Optical record carrier

In a run length limited block coding method in which blocks of m data bits are converted to blocks of n1 information and n2 separation bits (n = n1 + n2>m). The blocks of bits satisfying the requirement of being (d, k) run length limited, the blocks of separation bits BSi between each of the blocks of n-information bits Bli are chosen, in those cases where the format is not prescribed by the (d, k)-constraint, such that the low-frequency spectrum and particularly the direct current unbalance is as low as possible. The direct current unbalance may be minimized within each block independently or cumulatively over a series of blocks. A demodulator for decoding data encoded as above is described.