Baseband codes are distinguished from passband codes (See Section 2.3) in that baseband codes can transmit energy at DC (f = 0), while passband codes transmit at a frequency spectrum translated away from DC. Baseband line codes in DSLs have N = 1 and appear in the earliest DSLs: Examples are T1, ISDN, and HDSL.
The 2B1Q baseband line code is heavily used in early DSLs. The 2B1Q line code ideally uses a basis function
 The sinc function is
which leads to the independence of ...