Abstract: Telecommunications (e.g. ISDN) wiring is tested by sequentially supplying to each wire at a first location a pulse sequence identifying that wire while connecting to a common potential other wires, and detecting pulses on the wires at a second location while simultaneously providing a return path connection for the pulses to the other wires. The return path connection is provided by connecting between each wire and a local ground at the second location a resistor and a diode poled to be reverse biassed by pulses on the wire. The detected pulses are counted and the result used to drive a LED (light emitting diode) display comprising N green LEDs indicating correct wiring and N(N-1) red LEDs indicating incorrect connections, the LEDs being arranged in an N by N array with the green LEDs along one diagonal.

Abstract: An electronic hybrid circuit includes a differential amplifier having its non-inverting input coupled via a resistor to a four-wire input port and via a coupling transformer to a two-wire input/output port, its output coupled to a four-wire output port, and its inverting input coupled via a resistor to the input port and also coupled to an impedance network. The impedance network includes an inductive impedance which is coupled either to the amplifier output or to circuit ground, and which may be constituted by a gyrator or other electronic circuit which simulates an inductor.

Abstract: A two-wire telephone subscriber line protection arrangement consists of two protection devices each comprising a pair of complementary thyristors connected in reverse parallel with one another between a respective wire and a reference point, with their gates connected together and directly to a control terminal which has a connection to the wire via a resistance which determines an overcurrent threshold level for the device. The reference point is ground for the ring wire protection device, and is desirably the ring wire for the tip wire protection device, whereby a directional sensitivity of the protection devices is taken advantage of and the overcurrent threshold level can be reduced. Zener diodes connected between complementary thyristors provide for convenient overvoltage protection.

Abstract: Digital signals are multiplexed in tdm (time division multiplex) frames each consisting of an integral number of sub-frames each consisting of an integral number of sub-sub-frames, whereby the tdm frame, sub-frame, and sub-sub-frame periods are harmonically related. The signals are switched in a hierarchy of time switches which are operated sequentially and cyclically with frame periods corresponding to respective ones of these harmonically related periods, whereby the switches switch services at respective harmonically related bandwidths. The harmonic tdm frame structure and the hierarchical switch arrangement combine to facilitate the switching of many services with various bandwidths using relatively small switches.

Abstract: An optical busbar, for optically interconnecting electronic and/or photonic equipment, comprises a rod of light-transmissive material, for example polycarbonate, having a longitudinal planar surface. Preferably the rod is of polygonal cross-sectional shape. Distributed along its length, opposite the planar surface, the rod has a plurality of inclined reflector surfaces for diverting light travelling along the rod so that it emerges laterally at intervals corresponding to the spacing of the reflector surfaces. When the rod is installed on a backplane of the equipment, with a plurality of circuit cards projecting perpendicularly from the backplane, optoelectrical devices on the circuit cards are arranged to be in register with the inclined reflector surfaces. Optical signals transmitted along the optical busbar will be distributed to the circuit cards. The optical conductor will, of course, function with the light travelling in the opposite direction, i.e.

Abstract: An optical communications system comprises a pluarality of optical transmitters coupled by a common channel or bus to an optical receiver. The optical transmitters are operated using time division multiplexing (TDM), each TDM period being word or message length. The optical transmitters each comprise a least one current source to drive the light source, for example a laser, in response to the data signal, to vary its output power about a steady level which is the average of the maximum and minimum values. Therefore each current source may be controlled as to increase its output gradually (preferably sinusoidally) to reach the aforesaid mean level at the beginning of a TDM period and to decrease it in the converse way after the period ends. Such an arrangement avoids the need for guard time before and after the TDM time period, thereby increasing channel efficiency, and improves receiver sensitivity by reducing quantum shot noise due to steady light from inactive transmitters.

Abstract: A phase detector includes a bridge of FETs which are simultaneously rendered conductive by pulses of a reference signal having a very small duty cycle. A higher frequency signal, whose phase in relation to the reference signal is to be detected, is supplied to a gate of one of the FETs. A bridge output signal is low pass filtered and differentially amplified to produce a phase detector output signal. The phase detector output signal can be used directly to control the frequency of a voltage controlled oscillator which produces the higher frequency signal.

Abstract: A bandwidth limiter for an optical fiber transmission path comprises a four-port optical coupler via which an optical signal is passed. A spliced optical fiber provides a loop, from an output to an input of the coupler, with a predetermined propagation delay and attenuation. An alternative arrangement uses two couplers which are coupled together via fibers of different length and hence propagation delay.

Abstract: A square array of signal points forming a QAM signal point constellation is modified by relocating signal points from corners of the square to positions extrapolated from the square array and at reduced distances from the origin, corresponding to reduced peak amplitude levels. The relocation 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 points, with n=4 or more and points with I and Q amplitudes 1, 3, 5, . . . 2.sup.n -1 units in the square array, a plurality of points in each quadrant are relocated to positions, relative to positions which they would have in the square array, rotated through an angle of 180.degree. about predetermined reference points having I and Q amplitude co-ordinates of (2.sup.n,2.sup.n-1) and (2.sup.n-1,2.sup.n). Specific examples are described for 256 QAM (n=4) and 1024 QAM (n=5).

Abstract: An optical fiber communications system with a plurality of bidirectional channels and a protection channel is disclosed. Each channel has a respective identity which is transmitted in the traffic of that channel and in the event of a protection switch is identified in the protection channel traffic. A protection switch is effected by detecting a channel failure at the receive end of a span, transmitting a protection request on the return channel back to the transmit end of the span, and using this request in a controller for the channel to effect a protection switch, using an individual selector for the channel, if a priority scheme allows. The controllers are enabled or disabled under software control following a protection switch in accordance with the prevailing priority scheme. The arrangement provides for rapid protection switching.

