Abstract: A telephone set adapted for use by hearing impaired and visually impaired persons is disclosed in which the frequency response of audio signals produced from the handset receiver of the telephone is shaped to have enhanced high-frequency characteristics. The telephone is further provided with indicator lights which flash in conjunction with the telephone's electronic ringer to indicate an incoming telephone call. An array of light-emitting diodes is disposed beneath a dialer keypad to facilitate viewing of the dialing buttons in darkened environments. The dialer keypad's buttons are of an enlarged size, so that they may be more easily used by persons with impaired vison or motor skills.

Abstract: An amplification device which enables an electret handset microphone, a dynamic handset microphone, or a carbon handset microphone to be connected to either an electronic telephone or to a hybrid coil telephone. The amplification device is powered from current from the line connecting the telephone to a KSU, PBX, or other telephone servicing system.

Abstract: A paging system for a PBX comprises an amplifier/speaker module adapted to remain coupled across the Ring and Tip lines of a given phase of the PBX. The module responds to audio signals sent through such lines when the conventional polarity of same are reversed and a DC operating bias is applied thereto. The audio signals are superimposed on the DC bias. A relay controls the polarity of the Ring and Tip lines and when activated, reverses the polarity of such lines whereupon the module amplifies the audio signals and applies them to a speaker. The module includes circuitry to prevent the amplifier from operating when the relay is de-energized and Ring and Tip lines are of conventional polarity.

Abstract: An operational amplifier for producing an output signal representing the voltage differential between two input signals is powered by first, second and third power sources respectively having high, intermediate and low output voltage levels. A voltage buffer circuit, which is connected to the second power source, interconnects a constant current source, which is connected to the first power source, and an amplifier means, which is connected to the third power source, and separates the constant current source from the third power source. A differential amplifier in the amplifier means responds to the two input signals for producing a voltage differential output signal, and an output circuit connected between the second and third power sources receives the voltage differential output signal of the differential amplifier for producing the voltage differential output signal of the operational amplifier.

Abstract: An electronic telephone terminal having a transmitter and a receiver, both having an approximately linear acoustic-to-electric transduction characteristics, and having a surrounding noise suppression function, and including a variable attenuator for controlling a gain of a transmission system; a noise detection device for detecting surrounding noise; and a control device for controlling the variable attenuator in such a manner that when a sound pressure level input to the transmitter exceeds a predetermined threshold value, the gain is fixedly set to a constant value, and when the input sound pressure level is equal to or below the predetermined threshold level, the gain is controlled in response to a change in the surrounding noise level detected by the noise detection device.

Abstract: A method for gain control of telephone sets having an electronic speech circuit and an apparatus for carrying out the method include a modulation current source. Two line terminals connect the modulation current source to a telephone line. A microphone amplifier has an input connected to a microphone terminal and an output controlling the modulation current source for generating a line signal. Signals are derived from the modulation current source and converted into mutually associated measurement signals and threshold value signals. The mutually associated measurement and threshold value signals are compared with one another. The gain of the microphone amplifier is regulated as a function of the result of the comparison.

Abstract: The Invention relates to a signal compression circuit, more particularly for a telephone set of the handsfree-type, comprising a preamplifier (10), the input of which is connected with an input signal to be processed, and a preamplifier gain control device (11) for servo-controlling, to a predetermined value, the output signal amplitude of the preamplifier, in which the gain control device (11) is associated with a circuit (C, 15, 16) determining two time constants, which respectively control the increase or decrease speed of the preamplifier gain (10) in relation with the input signal amplitude (Vin).

Abstract: Apparatus is provided for interfacing a voice communication instrument, e.g. headset, to a communication link, e.g. a telephone line. The interface includes a transmit channel having amplification, frequency shaping and voice switching to suppress background noise. The interface includes a receive channel having compression limiting as a function of the received signal amplitude and frequency. The interface further includes a power-down circuit and a low battery indicator.

Abstract: An electronic speech circuit configuration includes first and second line terminals to be connected to a telephone line. An AC circuit portion includes a modulator circuit for generating a line transmission current. A DC circuit portion supplies energy and generates a DC current/voltage characteristic curve. The DC circuit portion includes a reference potential source and a decoupling capacitor. A current sensing resistor is provided. A high-speed control amplifier has a control circuit and controls current through the current sensing resistor. A buffer capacitor is provided. Output terminals are connected to the buffer capacitor for the connection of external components. The DC circuit portion also includes a current toggle circuit connected upstream of the buffer capacitor. The current for feeding the buffer capacitor flows through the current sensing resistor connected to the first line terminal.

