I'm a keyboardist in my band at church, and I guess I'm somewhat behind the times, because every other keyboardist I see in the media has a Macbook hooked up to their keyboard/synth. I'm guessing this is for midi?

I've done some stuff with midi (mostly midi files) for playback on the keyboard.

I'm just wondering of what the purpose is of having the comp hooked up to the keyboard during performance? If so, what is the common program they are using with the purpose?

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Probably CakeWalk (I use Sonar 8) which allows, amongst other things, live multi-tracking.

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Pone ubi sol non lucet!

Excuse my ignorance on this - hopefully I'm not going too far off subject.
Does anyone know, can Sibelius be hooked up to a keyboard for composition?

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"I don't see much sense in that," said Rabbit.
"No," said Pooh humbly, "there isn't. But there was going to be when I began it. It's just that something happened to it along the way."