Abstract: A call signal generating circuit includes a DC/DC converter, an ON/OFF control unit, including a control circuit and a phototransistor, for ON/OFF-controlling an output voltage from the DC/DC converter, and a polarity inverter cooperating with the control circuit, for switching polarities of an ON/OFF-controlled voltage. The ON/OFF timings of the ON/OFF control unit and a polarity switching timing of the polarity inverter are adjusted to generate a call signal having an arbitrary frequency.

Abstract: A central office line card with distributed signaling voltages comprised of a plurality of subscriber line interface circuits connected to a common signal bus. Standard ringing signal is received from a remote generator within the central office and applied to the common signal bus via a first relay. A receptacle is provided on the line card for receiving a removable module containing circuitry for generating further signaling voltages such as party line ringing and coin telephone test and control voltages. Signals from the module are applied to the common signal bus via a further plurality of relays. Each of these subscriber line interface circuits is connected to the common signal bus via additional respective relays.

Abstract: The functionality of the keypad of a telephone test set is augmented by an interface circuit arrangement which enables keys of the keypad to be used to control both dialing signal generation and prescribed test functions, while effectively preventing interference between the operation of a dialing signal generator, to which the keypad is coupled during a first mode of operation, and that of test function signal generator circuitry, provided exclusive of the dialing signal generator, during a second mode of operation. Additionally, the test set includes a ringing signal coupling circuit for enabling the test set microphone, through which a telephone craftsperson voices audio speech signals to a speech signal processing circuit, to be used to audiblize the ringing signal, without impacting the normal voice input functionality of the microphone and associated speech signal processor.

Abstract: Detector circuitry monitors the line voltage on a telephone line to detect an off-hook condition. In order to accomodate varying line voltages and noise conditions, the circuitry automatically selects a threshold off-hook voltage by an iterative initialization procedure which is initiated on circuit power-up. The intialization procedure repeatedly compares the threshold voltage to the actual line voltage over a period of time to eliminate false off-hook detection due to noise.

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: A signal transmission control apparatus connected between a telephone line and a terminal device controls signal transmission therebetween in no-ringing service. First DC and AC circuits of the apparatus become conductive in response to a DC control voltage from the telephone line. The first DC and AC circuits are connected to the telephone lines and the leads of a primary coil of the transformer of the apparatus. The secondary coil of the transformer is connected to the terminal device and an amplifier and first detection device of the apparatus. A subsequent AC calling signal from the telephone line, received on the first AC circuit, effects conduction in second DC and AC circuits through serial operation of the first detection circuit, a control circuit, a switching transistor and closure of relay contacts.

Abstract: An improved offhook supervision circuit including an amplifier connected to a subscriber loop through the tip and ring conductors of a subscriber line circuit. The amplifier is arranged to develop and output a loop voltage signal representing the level of the DC signals applied to the subscriber loop circuit. A comparator device has a first input connected to the amplifier and is arranged to receive the loop voltage signal output by the amplifier. A second input of the comparator device is connected to a reference voltage device which outputs a predetermined reference signal voltage. Any significant reduction of the DC signal level on the loop circuit will cause the loop signal voltage output by the amplifier to fall below the reference signal voltage. An offhook signal is then output from the comparator device to a control device signaling an offhook condition.