| 摘要 |
The present invention discloses a reduced power dissipation telephone subscriber line feed circuit in which the battery feed current from the central office is determined by the subscriber loop resistance for medium to long loops, and for shorter loops, for which the battery feed current would exceed a given predetermined threshold, the feed current is limited to the threshold value. The invention also relates to compensation for loop and subset attenuation characteristics. During operation in the current limited mode, attenuation is automatically inserted into the signal transmission and receiving paths in order that normal equalizer operation will not be impaired by current limiting of the battery feed current. A multiple telephone subscriber system is disclosed wherein the circuitry used in deriving current limiting control signals is shared in a multiplexed arrangement by a plurality of subscriber lines. Line feed current is limited in discrete steps to a predetermined threshold value in each subscriber loop upon the detection of an OFF HOOK condition when the feed current drawn by the subset exceeds the threshold value. A plurality of discrete signals are derived by sensing the amount by which the line voltages exceeds a predetermined selected reference voltage value. These discrete signals are used to determine both the effective battery voltage which will produce a d.c. feed current corresponding to the limiting value and the amount of attenuation to be added at the central office by the line circuit to compensate for the equalizer effect in the subscriber set which occurs as a result of the current limiting. |
