| 摘要 |
<p>A substation circuit in accordance with the present invention includes a light emitting diode (LED) connected in parallel with the speech network of the associated telephone set, the speech network including a carbon transmitter, a receiver, and a hybrid coil The LED is connected in series with a high ac impedance to prevent voice signals from being attenuated by the shunt path, and the LED is shunted by a varistor to provide high current and surge protection. The relative resistances of the illumination circuit and the speech network are such that at least one-fifth of the line current derived when the telephone set is connected to the longest telephone line loop is shunted through the LED path The speech network compensate are selected so as to substantially compensate for the reduction in transmit level resulting from the reduced current flow through the transmitter.</p> |
