Abstract: Co-channel interference of a frequency modulated signal represented by complex signal samples is reduced by amplitude limiting each sample and non-linearly filtering the limited samples. The non-linear filter low pass filters the limited samples, multiplies the filtered samples by their complex conjugates, and high pass filters the products to produce resultant signal samples. It also squares the filtered samples, low pass filters the squared samples, multiplies the filtered squared signals by the complex conjugates of the filtered samples, combines the result with said resultant signal samples and the filtered samples, and low pass filters the combined signal to produce samples having reduced co-channel interference. The limiter and non-linear filter can be constituted by a digital signal processor.

Abstract: A central station of a cable television distribution network transmits TDM frames comprising distribution information and overhead information downstream to customer terminals, and determines corresponding upstream TDMA frames by including an upstream frame identity in the overhead information of each downstream frame. Each terminal buffers asynchronous information for upstream transmission, and stores an upstream frame identity which it is assigned. On detecting this frame identity in the downstream overhead information, the terminal transmit a buffer queue size in an assigned time slot of the corresponding upstream frame. The central station uses the queue sizes from the terminals to allocate time slots in the upstream frames to terminals, by including their terminal addresses in the downstream overhead information, for transmission of the buffered asynchronous information. The central station can also assign time slots to the terminals for transmission of isochronous information.

Abstract: A digital receive filter comprises first and second filter parts with a down sampler between them. Each filter part is formed by a plurality of cascaded filter stages each comprising a second order finite impulse response digital filter having symmetrical coefficients which in most cases are integer powers of two, and in other cases are zero or a sum of integer powers of two. The filter stages are implemented by delay units and add units with shifting of bits at the inputs of the add units to implement the coefficients, whereby multipliers are avoided. A described receive filter has two filter stages in the first part, a down sampling factor of two, and seven filter stages in the second part, with scaling factors and ordering of the filter stages to facilitate filtering and minimize noise.

Abstract: In a cellular radio communications system, handoff of a mobile terminal from a serving base station to an adjacent base station is performed using two separate fuzzy logic procedures for handoff triggering and handoff targeting. For triggering, inputs to the fuzzy logic procedure are the signal strength for communications with the serving base station, a time since a failure to complete execution of a handoff, and, for mobile-assisted handoff (MAHO), signal strengths of communications between the mobile terminal and each of the adjacent base stations. If handoff is triggered, then handoff targeting is performed at a central unit. For targeting, inputs to the fuzzy logic procedure are the monitored signal strengths between the mobile terminal and each of the adjacent base stations, supplied either from the mobile terminal using MAHO or from the base stations, and the current loads of the adjacent base stations.

Abstract: In an echo canceller, an estimated echo signal is produced by an adaptive filter from a receive path signal and is subtracted from an incoming transmit path signal to produce an outgoing transmit path signal. Averages of signal levels on the receive path and the incoming and outgoing transmit paths are produced by exponential averaging. Adaptation of the adaptive filter is inhibited after initial convergence. Adaptation is enabled again in response to an echo attenuation parameter being less than a first threshold value and a variation of an echo attenuation parameter being less than a second threshold value, this situation representing a detected echo path change. The detection of an echo path change is inhibited in the presence of a near end signal.

Abstract: Each complex signal sample a(k)+jb(k) of a frequency modulated signal is limited in order to reduce co-channel interference. The limiting is performed efficiently in a digital signal processor by determining a ratio r with a magnitude less than one, equal to b(k)/a(k) in a first case and a(k)/b(k) in a second case, determining from a look-up table a variable p with a magnitude equal to 1/.sqroot.1+r.sup.2 , and determining a number equal to sgn(a(k))p(1+jr) in the first case and sgn(b(k))p(r+j) in the second case as the limited output signal sample.

