Using a binary-mapped constant weight modulation code (BCWMC) for the recording channel of a digital holographic data storage system can facilitate the decision on the detected signal.The authors propose a new modulation coding scheme

6:8 Gray-mapped constant weight modulation code (GCWMC)

in which each digital data having length 6 is viewed as a Gray codeword and the constant weight code is designed with a characteristic of binary 2-unit distance.Thus the 6:8 GCWMC scheme can possess the ability of its all the 8-bit codewords to bear some error when a two-digit swapping event occurres in an estimated codeword.Simulation results show that the 6:8 GCWMC scheme can outperform the 6:8 BCWMC one in the bit-error rate (BER) performance at low signal-noise ratio (SNR) for the system.