Telephone test set-installed low pass filter circuit for preventing corruption of digital communication signals

An arrangement for preventing corruption of digital data that may be transmitted over a telephone line in the course of coupling a voice band telephone device to said telephone line, said arrangement comprising:

• line coupling communication ports adapted to couple said voice band telephone device with said telephone line;
• and
• a low pass filter circuit that is effective to provide a low impedance path and pass therethrough only voice band signals, while providing a high impedance to, and thereby effectively blocking, the transmission of signals that fall within the high frequency range of digital data that may be present on said telephone line, said low pass filter circuit having first ports thereof coupled to said line coupling communication ports, and second ports thereof coupled to said voice band telephone, and including inductors coupled with respective tip and ring line segments of a lead pair by way of which said voice band telephone device is connectable to said telephone line, and a controllably switched, shunt-capacitor circuit coupled across said tip and ring line segments, and resistors that compensate for impedance variations across said lead pair and are controllably coupled in series with line-coupling leads adapted to be coupled to said telephone line.