How many octaves can a Midi keyboard record? Please help!!!
Well, the limit are the 128 notes of Midi, a bit more than 10 octaves. If you want to be obnoxious, you can pitch bend outside of that range, and also use tuning commands. But that's more or less what the format will support, and it depends on the physical keyboard range and its abilities for transposition in octaves and single notes how far you'll get.
It's probably worth pointing out that the 10.5 octaves of Midi are not entirely useful for musical purposes. The lowest note, C-1, is about 8Hz. Even C0, 16Hz, is outside the range of human hearing although its overtones aren't. At the upper end of the range, G9 corresponds to a frequency of about 12.5kHz. This is somewhat below CRT whine but not very much so. While not inaudible to good hearing, the note is of little usefulness for music. A standard 88-key piano runs from A0 to C8, meaning that Midi still has 21 more notes at the bottom, and 19 more at the top.
A full-size Midi piano keyboard will also have 88 keys. Transposition facilities, when available on the keyboard, often span additional 11 notes (sometimes only 6) in either direction. Exploiting that range would rarely seem to make a lot of sense, and will be out of the range of actually sampled notes (where acoustic properties correspond reasonably to a real instrument) anyway.
128 notes (abot 10 octaves and half) are defined in MIDI protocol (see below). It means that you can record from C0 to G10, where C4 is the "central C" of the piano. These are the extreme boundaries of notes that you can notate through MIDI protocol. It does not mean that these are the notes that you will hear in terms of frequency. Think, for example, to a - virtual - percussion set driven through MIDI: each note corresponds to a timbre, not to a frequency. Take also into account that your instrument could not carry out MIDI data all over that range. In case you are recording to play deferred performances (not in real time) or to carry out music sheet and you are using a sequencer or digital notation software, you can always transpose your execution by the range you need.