Well I don't own an arranger..... but I do use a drum machine a lot. Which IMHO is almost like an arranger (Don't flame me please!! Obviously it's missing a lot of instrumentation [rhythm guitar, horns, bass, piano, strings, etc.], but the basic function is the same) If I were to design the perfect arranger keyboard it would have the following: (some of these things may already exist BTW)
1) A full 88-key semi-weighted keyboard (being a piano-player, I really would need the 88-keys; semi-weighted so it doesn't weigh 60+ lbs.)
2) NO ONBOARD SOUND SYSTEM!!!! (I might get flamed here too!
) But most of us keyboard players have amps already...... and I would much rather have that much less weight to lug around (seeing as I already added a lot more with the 88-keys)
3) Tons of pre-programmed styles (around 300 or so......) with many different styles (but not just 1 or 2 of each style.... around 5-10 of each style)............ with a "style-category" button that would let you easily flip through different kinds of styles (Soft, Rock, Jazz, Ethnic, etc, etc.....) Maybe even another level beneath those broad categorys. (Hard Rock, Classic Rock, Heavy Metal, etc, etc)
4) Very easy to program (but still very good quality) user styles.......... and lots of room to store them!
5) A good expansion library for more patches (and styles!); These expansion boards would be designed for specific styles of music (rock boards, soft rock/slow boards, piano based music boards, etc, etc) similar in size to the Roland SR-JV series library. These boards would include approximately 150 or so styles in the genre of the board.
6) A very, VERY good vocal harmonizer built-in...... with the ability to change your voice's gender and pitch (soprano voice, alto voice, etc, etc........ country style voice, hard rock voice, etc, etc) These voice set-ups could be assigned to each style... also each style would have a second set-up that would by-pass the gender and pitch change, but would still add effects such as reverb.
7) Along the same line as the vocal harmonizer...... an input for an external mic, guitar, another keyboard, etc. This input would be able to add effects that are suitable for whatever you plugged in (reverb for mics, distortion/chorus/etc for guitars, etc etc etc for any other instrument you could want to plug in) You could then mute this part in the style and have someone else play along with you. (Actually thinking about it now.... make it about 5 instrument-specific inputs, so you can have a whole band play with you if you wanted!)
8) At LEAST two different fills for each variation of a style (one to go back to the same style [going from one verse right into another one] and the other for going into the next part [into the chorus]) And AT LEAST three different variations (Verse, Chorus and BRIDGE!!!!!!) I don't think I've seen a drum machine yet that has three different variations...... almost all bridges sound completely different from the verse or chorus. Also endings and intros, of course.
9) An input for a specially designed pedalboard that has pedals that go to any of the different parts of the style (one pedal to go to variation A, one for B, one for C.... two pedals above "variation A" pedal, one for fill A (that would go back to variation A) and one to do fill B which would go on to the next part (the chorus). Actually I'd have three fill pedals per variation.... so you could go to any of the the three variations. (Verse, Chorus, Bridge) And then a couple other pedals to do things such as style changes, patch changes, and an expression pedal tossed in for fun!
10) And last but not least..... all the basic specification things: LOTS of polyphony, LOTS of waveform memory, LOTS of onboard controllers (pitch bend, mod., ribbons, knobs, etc, etc). A nice sturdy metal chassis. You get the picture!
That's about it....... I probably left out all kinds of stuff..... but oh well.
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Romans 6:23 For the wages of sin is death, but the gift of God is eternal life through Jesus Christ our Lord.