Abstract: A data transmission system is described which includes a switching circuit that enables the connection of a tributary modem to a control modem via the public switched network. Switching occurs when it is no longer possible to transmit data on the leased network because the line connected to the tributary modem has failed. The switching circuit can connect either the permanent leased line or the switched network to the receiver of the control modem. It comprises a narrow pass-band filter and detector which is permanently connected between the reception side of the switched network and the control modem receiver. When the tributary modem answers a call from the control modem, it sends a tone ahead of the data message, the frequency of which tone corresponds to the frequency of the pass-band filter and is out of the data band.

Abstract: Echo cancelling device for cancelling echoes affecting the incoming signal received over a two-wire telephone line, comprising near echo canceller 20, far echo canceller 22 and an adder 26 providing the estimated echo signal. A circuit path including a digital-to-analog converter 48 for converting the digital estimated echo signal into analog form and a subtracter 52 for subtracting the resulting analog signal from the analog incoming signal, is separated from a circuit path including an analog-to-digital converter 42 for converting the analog incoming signal into digital form and a subtracter 44 for subtracting the digital estimated echo signal from the resulting digital incoming signal thereby providing a digital signal used as a control signal for adjusting the echo cancellers 20 and 22.

Abstract: Echo cancelling above for providing an echo-free incoming data signal with a phase-roll tracking device comprising phase rotating means (58) for rotating the phase of the generated echo signal by a value X which is the estimated phase-roll value, thereby providing a complex phase-roll corrected echo signal a multiplier (124) for multiplying the error signal; between the incoming signal and the real component (112) of the phase-roll corrected echo signal by the imaginary component (120) of the same signal, thereby providing a value Y function of the variation of the phase-roll; a phase-roll variation estimator (128) providing an estimation Z of the phase-roll variation in half duplex mode or in full duplex mode, in response to value Y; and a phase filter (126) providing the estimated phase-roll value X in response to the estimation Z.

Abstract: The purpose of the receiver is to indicate the reception of a multifrequency signal and to identify said signal. A multifrequency signal is a signal combining two predetermined pure tones, one belonging to a high-frequency range, the other to a low-frequency range. The received signal is sampled at 8 KHz and digitally converted. The signal provided by the converter is simultaneously processed by two tone detectors. Each tone detector comprises: a bandpass filter covering the high- or low-frequency range and supplying the components (x, y) of the filtered signal in Cartesian coordinates; decimation means at the output of said filters; a converter converting the Cartesian coordinates (x, y) into polar coordinates (.rho., .theta.); and logic circuits analyzing the variations of said .rho. and .theta. over fixed periods of time to derive therefrom an identification of the received tone.

Abstract: A switching network includes electronic crosspoint switches for providing electronic connection or disconnection between two voice stations. Each station is connected to the network through a line transformer. When two stations are connected to each other, the voice path in the network includes the secondary winding of the 1st station transformer, a series of crosspoint switches, a middle junctor, a series of crosspoint switches, and the secondary winding of the 2nd station transformer. Each middle junctor includes a negative resistance circuit which introduces a negative resistance in series with the voice path, thus compensating for the series losses in the network, and two negative inductance circuits connected on both sides of the junctor in parallel with the voice path, to compensate for the parallel losses, especially those introduced by the line transformers.