A circuit for detecting a digital data stream includes a first detecting circuit, a second detecting circuit and a decision unit. The digital data stream includes a predetermined symbol having a first data and a second data. The first detecting circuit detects a correlation between the second data and a first frequency-shifted data to generate a first correlated data; and the second detecting circuit detects a correlation between the second data and a second frequency-shifted data to generate a first correlated data, where the first frequency-shifted data and the second frequency-shifted data are generated by performing first frequency-shifting and second frequency-shifting upon the first data, respectively. The decision unit determines a signal status of the digital data stream according to the first and second correlated data.