Abstract: An automatic protection and recovery method for a telephone line interface circuit is described. The interface circuit monitors current on the line, and in the event of a fault producing an excessive current a protection relay interrupts a connection of the interface circuit to the line, and the interface circuit monitors common mode voltage on the line. The interface circuit is reconnected in response to the monitored voltage being below a relatively high threshold. If a second interruption results, the interface circuit is then reconnected in response to the monitored voltage being below a lower threshold. If a further interruption results, the interface circuit enters a ground fault recovery process. The method facilitates protection and rapid recovery from arbitrary fault conditions on the telephone line.

Abstract: A double mode surface wave resonator comprises two IDTs (inter-digital transducers), arranged side by side between reflection gratings, on a surface of a piezoelectric substrate to provide for coupling of surface waves therebetween, each IDT having interleaved electrodes extending from a common rail between the two IDTs and from a respective outer rail of the IDT. One or each of the IDTs and its outer rail is divided into two halves for providing a differential signal connection to the resonator. The electrodes of the IDTs can be arranged in spatial synchronism with adjacent fingers of the reflection gratings to reduce spurious longitudinal modes.

Abstract: A two-wire telephone line interface circuit comprises a driver circuit for supplying direct current on the line, a controlled voltage generator, a sensing circuit for monitoring the line current, a control circuit for controlling an output voltage of the voltage generator, and a switching circuit. The control circuit controls the switching circuit to supply, as a supply voltage for the driver circuit, a battery voltage in an on-hook state of the line or the controlled output voltage of the voltage generator in an off-hook state of the line, this output voltage being controlled to provide off-hook current limiting. The control circuit also controls the switching circuit to selectively supply the controlled output voltage of the voltage generator as a signalling voltage to at least one wire of the line for high voltage signalling, e.g. ringing, on the line. Desirable forms of the switching circuit are described.

Abstract: A electronic circuit board arrangement includes a main circuit board on which components are mounted, with component leads passing through holes in this board. Additional components are provided on an auxiliary circuit board at least part of which is directly against the main board, either on the component side underlying components or on a soldering side of the main board. The additional components are connected to the component leads via conductive tracks on the auxiliary board. The auxiliary board can be flexible to also extend over components on the main board. The arrangement is particularly useful for adding additional components to an original circuit design.

Abstract: Changes in a telephone subscriber line voltage are detected in a telephone set by supplying a voltage dependent upon the subscriber line voltage to a potential divider having first, second, and third tapping points, the second tapping point being between the first and third tapping points, and comparing the voltages at the first and third tapping points with a smoothed version of the voltage at the second tapping point to produce pulses at respective terminals when the line voltage is falling or rising. An algorithm is described for responding to the pulses to distinguish changes in hook state of another telephone connected to the same subscriber line from interfering signals. The arrangement facilitates remote release from hold and privacy indication functions for the telephone set.

Abstract: An active impedance line interface circuit for connection to a two-wire telephone line comprises tip and ring unity gain d.c. amplifiers having outputs d.c. coupled via feed resistors to tip and ring wires respectively. Loop current and common mode current conducted via the feed resistors are monitored individually and converted to digital signals supplied to a digital control circuit, which can be programmed to provide desired operating conditions. Digital control signals produced by the control circuit are converted into analog signals and used to control currents passed by current sources through resistors at inputs of the d.c. amplifiers, thereby determining voltages at the d.c. amplifier inputs and hence on the tip and ring wires. These d.c. voltages are smoothed by capacitors used for coupling an a.c. signal to the d.c. amplifiers.

Abstract: An impedance generator, especially suitable for determining a terminating impedance of an active impedance telephone line interface circuit, comprises a subtracting circuit for forming a difference between an input voltage multiplied by a multiplier and a high pass filtered version of the difference multiplied by another multiplier, and a summing circuit for combining the input voltage multiplied by a further multiplier with a low pass filtered version of said difference to produce an output voltage. In one arrangement, the input voltage multiplied by the further multiplier constitutes a third input to the subtracting circuit. The high pass filtered version of the difference is produced by subtracting the output of a low pass filter from the difference. The multipliers are constituted by controllable gain elements, enabling the generated impedance to be easily programmed under digital control.

