The carrier telephone system is mainly composed of terminal equipment, sound-increasing equipment and transmission lines. ① Terminal equipment: It is composed of sending part, receiving part and corresponding carrier frequency supply device. The transmitting part converges each channel of audio signals after one or more modulations to form a line transmission frequency band. The receiving part demodulates the line frequency band transmitted by the other party one or more times, and restores them to each audio signal. The carrier frequency supply device supplies the carrier frequency and pilot frequency used for each level of modulation. ②Amplification equipment: mainly composed of line amplifier, equalizer (or equalization system) and automatic level adjustment equipment (or adjustment system). Line amplifiers are used to compensate for signal loss in line transmission. The equalizer is mainly used to correct the frequency characteristic deviation caused by the line and equipment during the transmission process. The function of the automatic level adjustment device is to adjust the deviation that changes with time by means of the line pilot. Amplifying equipment buried in the ground or installed in an underground manhole does not require maintenance personnel to take care of it frequently, and is called an unmanned amplifier (referred to as an unmanned amplifier). Amplifying equipment is generally equipped with a long-distance power supply system (also known as a remote supply system), a fault location system (also known as a remote measurement system) and a cable leak alarm system (also known as a remote signal system). According to needs, the sound-increasing equipment can be equipped with an automatic conversion backup system to improve the reliability of the system. ③ Transmission lines: there are overhead open wires, cables, submarine cables, power lines, wireless UHF and microwave relays, etc.