Abstract: A characteristic of a signal received via a receive line is monitored and used for automatic control of a line buildout circuit in a transmit signal path. The characteristic is conveniently an amplitude level, or differential amplitude levels for different signal bandwidths, of the received signal. The transmit line buildout circuit includes a transversal filter in which multiplier coefficients or delay times are controlled. The arrangement is particularly useful for digital signal interfaces on a digital cross connect.

Abstract: A digital signal, such as a t.d.m. PCM signal, is redundantly encoded in that sequences of n bits of the signal are transmitted as sequences of m bits, where m>n. Different sets of m-bit sequences are used to transmit the n-bit sequences in dependence upon the state of an additional signal, which may be a superframe indication signal. At a receiver, the digital signal and additional signal are reproduced. In the described embodiment =n+2, providing further redundancy to facilitate providing such desirable transmission code properties as maintaining d.c. balance and detecting errors on the transmission link.

Abstract: A band limited composite video television signal has summed with it, at a reduced amplitude, a frequency modulated audio carrier signal at a higher frequency. The sum signal is sampled and digitized at a sampling frequency which is at least twice the audio carrier frequency and is more than three times the frequency of a color subcarrier in the composite video signal, and is not related thereto. The sampling frequency is derived from a transmission network, whereby stuffing of the digitized signal for transmission is eliminated, and multiplexing of several digitized signals is facilitated. At a decoder, the digital signal is converted back into an analog signal and the audio carrier signal amplitude boosted using filtering. Picture quality of a reproduced television signal is enhanced in that quantizing noise in the video signal becomes uncorrelated from line to line and frame to frame, due to dithering effects by the audio carrier signal.

Abstract: A binary data signal is regenerated with respect to three threshold levels, one of which is derived from and lies between the other two so that it is an optimum level for producing a regenerated output signal. Two control loops are used to produce the other two threshold levels to produce regenerated signals with predetermined error rates, the errors occurring on the two control loops for the two respective states of the binary data signal, whereby the optimum threshold level is maintained regardless of the nature of signal degradation. The arrangement provides rapid performance monitoring which can also be used to adjust other parameters, such as detector bias, equalizer tuning, and clock phase, for optimum performance in an active and ongoing manner.

Abstract: A monolithic transformer comprises two windings arranged in a plane with conductors of the windings interleaved with and crossing one another, whereby each winding is symmetrical with respect to a center point of the winding, to which a center tap connection can be made. The conductors of the windings and crossings therebetween can be arranged so that adjacent conductors belong to the different windings for maximum coupling. An unbalanced circuit can be connected between the center tap and one end of a winding, the other end electrically floating, to compensate for the effects of the unbalanced circuit on interwinding capacitance. The transformer can be constructed using air bridge technology with conductors supported by and spaced from a surface of a substrate.

Abstract: A SAW device includes aligned IDTs (inter-digital transducers) with a continuous pattern of fingers between their front edges. Each IDT may be symmetrical about its center, and includes front and back edge reflection suppressing regions of fingers of successively decreasing length, which serve to suppress surface acoustic wave reflections at the respective edges. Optional shield regions between the two front edge suppressing regions, and between the back edge suppressing regions and acoustic absorbers, include fingers of successively increasing length complementary to and adjacent the fingers of successively decreasing length of the reflection suppressing regions. The IDTs may be apodized or unapodized, and each IDT may include dummy fingers between its active region and each reflection suppressing region.

Abstract: In a synchronous network, asynchronous data signals are synchronized using positive/negative stuffing under the control of stuff request signals which are produced in dependence upon the phase difference between the asynchronous and synchronized data signals as compared to respective threshold values. The phase difference is, or the threshold values are, cyclically changed in a manner to produce additional stuffing, not necessitated by the asynchronous frequency difference, whereby the frequency of jitter, due to stuffing, in the synchronized data signal is increased. The increased frequency jitter is filtered out in an already-provided phase locked loop.

Abstract: An asynchronous data signal is synchronized by stuffing in dependence upon phase differences between the asynchronous signal and a resultant synchronized signal. The synchronized signal comprises tdm frames each including data bits and a stuffing opportunity bit, and a plurality of frames constitute a superframe in which different frames have different numbers of data bits. To compensate for the biassed timing of stuff requests which would otherwise occur relative to the frame timing in the superframes, the threshold value with which the phase difference is compared to produce the stuff requests is modified with a stepped sawtooth waveform having a period equal to that of the superframe.

Abstract: A two-wire transmission path repeater includes, between two hybrid circuits coupled to the path, an echo canceller and an amplifier for each direction of transmission, each echo canceller including a transversal filter and a subtracting circuit. The power ratio between signals before and after the subtracting circuit of each echo canceller is used to distinguish between simplex and duplex signals. If the power ratio exceeds a given value, which is dependent upon the input signal power, for only one transmission direction then a simplex signal in the opposite direction is determined, the gain of the amplifier for the one transmission direction is reduced, and adaptation of the transversal filter coefficients for the opposite direction is prevented.

Abstract: Data to be switched is preceded by a header containing routing information for establishing a connection via a switch, and is accompanied by additional information which indicates the start of the header, in response to which a connection is established, and the end of the data, in response to which the connection is terminated. The switch is a non-saturating, non-blocking, full matrix time switch which uses the routing information to establish a connection within one tdm frame, whereby connections for data of any type (including switch control information) and of arbitrary duration can be handled efficiently. An incoming channel can be connected to any free channel or to a specified outgoing channel of any output port, and contention resolution is provided for simultaneous requests for connection to a single outgoing channel. Interconnections among crosspoint nodes are reduced and simplified using token ring and systolic interconnection techniques.