| 摘要 |
A power-conserving, loop start signalling, dual-voltage line-feed circuit that maintains ac longitudinal balance on the loop conductors and which limits average and peak loop current not only when the loop is in the off-hook transmission state but also during part-time on-hook transmission. The current-limiting circuitry employs feedback so that a virtual ground is presented toward the line feed transformer at voice frequencies to reduce battery noise and potential crosstalk that might otherwise couple into the circuit because of non-zero battery impedance. The circuit provides for injecting a voltage-controlled feed current into the current regulator to maintain sufficient drive current so the current regulator can continue to maintain the virtual ground when the loop is on hook. The circuit employs a battery switch and switch hook state detector which controls the connection of a higher voltage battery to the loop when the loop is on-hook and a lower voltage battery when the loop is off-hook to conserve power. To reduce the likelihood of oscillation between on and off-hook indications, the switchhook status detector responds to a loop resistance threshold that is lower when the high voltage battery is being used than when the low voltage battery is being used.
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