Abstract: A plurality of terminals, each responsive to RR (receive ready) and RNR (receive not ready) messages for respectively enabling and inhibiting the transmission of data packets by the terminal, are coupled to a packet handler via a multiplexed path. In order to limit each terminal to an allocated bandwidth, for each terminal a number is stored representing a number of data bytes which the respective terminal is permitted to transmit within a timed period at its allocated bandwidth, and in each timed period a counter counts down from this number in accordance with the number of data bytes transmitted by the terminal. An RNR message is transmitted to the terminal if and when a zero count is reached, and following such an RNR message na RR message is transmitted to the terminal to enable it for the next time period. The arrangement provides a relatively instantaneous limiting of the bandwidth of each terminal to its allocated bandwidth, and can be applied independently for each direction of transmission.

Abstract: In a QAM microwave radio communications system an IF (intermediate frequency) input signal is amplified and predistorted, and the amplified IF signal is mixed with a local oscillator signal to produce an RF (radio frequency) signal which is amplified in a power amplifier for transmission, the predistortion compensating for non-linear gain of the power amplifier. Clipping in the power amplifier is controlled by down-converting part of the RF signal to produce an IF output signal, monitoring amplitude distortion of the RF signal by comparing the IF output signal with the IF input signal, and controlling the IF amplifier gain in dependence upon the monitored amplitude distortion.

Abstract: A SAW (surface acoustic wave) device tapped delay line comprises a continuous pattern of bifurcated inter-digital fingers having a constant pitch of .lambda./2 and forming a first IDT (inter-digital transducer), a plurality of second IDTs, substantially identical to one another and arranged with a predetermined pitch P for consecutively receiving a SAW propagated from the first IDT with respective propagation delays, and grounded dummy fingers in regions between adjacent IDTs. The first IDT is apodized in accordance with a Hermitian response to provide it with a pass band centered at a first frequency, and .lambda. is the wavelength of a SAW at a second frequency different from the first frequency and within the pass band. P is an integral multiple of .lambda./2, so that there is a constant finger periodicity throughout the SAW device whereby reflections between the second IDTs are avoided.

Abstract: For a public key encryption system, for example using the RSA algorithm, large numbers, for example each of the order of 256 bits and represented by a plurality of sequentially stored words, must be multiplied together. To achieve this at high speed there is described a method using a digital signal processor which has two accumulators for accumulating products of words, each accumulator having a capacity of more than twice the size of the words which are multiplied. The two accumulators are used alternately for accumulating partial products of the same significance in a multiplication of two numbers. Pointers to the sequentially stored words are incremented in opposite directions to point to words to be multiplied to form partial products of the same significance, and the pointer incrementing directions are changed for sets of partial products of increasing significance. The pointer control and alternating use of the accumulators enables a significant increase in multiplication speed to be achieved.

Abstract: A switching node for an optical communications switched network includes a circuit switch for making relatively long duration circuit-switched connections and an ATM message switch, which is coupled in the network via the circuit switch, for communicating relatively short duration messages or datagrams. The messages include signalling messages for establishing the circuit-switched connections, the ATM switch being coupled to control circuitry of the circuit switch for communication of such messages. The control circuitry includes free channel queues which enable the communication of signalling messages throughout a network to be effected rapidly without delays due to the time division multiplex frames in accordance with which the circuit switch is operated. In a preferred form, the control circuitry is constituted by a micro-controller and random access memory.

Abstract: Asynchronous DS-1 data is byte synchronized and converted to the SONET VT1.5 format by storing the DS-1 data in a store from which it is read in dependence upon a gapped clock signal which is produced by gapping a first gapped clock signal with a ratio of 208/193, which is the ratio of VT SPE bits per frame to DS-1 bits per frame. The first gapped clock signal is produced by gapping a VT1.5 synchronous clock signal. A frequency difference between the first gapped clock signal and the asynchronous data rate, multiplied in a frequency multiplier by the ratio of 208/193, is monitored by comparing the counts of modulo-208 counters, and, in dependence upon the monitored frequency difference, the gapping of the synchronous clock signal is controlled to achieve positive or negative stuffing and hence to compensate for the frequency difference.

