant independently of supply and signal conditions and the feedback loops adjusts the DC bias of the detector to compensate for varying signal levels, thereby keeping any input noise signals below the detector threshold independent of the level of slow varying carriers. Any carrier level variation faster than the feedback loop time constants are detected.

Abstract: A radio receiver including an RF amplifier stage arranged to receive transmitted RF signals and noise signals from an antenna, a mixer stage for converting the RF signals to intermediate frequency (IF) signals and supplying the IF signals to a blanker gate receiving blanking signals from a blanking signal source, the gate normally passing IF signals to discriminator means to demodulate the IF signals to audio signals, but decoupling the mixer stage from the demodulator when the blanking signal is present. The audio signal output from the discriminator is applied to a noise squelch circuit which detects noise frequencies above a predetermined frequency. If the predetermined frequency is exceeded, a squelch signal is generated to block or mute the audio signals. In the present invention, the squelch circuit employs an averaging detector instead of a peak detector in order to eliminate short duration spikes in the squelch circuit.