A powerline communication (PLC) power supply and modem interface can be implemented using a power supply processing unit coupled with a PLC modem unit. The power supply processing unit generates a composite PLC signal comprising a PLC signal and a DC power signal modulated with a zero cross signal (all determined from an AC powerline signal). High-powered components of the PLC modem unit can cause signal distortion in the zero cross signal component of the composite PLC signal making it difficult to extract zero cross information. An error correction unit can be implemented at the PLC modem unit to minimize the signal distortion and generate a zero cross signal with little or no error. The PLC modem unit also extracts the PLC signal and the DC power signal from the composite PLC signal, and processes the PLC signal using the zero cross information extracted from the corrected zero cross signal.