Telephone subscriber line interface circuit - has level shifting function which makes it transparent to speech frequencies

摘要

The interface circuit has four cascaded bi-directional amplifiers (1,2,3,4). The subscriber's appts. (Zsub) is connected in parallel to two amplifiers (1,3), one (3) of which provides a high impedance shunt to high frequencies and a low impedance, e.g. 300 ohms, shunt to low frequencies. The other amplifier (1) connects to the intermediate circuit (2,4). The intermediate circuit has three amplifier (1,2,4) terminal pairs in parallel. One is the other side of that (1) connected to the subscriber, another is the one (2) connected to the PABX switch matrix (Zpabx). The third amplifier supplies a frequency-dependent shunt impedance. The level shift function of the circuit makes it transparent to speech frequencies, i.e. 300 to 3400 Hz, but also enables detection of on/off hook conditions.