Abstract: A paystation telephone level control system is disclosed including a telephone receiver driven by a local amplifier and wherein two sets of attenuating resistors, a coarse attenuation set and a fine attenuation set, interface with connectable terminals of a binary control crosspoint switch. The presence or absence of coin tones and dialing tones are detected and the crosspoint switch discretely controlled by their presence to effect discrete levels of attenuation of signals fed to the receiver. In addition, the lever of attenuation during voice communications is a function of a pre-adjusted attenuation and a user selection.

Abstract: A circuit is disclosed for use in telephone handsets having a line signal node (1), a microphone signal input (20) and an output to an earphone amplifier. There is provided two voltage feedback loops via respective impedance networks (10, 11) and a current feedback loop (5, 7, 14). These feedback loops determine the impedance presented to the line at the signal node with specific voltage/current characteristics and signal frequencies. The first voltage feedback loop in conjunction with the current feedback loop defines the signal frequency impedance and the second voltage feedback loop defines the DC mask presented to the line.

Abstract: A signal processing circuit for a telephone set. This circuit includes signal sending means that sends to a transmission line a sound signal produced from a telephone transmitter, a first adding circuit that adds the signal from the transmission line to the sound signal from the transmitter, an inverting circuit that phase-inverts the output signal from the first adding circuit, an impedance element provided between the transmission line and the output of the inverting circuit, and a second adding circuit that adds the output sound signal from the transmitter to the output signal from the inverting means and supplies the resulting composite signal to a telephone receiver.

Abstract: An interface circuit interfaces a four wire telephone circuit to a telephone headset. An adjustable receiver portion of the interface circuit receives a line input signal from the four wire circuit and produces a headset input signal limited to a predetermined signal level. An adjustable output portion of the interface circuit accepts a headset output signal and conditions it for use as a line output signal on the four wire circuit. A DC voltage is also supplied to a headset microphone through the output portion of the interface circuit.

Abstract: A key-in sound-generating circuit comprises a keyboard including dial number keys and function keys, a DTMF signal generator coupled, at the output terminal, with a telephone network, a sound generator for converting, into a sound, the output signal from the DTMF signal generator, and a data processor circuit for executing the key data processing according to the keying operation performed using the keyboard, and supplying a control signal to the DTMF signal generator, the data processor being responsive to the operation of the dial number key, to supply a DTMF signal to the keyboard and the sound generator, and responsive to the operation of the function key, to supply a single-frequency signal to the keyboard and the sound generator.

Abstract: An amplifier for controlling the volume in a telephone instrument of the general type including coin-operated and pay type telephone instruments is enclosed in the telephone housing and provides a number of discrete, predetermined amplification levels selectable by operating a push-button switch also located in the housing so that its plunger extends through an opening in the telephone instrument face. The amplification level is automatically restored to a volume associated with a normal hearing sensitivity at the start of a new call and a dial shunt circuit suppresses the DTMF tones heard by a user when tone dialing is used.

Abstract: An integrated circuit for the transmission of telephone signals is designed so as to be inserted in a speech circuit of a subscriber's telephone set and includes a threshold comparator having first and second input terminals coupled to the terminals of a two-wire telephone line and an output terminal connected to a point of the signal path of the circuit. The threshold of this comparator is set such that when the voltage at the terminals of the line drops to a predetermined value, which is not lower than the minimum voltage value required to ensure the correct operation of all the components of the circuit, it generates an output signal used to limit the amplitude of the negative peaks of the waveforms of the transmitted signals.

Abstract: To provide an adequate dc supply to the electronics of a telephone, when on hook or off hook, there is a transistor (T2) with its emitter-collector path in one line wire, and a storage capacitor (Cpsu) across the input to the telephones's circuits. This capacitor is charged up and maintains dc supply when the line voltage (at A) falls below a preset level. The base of this transistor is connected via another transistor (T1) to the other line wire. The speech output (V.sub.IN) is connected via an operational amplifier (A) to the base of the second transistor (T2). Hence the dc supply and the speech output are in parallel.

