Abstract: An apparatus compensates for an attenuation of a signal on a telephone line in a communication system connected to the telephone line via a transformer. A photo coupler detects a loop current at a primary winding of the transformer. A controller generates switching control signals corresponding to an output of the photo coupler based on a predetermined switching table, and controls an overall operation of the communication system by receiving coding data. A level amplifier having different gains amplifies a signal at a secondary winding of the transformer by the gain corresponding to the switching control signal. A coder codes an output from the level amplifier to generate the coding data.