Abstract: A surface wave device, for use in differential coherent detection in a commutation signaling communications system using N predetermined M-bit codewords, propagates surface waves for each codeword to a first coded output IDT (inter-digital transducer) comprising M parallel taps with a pitch t=T/M, the taps having phases representing bits of the respective codeword, and to a second output IDT which is at a distance from an input IDT greater by T than the first coded output IDT, to provide an additional delay of one information symbol. The input IDT, or each tap of each coded output IDT, has a length equal to t. The second output IDT can comprise a coded IDT, or a delay tap responsive to the surface wave propagated via the first coded output IDT.

Abstract: A telephone line interface circuit comprises a controlled voltage generator for selectively generating a ringing voltage waveform for a telephone line or a supply voltage for a drive circuit for the telephone line, and a switching circuit interconnecting an output of the controlled voltage generator, the drive circuit, and the telephone line accordingly. In order to avoid creating a potentially hazardous condition on the telephone line, an interlock is provided between enabling of the controlled voltage generator and switching of the switching circuit, and software and hardware monitoring functions monitor operation of the control software and the controlled voltage generator and disable the controlled voltage generator in response to fault conditions or unusual behavior.

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: A digital video network comprises a controller and network sites each coupled to the controller via communication paths. The network sites and controller include codecs for converting between encoded signals on the communications paths and video and audio signals in the network sites and in the controller. The controller also includes at least one unit for producing a quad split video signal from four video signals supplied thereto. The coders encode four audio signal channels and audio mixing control signals in each encoded signal. The decoder in each network site decodes the audio signal channels from the encoded signal which it receives, and is responsive to the control signal in the encoded signal to mix the decoded audio signal channels in a predetermined manner to provide a continuous audio presence at the network site, independently of whether or not the video signal in the encoded signal comprises the quad split video signal.

Abstract: In an echo cancelling arrangement, an estimated echo signal is produced by an adaptive filter from a receive path signal and is subtracted from an incoming transmit path signal to produce an outgoing transmit path signal, to compensate for ERL (echo return loss) via a hybrid circuit. Short-term averages of signal levels on the receive path and the incoming and outgoing transmit paths are produced by exponential averaging. An ERLE (echo return loss enhancement) due to the subtraction of the estimated echo signal is calculated from the incoming and outgoing transmit path averages and, when this exceeds a threshold value, enables averaging of a ratio of average levels on the receive path to average levels on the incoming transmit path. The resulting long-term average constitutes a determination of the actual ERL.

Abstract: A voltage regulator for an electronic circuit card includes a FET with a source supplied via an input LC filter with a voltage to be regulated, a drain for delivering a regulated output voltage via a current-sensing resistor and an output LC circuit, and a gate which is supplied with a current drive from a linear transconductance amplifier having a differential input responsive to the regulated output voltage relative to a reference voltage. The regulator provides large bandwidth low voltage drop regulation without requiring higher voltage supply for the gate of the FET. A current limiting circuit reduces the current drive in response to excessive current through the current-sensing resistor, and a turn-on control circuit reduces the current drive to provide a controlled turn-on for hot insertion of the card into electronic equipment.

Abstract: A telephone line interface circuit with d.c. amplifiers coupled to tip and ring paths is protected against transient voltages by a crowbar-type protection device coupled via a diode bridge to the tip and ring paths, positive and negative terminals of the diode bridge being connected respectively to ground and via a diode to a negative supply voltage line for the amplifiers. The diode prevents discharge of a capacitor, coupled between the negative supply voltage line and ground, through the protection device when it conducts. A locked up state, in which the amplifiers maintain holding current to the protection device after a transient voltage, is avoided by further diodes coupled from the negative terminal of the diode bridge to inputs of the amplifters, or by controlling a current limiter to interrupt current to the capacitor when the diode is reverse biased, so that the capacitor discharges.

Abstract: A tree mixer includes a first differential pair of transistors which divide current from a first current source between the transistors in dependence upon a first differential input signal supplied to first input terminals. Two second differential pairs of transistors have controlled paths which conduct current, divided between the transistors of the differential pair in dependence upon a second differential input signal supplied to second input terminals, dependent upon current passed by a respective transistor of the first pair of transistors, to produce a product signal at output terminals. One or more second current sources provide for the total current passed by the first pair of transistors to be greater than the total current passed by the two second pairs of transistors, for improved linearity without increasing noise. A quadrature phase modulator can include two such mixers with common output circuitry.

