DIAL TONE DETECTING CIRCUIT FOR COMMUNICATIONS SYSTEM

摘要

The dial tone detector detects the level and frequency of the dial tone sent from an exchange. The detector comprises a first detector for amplifying input signal with a first gain and for generating a first level detect signal when the amplified input signal level is higher than a reference level, a second detector for amplifying input signal with a second gain and for generating a second level detect signal when the amplified voltage level is lower than a reference level, and a discriminator for generating input signal detection signal when a first and a second level detect signal is generated.