FL Studio will accept almost any midi controller surface if it is supported by your system as such. You may need driver software, but FL can use generic surfaces to have your keyboard as a controller, you only need to assign the keys and controls to what you want them to do in FL.
Writing trance music is another issue... generally, trance, techno and electronic music does not need a person to actually play an instrument in real time, it is often inserted into a song in step time. If you have a sound in mind, then you can probably have FL play to your midi device, though I don't see the point of this since you may be able to find an emulated CTK-2100 plugin with everything you need and use FL as the composition workstation for the plugin sounds.
If this CTK-2100 is a small digital audio workstation (DAW) in itself, then it is rather pointless to have FL Studio as the external DAW to send data to the keyboard. You may as well use a lawn mower to jump-start a jumbo jet. FL Studio is a fantastic program to create unique trance music right out of the box, but if you are a good keyboard player and you would rather play in real time, then your Casio will be a great control surface to use FL's built-in features.
Casio CTK 401
One could purchase a Casio piano keyboard from a local musical instrument shop. On the internet the best places to purchase a Casio piano keyboard is either eBay or Amazon.
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The best headphones that will work with Casio LK-120 Key Lighting Keyboard are stereo headphones.
If the Casio is has a MIDI output, you can use it as a MIDI controller, but you wont be able to use the patches in the unit. If the Casio just has an RC out, you can plug in into the PC and have FL Studio record the Casio output using the Edison plugin.
On the corner shop.
The notes for Happy Birthday on the Casio keyboard are quite easy. The exact notes depend on the key that the song is to be played in. If the song is played in the key of C then the first note is G.
I'm pretty sure this keyboard was manufactured in 2002
1/8 size or size 4
I don't know? but maybe a key or something
There are a wide range of companies that manufacture keyboard instruments. The most popular ones include Roland, Yamaha and Korg. Some other electronic companies produce keyboards too, like Sony.