Here is an FAQ I wrote that I'm trying to get pinned:
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The GP32 comes in 4 flavours: nlu (non-light unit), flu (front-light unit), blu (back-light unit) and blu+ (back-light unit, Taiwanese screen).
NLUs were manufactured first. They (and FLUs) require the user to register the GP online before non-commercial software can be used. Note that homebrewed and BLU firmware is available to update the launchers, so a NLU or FLU can run a BLU launcher.
FLUs have an issue with dust getting behind the screen. There are fixes. The front light washes out colours to some extent (this is common with front lights) but quality is still very good.
BLUs have an updated launcher that does not require registration to play non-commercial games. The launcher also boots faster. The back light makes colours very vivid, giving the blu and blu+ the best quality screen by far.
In late 2004 GamePark sourced their screens from a different (Taiwanese) manufacturer. These units are called BLU+. The BLU+ designation was devised by the GP32X board members to differentiate the units. On the box the BLU+ is still called BLU. If you are unsure if your unit is BLU or BLU+ you can hit L,R,R,L,R,R,L,L to enter test mode. If the unit is BLU+ you will see the following: "FW Version :1.6.6 (2004.10.10)"
Two issues were immediately apparent when the BLU+ was released:
1) A thick white bar about 1/3 of the way down the screen made some software unuseable.
2) The very top row of pixels (or 'scanline') was moved to the very bottom of the screen in most software.
Much was made (understandably) at the time about these problems, particularly the white bar.
HOWEVER, a software fix for the white bar issue has since been incorporated into almost all programs, apart from a few that are no longer being developed anyway. In addition, sometimes an older version of a program will work ok.
The following emulators have been tested and do NOT have the white bar issue:
Little John (NES)
FSMS32 (MS)
Park o rama (SC3000)
SNES9XGP (SNES)
CaSTaway (Atari ST)
GPEngine (PC Engine)
FGB32 (GBC)
Fgen32 (Genesis)
DrMD (Genesis)
FZX32 (Spectrum)
Doom
Mame (Arcade)
Fenix homebrew games
The 'moved scanline' issue has been harder to fix. Currently around half of the emulators do not APPEAR to move the scanline, despite no fix being incorporated into their code.
BUT as most games have borders it is almost impossible to tell whether the scanline has been moved or not. Even (rarely) when the scaline is clearly moved it is a VERY MINOR issue that is easily ignored as it is so far from the playfield. Generally, at worst a few lighter pixels will appear at the bottom of the screen, very rarely some movement can be observed. Also, the unit plays best when tilted slightly away from the eyes (so that the player is not reflected), which hides the bottom scanline, as the screen is recessed into the body of the GP.