A control unit for establishing a four-wire connection between two terminals via a pair of telephone lines comprises switching means which are operable to provide a connection to a pair of telephone lines, signalling means connectible to said lines via said switching means to enable another control unit to be signalled via a line, and line send and receive circuitry connected to the switching means. The line send and receive circuitry has an input for connection to the send channel of a terminal...

A temperature compensated, direct current feedback loop stabilizes the output level of an audio oscillator and gives the oscillator a negative effective output impedance, so that the oscillator can maintain a low distortion, constant level sine wave across the terminals of a remotely located load.

An electro-acoustic terminal unit for use in an audio teleconferencing system comprises a loudspeaker and one or more microphones each having a sensitivity which is directionally dependent and exhibits at least one null or substantially null position. The loudspeaker and microphones are rigidly mounted on a boom and the microphones are so located and orientated relative to the loudspeaker that the null position is directed towards the loudspeaker.

An audio amplifier including a non-inverting amplifier stage that provides linear amplification without loop feedback for low distortion audio systems. The amplifier stage includes a triode vacuum tube or a triode connected pentode vacuum tube and a P channel field effect transistor (FET). The vacuum tube and the FET are connected in a series combination in which the source of the FET is connected to the cathode of the tube through a suitable resistance. The drain of the FET is connected to a ne...

Disclosed is a Class A power amplifier circuit which includes means for controlling the quiescent bias current of the output devices of the amplifier and providing absolute overload and short circuit protection without the need of extra protective devices of any kind. The circuit includes a driver stage which amplifies an input signal and provides two outputs which are out of phase with respect to each other by 180.degree.. One output of the driver stage provides a drive signal to the control in...

In active element of an audio amplifier, there exists a neutral point in which the amplification is minimum. When the middle point of AC mode in a circuit is coincident with the neutral point, noise elements accompanying with input signals are not amplified, i.e. the signal-to-noise ratio of an audio amplifier is improved. The middle point of AC mode in a circuit can be varied by adjusting resistors or capacitors of the circuit.

An audio distributor interconnects a source of at least one electrical audio signal and a plurality of sound-producing speakers to sequentially apply the signal to each speaker in serial fashion. The distributor includes a support member having mounted on an outer surface thereof a number of interconnected conductive segments, each segment corresponding to one of the speakers. A motor rotates the support member, moving each conductive segment into contact with an electrical pickup to establish e...

An audio amplifier which processes mainly dynamic signals and which is provided with a plurality of amplifying systems each having a preamplifier and a main amplifier. A plurality of power sources are used to separately supply electric power to the respective amplifying systems, resulting in elimination of transient crosstalk.

An audio dosimeter for individual use determining exposure to sound energy as a function of both frequency and pressure level, with integration over the time of exposure and incorporating storage means preserving a quantitative measure of the exposure.

The audio compass includes a first polarized transparent sheet affixed to a magnetic needle of a magnetic compass, and second and third polarized transparent sheets affixed to a graduated compass ring rotatably mounted on a housing for the magnetic compass and having their planes of polarization offset from each other by an angle which is bisected by the polarization plane of magnitude first sheet when the zero mark of the compass ring is set at north. Light from a first light source shines thro...