string-make-unibyte

string-make-unibyte is a built-in function in `C source code'.

(string-make-unibyte STRING)

Return the unibyte equivalent of STRING.
Multibyte character codes are converted to unibyte according to
`nonascii-translation-table' or, if that is nil, `nonascii-insert-offset'.
If the lookup in the translation table fails, this function takes just
the low 8 bits of each character.