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This paper addresses low frequency range power-line communications (PLC), where the transmitters are output voltage and bandwidth limited. We propose an efficient transmission method of combining Reed Solomon (RS) coding with run length limited (RLL) codes. Our proposed transmission model consists of three main parts: RS encoder followed by a one or two bit(s) look-ahead RLL encoder and 2-FSK modulator. The resulting transmission has a constant envelope signal modulation, no bandwidth expansion and more concentrated spectra in the carrier frequencies. This is an important result, directly applicable to the CENELEC band. In addition, a soft decoder based on minimum distance criteria for the RLL codes is introduced to yield an additional coding gain. We present the spectral analysis of RLL coding. Finally, the performance of the detectors is analyzed theoretically as well as through simulations.