To provide a method by which data are transferred from a digital data modulator MOD to a digital data demodulator DMOD via a digital subscriber line TL by means of the ADSL(asymmetrical digital subscriber line) technology or the VDSL(very high-speed digital subscriber line) technology.

The digital data modulator MOD includes a control circuit MCTR that controls operations of a zero data cell generator, a rotation circuit, and a shift circuit when detecting idle data. The digital data demodulator DMOD includes cascade connection of a detection circuit DDET to detect received zero data, an inverse shift circuit DTRN that shifts a constellation point having the lowest power to a point corresponding to the zero data cell, an inverse rotation circuit DRAN that inversely rotates the constellation point, a data decoder or an inverse mapping device DMAP and a trellis code demodulator TCD.