Abstract: A method for detecting the presence of a predetermined dual tone signal on a conductor including steps of performing threshold detection on the conductor for detecting the presence of signals having an amplitude greater than a predetermined threshold signal, and in the event of detection of a signal having substantial amplitude, detecting the periods of a predetermined number of successive cycles of the signal, and in the event the successive detected periods are different, detecting an average value of the periods. In the event the average period corresponds to a frequency equivalent to the average frequency of the predetermined dual tone signal, an output signal is generated indicative of the presence of the dual tone signal. This method can advantageously be used for detection of dial tone on a telephone conductor. A method is straightforward and can be easily implemented by a microprocessor program in conjunction with simple threshold detection circuitry.

Abstract: A digital progess tone detector, for connection to a telephone line and a PABX, comprising circuitry for generating one or more reference tones having user definable frequencies and performing single frequency discrete Fourier transforms on one or more signals received from the telephone line at the defined frequencies, and providing an output signal to the PABX in response thereto. The output signal provides an indication of whether or not a predetermined one or more progress tone frequencies have been detected in the received signals. Because the generated reference tone frequencies are user definable, the present invention can be easily adapted for use in various foreign countries having different progress tone frequency requirements. A preferred embodiment of the invention is implemented on a single microcomputer chip in conjunction with peripherals (such as codecs, crosspoint switches, etc.).

Abstract: A digital progress tone detector, for connection to a telephone line and a PABX, comprising circuitry for generating one or more reference tones having user definable frequencies and performing single frequency discrete Fourier transforms on one or more signals received from the telephone line at the defined frequencies, and providing an output signal to the PABX in response thereto. The output signal provides an indication of whether or not a predetermined one or more progress tone frequencies have been detected in the received signals. Because the generated reference tone frequencies are user definable, the present invention can be easily adapted for use in various foreign countries having different progress tone frequency requirements. A preferred embodiment of the invention is implemented on a single microcomputer chip in conjunction with peripherals (such as codecs, crosspoint switches, etc.).

Abstract: A transformerless, fully balanced, telephone line circuit is provided. Two differential amplifiers are employed, one for incoming signals and the other for outgoing signals. The latter drives the line with two out-of-phase, equal amplitude signals. A comparator detects grounding of either TIP or RING at the telephone to accommodate international applications.

Abstract: A subscriber line interface circuit for coupling a balanced communication line with unbalanced bidirectional or a pair of unidirectional communication lines. Line current is fed to the line via a low resistance path for maximum current supply, via a circuit which matches the line impedance, and also provides high resistance protection to the output of amplifiers feeding the balanced line. Since a low resistance d.c. current feed is used, a voltage boost is not required. However lightning protection is obtained for the outputs of the amplifiers which feed the line, at the same time as virtually optimum line impedance matching and sufficient line current feed. The circuit is comprised of a balanced line including a lead pair, a pair of matched feed resistors for providing a low resistance path, each connected to one of the pair for applying a d.c. line current from a power source to the line, the total resistance of the feed resistors being a fraction of the d.c. line impedance.

Abstract: This invention is a circuit for reducing the power requirements for a subscriber's line where the loop length is short. The circuit senses the line current in the off-hook condition, and if the line current is above a predetermined threshold, it decreases the power supply voltage. When the line is restored to the on-hook condition, the original high voltage is restored. A substantial reduction in the cost and size of the power supply is thus obtained.

Abstract: A telephone switching system for interconnection of a plurality of lines with a number of trunks in which telephone sets having no trunk access pushbuttons can access particular trunks upon command. Upon initiating a command signal, i.e. by a subscriber dialing particular access codes, the switching system provides a voiced enunciation of the number of an idle trunk or the number of which trunk is on "hold". The subscriber can then dial the trunk number followed by the number of an outgoing call, or an access code followed by the held trunk number whereby the line is connected to the particular accessed trunk.

