1,172,089. Automatic exchanges; line transmission systems. SUPERIOR CONTINENTAL CORP. 24 Nov., 1966, No. 52747/66. Headings H4K and H4R. [Also in Division G5] A carrier telephone system (Fig. 1) includes a central office multichannel carrier terminal unit 21 linked via a two-wire transmission line 23 to remote single-channel secondary terminals 24, so called line tap units, each of which attenuates outgoing carrier signals in dependence upon the level of incoming carrier signals to the unit, whereby all signals arriving at the central terminal from whatever line top unit are of the same level. Several subscribers 25 share a line top unit and are selectively rung. Channel units 21 of the control terminal define particular carrier frequency channels and each serves a particular one of the line tap units. Low frequency power is fed over the line to all line tap units where it is rectified to provide subscriber's battery voltage. Low pass filters 49 by-pass the low frequency power around line repeaters D, and provide an inductive loading to maintain line voltage. Central office terminal (Fig. 1). The lines from the central switching equipment 52 are coupled via transformer 30 to the receive and transmit sections of corresponding channel units 22. Outgoing speech signals are compressed before modulating a carrier from oscillator 37 which passes via band-pass filter 34 to a two-wire transmission line 23. Incoming carrier signals from a line tap unit are received in the corresponding channel unit receiver section 42 through band-pass filter 41 and after amplification are demodulated in detector 39 and then expanded. Ringing signals generated in the office signalling equipment 51 consist of normal ringing current on which is superimposed one of five marker tones to distinguish one of five subscribers sharing a line tap unit; capacitor 307, however, blocks the ringing current so only the marker tone modulates the carrier. Off-hook condition and dialling impulses are provided by the transmission of carrier from a line tap unit; upon the detection of carrier at 39 a relay 309 operates to close the D.C. loop to the central switching unit 52. Line tap unit 24 and subset circuits (Figs. 1 and 6). Low frequency power is rectified and regulated in 143 to provide 6 volts D.C. voltage over the secondary of transformer 138 to five subsets 25. Off-hook condition of a subset biases diode 149 into conduction, thereby enabling amplifier 146 to transmit carrier to the central terminal unit. Dialling impulses successively disable the amplifier to transmit a train of carrier bursts. Marker tone transmitted to a line tap unit operates via a uniquely tuned circuit in each subset, one of the five subsets' bell circuits (described in abridgment of Division G5). A local battery powers the bell circuit of a subscriber which is trickle charged from the line during off-hook conditions. Subscriber speech signals pass to the transmission section 24 of a line tap unit for compression in 141 and modulate a carrier from oscillator 142. The level of the outgoing carrier signal is adjusted by attenuator circuit 145 in dependence upon the level of the incoming carrier signal. In the receiver section 129 incoming carrier signals are demodulated in a detector 133 and expanded at 136. Automatic gain control is provided by a regulating circuit 134 in which a signal from the detector after comparison with a reference voltage 174 controls via transistor 171 the conduction of diodes in the attenuator circuit 167 in the receiver section and the attenuator circuit 145 in the transmitter section. Compressor and expander (Fig. 6). The expander 136 of the receiver section of a line tap unit, which is similar to those in the central terminal unit, comprises a transistor 189 whose gain is controlled by capacitor 195 and diode 196 whose series impedance is controlled by the D.C. amplifier 188 responsive to the V.F. energy input. The compressor 152 of the transmitter reaction of a line tap unit, which is similar to those of the central terminal unit, comprises a V.P. amplifier 207 whose output is shunted by diode 204 whose impedance is controlled by the D.C. amplifier 205 responsive to the signal level at the input of the V.F. amplifier. Repeater or line amplifier (Fig. 9). Outgoing carrier signals from the terminal unit pass through band-pass filters 257 and 259 and the lower frequency incoming carrier signals pass through low-pass filters 260 and 261. An amplifier 258, with a filament lamp 270 and tuned circuit 267-269 in its feed-back path, provides frequency dependent automatic gain control.