How to unpack content of UMD image

[Junisha1]

Hello everyone,,,i have a general question. I want to examine the contents of a PSP game.
The Image i have is in ISO format, which i can simply look into by double clicking it in my Windows 10 explorer.

Inside i see some small files but one who gets my attraction, it is called "ROCKY6.BIG". This file is about 400 MB in size and i suspect that it contains the game itself.

Further inspection with HxD Hexeditor shows "[ GEAR BigFile ]" as first bytes, which may be some sort of magic-bytes.

I then search the net to find infos for this type of file. Only one tool, the Dragon Unpacker, in combination with a special script was able to unpack the stream into single files. All tools handling BIG file extenion where not able to unpack this.

The files are named with hex-names and have extensions like .wav, .dat, .xml and such.

I then tried to play those wav files, but no success. The file signature is RIFF...WAVE... but it seems to have the waveforms coded in an unusual manner.

Now i'm stuck and wonder how those wav files can be decoded and played. I tried to import them with Audacity as raw data as 8 bit mono 44khz but it only plays noise.

Can someone please give me some hints how to go on from here!?

What is this special Gear Bigfile and how to deal with it?
How can i find out the right codec of those wav files?
Where are the graphics objects or 3D models in?

Thanks a lot, from an absoute beginner....