We have developed a track circuit using cyclic transmission of digital codes in a low frequency range. Each digital codes indicating allowable speeds for trains, i.e. ATC signal, is composed of 10 bits. By using the low frequency range as a carrier frequency for digital code transmission, we can control track circuits longer than 1.5km. Furthermore, we have selected the carrier frequency to avoid harmonics of 50/60Hz power source. Thus, we will be able to reduce construction costs by introducing ATC and a cab signalling system to the projected Shinkansen lines and the high speed sections where trains run at 160km/h or over on the conventional narrow-gauge lines in the near future. In this paper, we explain the feature of this system, and report model simulation and field test results.