EEPhil said “I suspect the .hjm is a JSON file describing a class/structure "hjm" in a programming language.”. I don’t think the j represents json. Wikipedia says “Humax is a consumer electronics company. Founded in South Korea in 1989, it manufactures set-top boxes, digital video recorders and other consumer electronics. It is publicly traded on KOSDAQ.” and the Korean word I thought of makes nts roughly translate to “index of ts file”. Using the same logic on the hj* files leads me to believe I remember the file format :-
{"<structure name>":"<preamble><data>"}
, where non-printable bytes were represented by the escape sequence %nn, where nn is a 2 digit hexadecimal number with the alphas in upper case. I remember using it because you could compile a program for the big endian mainframe and the little endian PCs and use it to transfer data bidirectionaly between the two. The preamble described the data structure in tearms of ‘symbols’ (not to be confused with the symbols used by ld and rtld) because the data was compressed to not contain any zero bytes.
If anybody reading this was programming at the time when mainframes and PCs overlap, can you remember the language? I remember getting a crash course in mixed language programming in order to make a program written in C perform the same trick.
Edit 24 April 2022: The language was PL/1. Looking through my stone tablets (sorry 5.25” disks) and I found the C wrappers and the 286 compilers. Dead end I’m afraid. 49/50 returned an error meaning corrupt file, the other one returned an error meaning bad compression!