Data transmission system and method, and data sending apparatus and receiving apparatus

A data transmission method comprising:

• a step that delays a signal having a predetermined base frequency so as to generate a reference-phase signal having a reference phase, a delay signal which is more delayed than the reference signal by a predetermined unit of time, and an advance signal which is more advanced than the reference signal by the unit of time;
• a step that selects one of the reference-phase signal, the delay signal, and the advance signal so as to output a data signal in which a phase of one of the rising edge and the falling edge of the data signal varies in accordance with values of transmitted data, and a phase of the other of the rising and falling edges is constant;
• a step that generates a reference signal at the timing of one of the rising edge and the falling edge of the data signal, said one having the constant phase;
• and
• a step that reproduces the data by measuring a phase difference between the generated reference signal and the data signal at the timing of the other of the rising and falling edges which has the varying phase.

A data receiving apparatus for receiving a data signal in which a phase of one of the rising edge and the falling edge of the data signal varies in accordance with values of data, and a phase of the other of the rising and falling edges is constant, wherein:

• the data signal is a signal output based on three signals which are generated by delaying a signal having a predetermined base frequency and are a reference-phase signal having a reference phase, a delay signal which is more delayed than the reference signal by a predetermined unit of time, and an advance signal which is more advanced than the reference signal by the unit of time;
• the apparatus comprises:
• a PLL circuit that generates a reference signal at the timing of one of the rising edge and the falling edge of the data signal, said one having the constant phase;
• and
• a data reproduction circuit that reproduces the data by measuring a phase difference between the generated reference signal and the data signal at the timing of the other of the rising and falling edges which has the varying phase.