Abstract: A rectangular array of signal points forming a QAM signal point constellation is modified by relocating signal points from adjacent corners of the rectangular array to positions extrapolated from the rectangular array and at reduced distances from the origin, corresponding to reduced peak amplitude levels. The location is effected in a manner to maintain, at least for a majority of the relocated points, a Hamming distance of one. To this end for a constellation of 2.sup.2n+1 points, with n=4 or more and points with I and Q amplitudes 1, 3, 5 . . . units in the rectangular array, a plurality of points in each quandrant are relocated to positions, relative to positions which they would have in the rectangular array, rotated through an angle of 180.degree. about a point having I and Q amplitude co-ordinates of (2.sup.n, 2.sup.n).

Abstract: A circuit for limiting switch-on surge current to a load including a large capacitance includes a MOSFET whose controlled path is connected in series with the capacitance across power supply terminals. A resistor and a capacitor, having a relatively small capacitance, are also connected in series with the controlled path, and a junction between the resistor and capacitor is connected to a gate of the MOSFET via a further resistor to provide a negative feedback path for charging the load capacitance with a constant current when power is applied. A further capacitor prevents initial turn-on of the MOSFET, and a zener diode limits the gate voltage. An alternative arrangement using a differential amplifier is described. The resistive part of the load can be connected in parallel with the load capacitance, or to the power supply terminals for which it can be separately switched in dependence upon the gate voltage.

Abstract: Speech signal components in a high band of frequencies between 4 and 8 kHz are transmitted via a digital transmission channel, which carries speech signal samples at frequencies below 4 kHz and sampled at a rate of 8 kHz, by replacing the least significant bit of the samples with bits of information derived from the high band by linear predictive coding. These information bits are transmitted in frames, each frame comprising a synchronizing bit and bits representing the power of and a set of filter coefficients for the high band signal components occurring in a period corresponding to the frame duration. Each such bit is transmitted redundantly three or six times in view of bit stealing techniques already used for signalling on digital transmission links. The resulting wideband speech signal transmission is compatible with existing limited bandwidth voice channel transmission arrangements.

Abstract: A decoder for Manchester encoded data includes an up/down counter which constitutes a state machine. Consecutive bits with the same binary value in the encoded data bit stream enable the counter, whic is incremented or decremented in dependence upon the relative phase of an output clock signal. Incrementing occurs in response to phase errors of the output clock signal, and decrementing to a count of zero occurs in response to phase assertions indicating a correct phase of the clock signal. The phase of the clock signal is reversed, and the counter reset, if a maximum count is reached in response to repeated phase errors. Such a phase reversal or phase slip is avoided in the presence of single bit errors in the bit stream. The decoded data is derived from the bit stream by sampling in dependence upon the clock signal.

Abstract: A SAW device tapped delay line includes an apodized input IDT (inter-digital transducer) and a plurality of substantially identical unapodized output IDTs arranged to receive, after delays which successively increase by an amont T, a SAW propagated from the input IDT. Each output IDT is terminated with a low impedance which is constituted by the low input impedance of a buffer amplifier, to minimize acoustic regeneration by the output IDTs. Grounded dummy fingers with the same metallization ratio as the output IDTs are provided between adjacent output IDTs to provide a constant SAW velocity. The delay line is particularly suited to use in an equalizer of a microwave radio transmission system, in which the delay T is the inverse of the symbol rate of the system. Intermediate frequency automatic time domain equalizer (IF ATDE) arrangements using the SAW device tapped delay line are described.

Abstract: A multiple path electro-optic transducer comprises a plurality of electro-optic elements arranged in an array in a device package with electrical connections thereto. A lens tube having a lens, such as a graded index (GRIN) lens, therein is supported so that an axis of the lens is perpendicular to and aligned with the array and so that the array is imaged by the lens in a plane of an outer end of the lens tube. An optical fiber connector comprises a ferrule having a bore therein and a plurality of optical fibers extending through the bore in an array corresponding to the array of electro-optic elements, the ferrule and fibers being positioned, by a cylindrical sleeve over the lens tube and ferrule, so that ends of the fibers lie in the plane of the outer end of the lens tube whereby light emitted by each of the electro-optic element is focused by the lens onto a respective one of the fibers. A mechanical coupling is provided by a bayonet connection between the optical fiber connector and the device package.

Abstract: In a QAM radio transmission system, stuffing indications for a transmitted stuffed data signal are transmitted only once rather than repeatedly. The two possible states of each stuffing indication are represented by respective sets of signal points in the signal point constellation, with a separation between any two signal points in the different sets which is at least three times the signal point separation of the system. The reliability with which the stuffing indications are sent is much greater than for other transmitted information.