A line filter is used in a speech-plus-duplex system for the purpose of?

A line filter in a speech-plus-duplex system is used to separate the outgoing speech signal from the incoming speech signal, thereby preventing echo and feedback. It ensures that the two signals do not interfere with each other, allowing for clear communication. By filtering out unwanted frequencies, the line filter enhances the overall audio quality and improves the user experience in duplex communication systems.