Well I own the SX... and played the GX for a bit when I was hired at a jazz camp and those were the differences I noticed. Pretty sure I'm not wrong on all of them, why not specifically tell me what I'm missing rather than being rude with a blanket statement like that? But okay, I will indulge. I've pulled up the RD-300sx and the RD-300gx on Roland's website to compare, just to be sure of some stuff.
1. The screen is different. You say I'm wrong? Umm... really? So to you, the LED numbers on the SX is the same as the screen on the GX that displays the name of the sound in letters and whatnot? Okay so on the site, for the SX it says the display is "7 segments, 3 characters (LED)" on the GX it is "16 characters, 2 lines (Backlit LCD)" (the stuff in quotes I copy and pasted).
2. The zones. In the SX, there is upper and lower. In the GX there is Upper 1, Upper 2, and Lower. While I did not specifically program any splits when I tried out the GX, I'm guessing this means you can have one sound in the lower range and two (layered, i.e. dual) sounds in the upper range. I could be wrong, I suppose I didn't try it, but Upper 1, Upper 2, and Lower seems pretty self-explanitory. On the site it says the SX has "Zone Level slider x 2" the GX has "Zone Level slider x 3 (Assignable)" (Assignable? Okay that's pretty cool, didn't know that).
3. The action. Well, my fingers don't lie, and having played an RD-300gx and an FP4, they seemed similar. However, since you demand research, on the website:
the RD-300sx keyboard: "88 keys (Compact Progressive Hammer-Action Keyboard)"
the RD-300gx keyboard: "88 keys (PHA alpha II Keyboard)"
the FP4 keyboard: "88 keys PHA alpha II keyboard"
notice it says PHA alpha II keyboard for both the RD-300gx and the FP4, whereas the RD-300sx says compact progressive hammer-action, meaning likely the first one... i.e. older technology.
As for the effects being mapped differently, I don't have a GX with me and wouldn't really be able to research it. Ok so on the roland RD-300sx, the second rhodes sound (the deeper one, sound number "2" on EP) doesn't have an effect mapped to it by default, meaning when you turn on the keyboard, hit EP and hit number two, if you check the multi-effect, it is 0... which is nothing. Pretty sure on the GX they put a tremolo on it, but I could be wrong, again I don't have one with me. Same thing with the wurly. The percussino organ, number 3 on the SX, has the leslie effect on it. When I tried one in the store briefly, I put it to that sound and there was chorus on it. In hindsight someone could have chaned that and left it before I got there, so there I could be wrong.
Anyway, if I'm mistaken in something specific, please, feel free. As you can see, I have done my research. Now where's yours?
