Well, thats a good question and theres a lot of choices out there and a lot of options that are available, basically you need to get two tracks from the keyboard and one good vocal track into a recording device. The simple way is to go into a mixer and into a two track deck of some sort, say a cassette for low cost or a DAT or ADAT , the ADAT will allow you to put a lot more than three tracks down.
If you already have a computer, youre partway there. You can get a good sound card they are from 299 and up past 1200 depending on how many inputs you get. Some soundcards will allow you to put in from four to eight inputs and you'll probably eventually want as many inputs as you can afford because you might want to go back and add more parts to your mix or you might get another keyboard later, you will also need software to run your sound card.
Another option is the simple and cheap four track, if all you want to do is create a simple demo they are great for it providing you have a good mike for recording.
There are also relatively inexpensive digital multi trackers out there now and some not so inexpensive ones. I have only grazed the surface here, but maybe you have some idea. It all really depends on your budjet and your preferences, if you are computer savy and dont mind doing everything with keystrokes and a mouse go computer.If you like tactile control get something hands on.
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Smitty
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Smitty