Higher order DC-constrained line codes

The authors consider the use of multilevel line codes which generate sequences with higher-order spectral nulls at DC for the suppression of low-frequency spectral contents of the digital signal so that a baseband speech signal can be inserted at low frequencies. The use of these line codes is proposed for the asymmetrical digital subscriber line (ADSL). The new line codes exhibit a large dead band at DC, and can thus be considered for use on the control channel of the ADSL. A further advantage of using the block codes is that the additional spectral nulls in the pass band of each of these block codes make it possible to insert pilot tones at these frequencies, to aid with functions such as clock recovery at the receiver