Abstract: A two-wire telephone line is supplied with d.c. feed from a drive circuit including two amplifiers via which a transmission signal is supplied to the line. Output potentials of the two amplifiers determine a loop current on the line, which is monitored and used to control a supply voltage for the drive circuit, the amplifier output potentials being offset from the supply potentials by headroom voltages to accommodate the transmission signal. The resulting current limiting voltage control acts iteratively to converge the supply voltage and a target voltage which is inversely dependent on an amount by which the loop current exceeds a predetermined value, the rate of convergence being fast for high loop currents for rapid response and slow for small differences to provide stable operation. The headroom voltages can be increased to nearly the supply voltage to reverse the polarity of the d.c. feed while maintaining current limiting.

Abstract: FIFOs having various depths are provided in a RAM in a plurality of groups. For each group, stored numbers B and D determine a base address 2.sup.B in the RAM for the group and a depth 2.sup.D of each FIFO in the group. Each FIFO is identified by a respective FIFO identity having a most significant "1" bit whose position identifies the respective group of FIFOs, less significant bits identifying the respective FIFO in the group. A count C is provided which is common to all of the FIFOs for identifying a respective location in each FIFO. Each FIFO in the RAM is addressed with an address comprising a sum of the base address 2.sup.B, a product of said less significant bits multiplied by 2.sup.D, and a number C mod 2.sup.D, using the stored numbers B and D determined by the respective FIFO identity. The sum can be provided by an OR function.

Abstract: A cable television network is supplemented with optical fibers and fiber termination units to communicate additional downstream digital signals and upstream signals. A drop unit for each customer premises is provided in the drop cable. The drop unit demodulates downstream signals and selects for delivery to the customer premises only those downstream signals addressed to the customer premises, modulating the selected signals using binary phase shift keying (BPSK) which provides relative immunity to noise for forwarding the signals to the customer premises. Upstream BPSK signals from the customer premises are demodulated and remodulated in the drop unit to isolate the customer premises wiring from the rest of the network.

Abstract: A phase lock loop (PLL) arrangement includes a voltage controlled ring oscillator (VCRO) including delay elements whose delay is controlled by a control voltage produced by the PLL. A phase error detector is provided which compares pulses of a PLL feedback frequency with pulses of a delayed reference signal, the delay being provided by further delay elements controlled by the same control voltage. The phase error detector produces an output signal which indicates when phase error exceeds a predetermined tolerance, and also indicates an absence of frequency lock.

Abstract: A plurality of ADSL (asymmetric digital subscriber line) terminals are coupled to the coaxial cable of a cable television distribution arrangement. Each ADSL terminal is coupled to a respective ADSL path and a respective customer premises telephone line for coupling of telephone signals. Each ADSL terminal comprises a transmitter for supplying asymmetric signals received via the respective ADSL path to the coaxial cable, and transmitters and receivers for communicating bidirectional data and control signals, communicated via the respective ADSL path, via the coaxial cable, in each case modulated at a frequency which is not used for television signals. Customer premises equipment includes corresponding receivers and transmitters. The arrangement permits the ADSL terminals to be used as a resource shared by a plurality of customer premises.

Abstract: A cable television distribution arrangement is supplied via optical fibers with additional digital signals which are delivered to customers via coaxial drop cables modulated at frequencies above those of the conventional analog television channels. A reduction in the required number of modulators is achieved, while delivering individually selected signals to individual customers, by supplying digital signals for a group of customers to a first modulator and a pseudo dam signal to a second modulator, and selecting between the outputs of the two modulators individually for each customer in the group. Switching between the outputs of the modulators is carried out during overhead information which is additionally supplied commonly to both modulators.