| 摘要 |
In our Application No. 8005000 (GB 2071461A) we have described a telephone line circuit which includes a programmable (i.e. adjustable) voltage source (VVS) connected in series with the line in one wire (B) thereof, and also a constant-current source (PCR) connected between the two line wires. Line conditions are monitored, and the line supply as defined by these two sources is so controlled in accordance with line conditions as to minimise power dissipation. Thus for a short loop, voltage can be reduced. The present invention relates to the supply of ringing current in such a circuit. The ringing current is applied from a low-level supply to the variable voltage source (VVS), so that it is amplified and modulates the DC line feed. In addition there is a further controlled current device (RS) on the line side of the voltage source (VVS) which is enabled when ringing current is present. This device by virtue of the current it diverts during ringing regulates the AC ringing current level as applied to the line. <IMAGE> |
