[Athanasy]

OS: MacOS High Sierra (I know, I know...)
ROM: .nds (came out in 2008)

I have an .nds ROM that I've unpacked through UniArchiver, and the types of data files the ROM uses seem to be on the more uncommon side of things. The voice clips all use .ahx (which I finally managed to convert to .wav, thank you Wine), the 'movies' use .vx, there's one nsb folder with nothing but .nsb files (still don't know how to access/open/convert those yet, but I'd like to if possible)...

...And the vast majority of the files are using the .nnt extension. All the files under [face] (which I assume are the character's faces, aka sprites), all the [bg] files (background?), all the files under [card] (possibly the character portraits?)...you get the idea. I don't want to edit the files, I just want to be able to open/access their contents. You know, I'd like them to be - well - usable.

(Note: I ended up extracting the files again using VGSC through Wine out of a flimsy notion that maybe it was simply something to do with the extractor, but came out the same way).

Google Searches and looking through utilities (both hex editors & unpackers), game databases and subject-specific sites - I've been searching to no avail. The .nnt extension seems to be mostly recognized as a Qualcomm Eudora Addressbook database file (of all things!)a and a few file extension sites claim that it's also used in the Close Combat III game data files, but other than that - I can't find anything on this file extension with respect to game ROMs. This surely can't be the only game or visual novel out there that uses .nnt, so what gives?

I'm grateful for any ideas/explanations/etc out there, because I definitely need external help with this. The .ahx files I finally managed on my own, but .nnt has me stumped. 

[KingMike]

If it's not something recognized by common NDS tools then it's likely a custom format.

File extensions are arbitrary and devs can call them whatever they want, with a good chance that SOMETHING else unrelated has already used that extension.

[Athanasy]

Ah, custom format is a good point. I did come across some games which had unique formats while researching, now that you mention it; one site I visited actually featured a person explaining what they thought each individual letter stood for in the extensions for one game's files. There was no reason for me to think that 'my' game would somehow be exempt from such a possibility...I don't know why that didn't click for me until now. Whoops. (It's not as if I don't know why designers might use unique formats; they might be deliberately trying to make the files less accessible for copyright reasons/so as to protect their assets, for one...)

So would my only option here be something along the lines of developing a decoding tool/attempting to reverse-engineer the proprietary file format? I fully recognize that something like that is probably overly ambitious/out of my current depth, I should say. When I saw your reply, I went and did some reading on the subject over at Wikibooks – it sounds like I'd have to closely examine the file via my hex editor--> identify/find potential structures --> document structures --> and then test theories by creating a manipulated file. Having some familiarity with encoding techniques, compression + checksum languages, and programming languages would be...(understatement upcoming) ah, highly recommended when carrying all of that out.

So - right. The question at hand isn't whether or not I'd have any chance of succeeding in the above scenario (I can...hazard a guess) – I just want to confirm if that really is the only recourse. 

(I'll end this reply by acknowledging the potential legal ramifications of reverse engineering a possibly protected file format, which the wikibooks page did make a point of mentioning. If reverse engineering the files really is the only way I'd be able to open/access them, then I'll probably lean more on the side of caution than risk. Like I said, I have no interest in modding the files or using them for monetary gain; I'd like them as reference material/for archival purposes...but I admit, the more I read about the subject the more tempting it is to see if I could [*someday?] be capable of it. At the very least, it would be a useful learning process/experience...  Yeesh, I'm getting ahead of myself. If it's the only way, it may be safer and wiser to accept the circumstances as they are.)

[FAST6191]

You do not necessarily have to do it yourself, someone else could reverse engineer it.

As far as legal woes go then I am not aware of any as far as console games go for reverse engineering, indeed legal woes in general are very few and far between.

I wrote a guide specifically focusing on the GBA and DS a while back.
http://www.romhacking.net/forum/index.php/topic,14708.0.html