Abstract: An integrated circuit includes an interface circuit for coupling with a relative high-level VOICE circuit and a relatively low-level DIALER circuit to a subscriber line. A first shunt transistor is coupled across the telephone line and has its collector current forced by a current sink driven from the VOICE circuit. A second shunt transistor is coupled in a current mirror configuration to the first shunt transistor and supplies current to the DIALER circuitry. The interface circuit includes a saturation prevention circuit coupled to the current sink and to the second shunt transistor for sensing a tendency of the second shunt transistor toward saturation and for increasing the collector current load on that transistor in response. The saturation prevention circuit includes a voltage offset element and a sensing transistor arranged to form a loop with the collector-to-base junction of the second shunt transistor.

Abstract: A voice circuit of a telephone is provided with a differential amplifier and an impedance setting circuit. An output of an amplifier to amplify an output signal of a transmitter is supplied to the inverting input terminal of the differential amplifier. An output of the impedance setting circuit is supplied to the noninverting input terminal of the differential amplifier. An output signal of the differential amplifier is supplied to a line driver. A positive feedback loop is constituted by the differential amplifier, line driver, telephone line, and impedance setting circuit. By controlling a feedback amount in this positive feedback loop, an AC impedance is controlled.

Abstract: An amplifier having properties of natural side tone balancing based on the utilization of a bridge arrangement, high efficiency with resulting high gain, high AC impedance and compatibility with electret microphones and electret microphone amplifiers. A matched pair of differential transistors arranged as part of a bridge circuit form a detector of the receive signal at one input versus a transmit signal which appears at both inputs in the same phase and therefore is rejected as a common mode signal. The resultant balancing of side tone and high gain finds particular application in telephone networks where the characteristics dictate the need for higher than usual receive gain or for use by auditorially handicapped users over a normal telephone network.

Abstract: A transmit amplifier which also stabilizes the line direct voltage of the audio transmission circuit of a telephone set with respect to a reference voltage, the amplifier gain being adjustable to a high value by means of a resistor which does not affect the further properties of the audio transmission circuit. Variation of the line direct voltage by the offset current of the microphone pre-amplifier preceding the transmit amplifier, and by temperature fluctuations of the reference voltage, is greatly reduced by increasing the reference voltage and adding a temperature-dependent current source to a voltage divider which is comprised in the amplifier.

Abstract: A transistorized current amplifier based on a cascode stage feeding into a composite transistorized output stage. The amplifier employs a constant current source, decoupling (isolation) from the power source and a signal limiting network to lower distortion on high signal input conditions. A two-wire device, the amplifier derives signal and power from a telephone line. Surge protection is provided by a signal limiting network and the inclusion of relatively high value resistors in the circuit.

Abstract: A signal limiting amplifier includes interstage limiting devices to restrict the range of output voltage signals. The devices are base-emitter circuits of transistors, one of which operates in response to one polarity of signals and has its collector terminal unconnected, and the other of which operates in response to the other polarity of signals and has its collector circuit connected (when limiting) to operate an appropriate utilization circuit. In one embodiment, the limiting amplifier is applied as a common mode feedback amplifier in a telephone line feed circuit, and the limiter signal is used to limit large longitudinal signals on a customer line and to power down the line feed circuit in the event the limited signals persist beyond a certain interval.

Abstract: The telephone amplifier of an audio transmission circuit comprises a class-B output stage as a result of which the required supply current of this amplifier is considerably lower than with prior art telephone amplifiers. A distorting voltage produced by the current signal in the supply current of the glass-B output stage does not affect the output signal of the telephone amplifier because the input stage and the output stage each comprise their own setting circuits which produce mutually independent setting voltages.

Abstract: The subject suppressor is an improved form of echo suppressor used in conjunction with adaptive sub-band speech coders. The suppressor analyzes and uses N contiguous sub-bands of a sub-band speech coder. The intermediate results calculated during the normal operation of such speech coders may be analyzed in the suppressor to determine predictions for the echo levels of each sub-band and to set break in and break out thresholds. A substantial performance improvement with little increase in complexity over current echo suppressors is achieved. The subject suppressor may also be used with adaptive transform speech coders in which the corner frequency points define the edges of "sub-bands" for purposes of this invention. Sub-bands are defined here as contiguous segments of the frequency-energy spectrum.