<p>A method comprises: receiving plural data portions; forming plural data frames, each data frame comprising one of the plural data portions conjoined with horizontal error correction data generated on the basis of the one of the plural data portions; and generating at least one vertical error correction portion on the basis of the data portions by creating plural elements of vertical error correction data, each element of error correction data being generated on the basis of corresponding elements in each of the plural data portions, each vertical error correction portion comprising plural error correction elements.</p>
