Data coding apparatus in which input data having a range of possible data values and a smaller sub-range of legal data values is coded by applying a coder mapping which maps input data values in the range of possible input data values and corresponding coded data values within the range of possible data comprises means for detecting, for each coded data value, whether the coded data value lies within the sub-range of legal data values; and means for remapping a coded data value lying outside the sub-range of legal data values to an unused coded data value lying within the sub-range of legal data values.