18. An apparatus as defined in claim 15 , wherein the data unit encoder is to change the first text character in the source data to the second text character representative of the symbol by selectively replacing the first text character in the source data with the second text character.
15. An apparatus to encode auxiliary data into text data, comprising: a data unit group assignor to assign source data to one of a plurality of groups, the source data comprising text data; a symbol group assignor to assign a symbol to be added to the source data to the one of the plurality of groups; and a data unit encoder to generate encoded data by changing a first text character in the source data to be a second text character that is representative of the symbol and represents a same alphanumeric character or non-alphanumeric symbol as the first text character in the source data.