Your question is difficult for me to understand.
If you have a number, then if you use num2str() you would end up with multiple characters unless the number was exactly one of 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9. If you use char() on a number then you would end up with one character if the number was an integer in the range 0 to 65535, and you would get warnings if you had negative values or you had any fractions.
Once you have your character or characters, you could arrange them in a matrix, but a matrix of characters cannot be used as an image by any MATLAB routine that I can think of. The MATLAB display routines or image-saving routines require matrices which are of the data type uint8, uint16, or double (some might permit single)
The connection between the numbers in the file and the conversion to characters "based upon maximum entropy thresholding" is not something I do not understand. You said that the coefficients were generated from "a data/signal", which implies that the data was not originally character, so I do not know why you want to output as character.
It would make more sense to me if you had not referred to characters at all, and had instead said that you needed to create a matrix of uint8.
