| 摘要 |
<p>PURPOSE:To minimize noise cased at polarity inversion and to attain miniaturization by decreasing gradually the talking current, inverting the polarity when the current reaches '0', increasing gradually the current succeedingly to bring the current to a required value. CONSTITUTION:When a pre-stage device is an exchange of a central office and an opposite party is a public telephone set and the set is busy, a DC loop of battery E-photocoupler 7-brake contact rv-choke coil 4-talking line terminal(b)- opposite device (telephone set)-talking terminal (a)-choke coil 4-brake contact rv-transistor 6-battery E is formed and a talking current is flowing. A coin deposition signal is given to a timing generating circuit, a control signal is fed to a base of a transistor (TR) 6 depending on the number of seconds from a cotnrol section at talking, and when the talking current is decreased gradually and reaches just '0', an output relay RV of a timing generating circuit is oeprated, a contact rv is closed to invert the polarity of the talking current. The control signal from the timing generating circuit is decreased, the talking current is increased again gradually and restores to the required current.</p> |