Abstract: A trunk circuit comprises a first lead pair for carrying two-way signals, a second lead pair for carrying two-way signals, a circuit for electrically coupling an incoming signal from the second lead pair to the first lead pair, a first transformerless circuit for applying an incoming signal from the first lead pair to the second lead pair, and a second transformerless circuit for preventing signals from the first circuit applied to the first lead pair from being reapplied to the second lead pair and for preventing signals from the first transformerless circuit applied to the second lead pair from being reapplied to the first lead pair. Preferably the coupling circuit is comprised of a transformer having a primary winding connected in a low impedance voltage source circuit to the second lead pair, and having a secondary winding connected in a very high impedance-to-direct-current circuit for applying the incoming signal from the second lead pair to the first lead pair.

Abstract: A telephone line circuit includes balanced tip and ring leads for connection to a telephone set and an unbalanced line output lead. A first differential amplifier has its input terminals connected through individual equal-valued resistors to the tip and ring leads, the values of each resistor being at least 10 times the off hook impedance of the telephone set. The output of the first differential amplifier is connected in a circuit path to a line output lead, for applying outgoing signals thereto. A second differential amplifier has its input connected in a circuit path to a line input lead for receiving incoming signals, and its output connected in a circuit path to one of the tip or ring leads, and connected by a further resistor to the input of the first differential amplifier which is connected to the other of the tip or ring leads.

Abstract: A multifrequency dialing tone to 2/8 decoding circuit which does not require individual filters for distinguishing individual tone frequencies. The circuit converts the period of each of the received multifrequency signals to individual voltage amplitudes which corresponds thereto, then compares the voltage amplitudes with a predetermined plurality of voltage ranges. Correspondence of a voltage amplitude level with one of the ranges causes production of an output signal, signifying the presence of a particular acceptable multifrequency tone. Individual correspondence signals operate latches which apply signals to decode leads which carry the decoded output signal.

Abstract: A non-blocking switching network which has considerably fewer crosspoint switches than would otherwise be expected. The network is comprised of a primary stage having n inputs and m outputs, a plurality of secondary stages each having m inputs and p.sub.1, p.sub.2, . . . p.sub.p outputs, output m.sub.x being connected via switching crosspoints to all of the outputs p.sub.x, p.sub.(x+1) and p.sub.(x+3), where x is an integral number of the m.sup.th output from 1 to m/2, output m.sub.y being dedicated and connected via switching crosspoints to all of the outputs of the y.sup.th secondary stage, where y=an integral number from m/2+1 to m.

Abstract: A transformerless two-way trunk circuit particularly useful for interconnecting a balanced lead pair and an unbalanced lead pair, with complete control over sidetone and signal level. The balanced lead pair is terminated with a first matching A.C. impedance and a second D.C. resistance means. A first transformerless circuit means applies an incoming signal from the unbalanced lead pair to the balanced lead pair, and a second transformerless circuit means applies an incoming signal from the balanced pair to the unbalanced lead pair. A third transformerless circuit means cancels signals from said first circuit means applied to the balanced lead pair to prevent them from being reapplied to the unbalanced lead pair, and a fourth transformerless circuit means cancels signals from said second circuit means applied to the unbalanced lead pair to prevent them from being reapplied to the balanced lead pair.

Abstract: A transformerless bidirectional amplifier useful in a balanced two wire to single wire line circuit, which blocks common mode signals and allows full control of signal levels, impedances, etc. A differential amplifier circuit is connected to the balanced wires, and has its output connected through a buffer amplifier to a single unbalanced wire. A second differential amplifier has its respective input terminals connected to the input and output of the buffer amplifier. The output of the second differential amplifier is connected to one of the balanced wires, and through a resistor to one of the inputs of the first differential amplifier which is connected to the other of the two balanced wires. The value of the latter resistor is such as to apply sufficient output signal from the second differential amplifier to the first differential amplifier so as to cancel a signal from the second differential amplifier which is applied to the first differential amplifier via the first wire of the balanced pair.