Subscriber line interface circuit which automatically adjusts the signal headroom on a telephone line and a method of doing the same

摘要

In a subscriber line interface circuit connected to a two-wire telephone line (A and B lines) for transmitting AC signals, one of the wires (e.g., the A line) is set to a DC voltage lower (i.e., more negative) than a first DC voltage (GND), while the second wire (e.g., the B wire) is set to a DC voltage lower than the DC voltage of the first wire, but higher than a second DC voltage (VREG). A first voltage generator is provided for setting a first threshold voltage (VTHA) between the first DC voltage (GND) and the DC voltage of the first wire. A second voltage generator is provided for setting a second threshold voltage (VTHB) between the second DC voltage (VREG) and the D voltage for the second wire. The first and second threshold voltages are utilized for transmission and reception of AC signals on the respective A and B wires. As long as the instantaneous value (VA) of the signal on the first wire (A) equals the first threshold voltage (VTHA) and/or the instantaneous value (VB) of the signal on the second wire (B) equals the second threshold voltage (VTHB), the subscriber line interface circuit automatically adjusts the signal headroom to the actual amplitude of the AC signals by controlling the voltage difference (VA) between the first DC voltage (GND) and the DC voltage of the first wire (A) and/or the voltage difference (VB) between the DC voltage of the second wire (B) and the second DC voltage (VREG).