Michael Bedesem's post explains everything there is to know about the megavoices. But also Yamaha explained this many times and if you have the manual it is explained too. The datasheet of Tyros and PSR3000 shows you exactly how the multisamples have been arranged across the keyboard.
Normally the velocity sensitivity only controls the volume (and perhaps filtering) of voices. Some effects, like the sweet! flute and some other voices, use velocity switching too, but in an easy controlable way.
You can argue if Yamaha should have programmed the megavoices in an other way. In fact they did that with the guitar Overdrive and Distoriton voices. You should be able to play this voice, because their buildup is simpler than for instance the Steel Guitar voice.
Overdive and Distortion voices:
C-B5: velocity= 1-55 is Open, 56-120 is Mute, 121-127 is Pick Harmonics.
Above C6 you have all the different effects for this voice.
Michel Voncken lets you hear these effects on the keyboard in his video.
Check the Data sheet of the PSR3000 and you can see why some megavoices are impossible to control by playing (mainly SteelGuitar and CleanGuitar).
With training you could try to play the following megavoices:
HiStringGtr, Overdrive, Distortion, 12StrGuitar, Fretless Bass and maybe the PickBass. (Note that Yamaha didn't use typical solo-instruments for megavoices, maybe that's why the Overdrive and Distortion voices have an easier buildup so they can be used in a solo).
Mind you, I didn't try it myself, but I draw my conclusion from the PSR3000 (and Tyros) Datasheet. But I also remember a mp3 played live(!) on Tyros, of someone playing a megavoice (it was probably the fretless Bass).
An advantage of playing these voices is, that you can use the special effects (above C6) too.
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drdalet [This message has been edited by drdalet (edited 09-04-2004).]