Abstract: A central office line unit (COLU) for a pulse code modulation (PCM) upstream and downstream system in a universal digital loop carrier (UDLC) includes a hybrid circuit for transmitting/receiving an analog signal to/from a switch. An analog-to-digital converter is provided for receiving the analog signal from the hybrid circuit and converting the analog signal to a converted digital signal. A digital transmission circuit transmits/receives a digital signal to/from a remote terminal. A quantizer is provided for evaluating and processing the converted digital signal so that only valid states of the converted digital signal are transmitted to the digital transmission circuit for transmission to the remote terminal. A receiver is used for receiving the digital signal from the digital transmission circuit and for evaluating and processing the digital signal so that only valid states of the digital signal are outputted from the receiver.

Abstract: A circuit attenuates echo caused by line variations and a transformerless, high DC impedance, two-wire line interface. To reduce echo, a test tone is introduced on the communication line with all station units connected to the line. The resultant receive signal is conditioned through a peak detector, digitized and read by a processor. The resistive and capacitive characteristics of a network are iterated by the processor and the results remeasured. The network is set to the best combination for least echo by the processor. An interfacing system capacitively couples a plurality of sources to a two-wire communication pair by means of a plurality of differential voltage-to-current amplifiers. Each source uses a differential receiving amplifier to receive signals from the line and is resistively coupled in parallel to all the sourcing entities coupled to the communication line at that particular interface. There is an echo balance network associated with each source.

Abstract: A small and economically fabricated CMOS voltage controlled crystal oscillator is provided by coupling three inverter amplifiers in series with a regenerative crystal controlled feedback loop. The first and second CMOS inverters have output nodes whose impedances are modified by a CMOS impedance modulating circuit. Also coupled to each of these two output nodes is a CMOS transistor shunt capacitor. The impedance of the output node is modified according to the magnitude of a voltage control signal applied to the CMOS modulating circuits. The self-bias of the modulating circuits is maintained substantially constant by adjusting the gate drive in each of the modulating circuits according to gate drives derived from a dummy modulating circuit.

Abstract: A low power, low noise driving circuit for a bank of LEDs utilized in the station units of the MAN system is provided by coupling each bank of LEDs in a series circuit between the voltage supply and a constant current source. Each LED has a controllable logic switch in parallel across it and the switches are further in series circuit with each other to form a ladder network. Any selected LED may be turned off by closing its corresponding logic switch. The current continues to flow then through the shunting switch into the remaining LEDs in the series circuit that are on. A plurality of such ladder networks may be coupled in parallel with each other and each ladder network controlled by a switching gate which selectively couples it to the constant current source so that the LED ladder networks are operated at a predetermined duty cycle.

Abstract: The invention is a telephone system capable of operating on a single internal telephone line as commonly found in the home or small businesses, which is economically implemented within an integrated circuit chip to provide a one chip telephone and which provides most of the components necessary for a full-featured, reliable and easy to install office communication system. The multiband audio network (MAN) transfers four audio bands and a digital data band over a single twisted pair of wire. A 6,000 bits per second full duplex digital data channel is also provided on the same twisted pair. Each station unit includes all the control and interface support necessary to perform conventional telephone functions. These functions include a combination of keyboard and display support circuitry such as strobe and debounce circuitry, LED buffers, piezo ring drivers, control registers and communication hardware.

Abstract: A telephone communication system for communicating between a plurality of exterior telephone lines and a single common in-house two wire line is comprised of a control unit for coupling to the plurality of exterior telephone lines and for controlling communication between the plurality of telephone lines and the common two wire line. A plurality of station units are coupled remotely throughout the building to the common two wire line. The control unit communicates on the digital channel with the plurality of station units in a time frame subdivided into a plurality of time slots. A specified portion of each time slot is reserved for control communication between the control unit and each one of the plurality of station units. A synchronization signal is transmitted for phase locking all the station units to the control unit clock and a common frame synchronization signal is used to align all units in time.

Abstract: 4800 B/S full duplex operation of a frequency-multiplexing modem over the switched telephone network without cancellation is made possible by combining 900-baud, 32-state operation with a combination of analog filtering and raised cosine digital filtering and stop bit robbing.

Abstract: 4800 B/S full duplex operation of a frequency-multiplexing modem over the switched telephone network without cancellation is made possible by combining 900-baud, 32-state operation with a combination of analog filtering and raised cosine digital filtering and stop bit robbing.

Abstract: Filter quality is substantially improved (or complexity substantially reduced) in a digital PSK modem requiring an overall filter characteristic of the percent raised cosine type by using, in both the transmitter and the receiver, a filter having the same percent raised cosine response but at a higher cutoff frequency which satisfies the formula ##EQU1## where f.sub.f is the cutoff frequency (6 db) of the new filter, f.sub.o is one-half the baud rate of the modem, and .alpha. is the percent raised cosine expressed in decimal form.

Abstract: Error multiplication in the differential decoder and descrambler of V.22 bis modems is avoided by replacing the differential coding with forward error correction and single symbol coding. The error syndrome bit of the error corrector is used in establishing phase and timing synchronization by causing the receiver to try all possible combinations of phase and timing, and to lock onto the combination which produces a minimum of error syndrome bits. The descrambler receives the error-free corrected bit stream, whereby the descrambler's inherent error multiplication becomes moot.