In MIDI, Middle C (~261.6 Hz, depending on how you tune Concert A and which tuning system you use) has a Note value of 60. If you aren't sure which key on your keyboard (normally) outputs a Note value of 60, you can check the manual to see if it says so, as the specifications page will often list the number of keys and what Note values they're (normally) associated with. Of course, if you transpose or octave-shift the keyboard then that will affect the Note values generated by each key-- and if you split or layer multiple voices, with each voice having different octave or transpose settings, then it's possible for a single key to generate multiple Note values on different MIDI channels, as well as for multiple keys to generate the same Note value on different MIDI channels.
If your keyboard's manual doesn't say what the (normal) Note range is for the keys-- which would mean it's not a very good manual
-- you can clear or disable any transposing and octave-shifting, then use a MIDI monitor to see which Note values are being sent. As Gary said, for a typical C-to-C 61-note keyboard, Middle C is always(?) the third C from the left.
One thing to beware of is the annoying (to me, anyway) practice that companies and software writers have of changing the "name" of the notes to suit their own personal preferences. In "Scientific Pitch Notation" (a.k.a. "American Standard Pitch Notation" and "International Pitch Notation"), Middle C is referred to as "C4." One explanation I've read is that this is because it's the fourth C key (starting from the left) on a standard piano keyboard, but the real reason is because the person who originally proposed this pitch notation-- Harvey Fletcher, I think (see
http://asa.scitation.org/doi/10.1121/1.1916017)-- recommended using ~16.35 Hz as the reference pitch, or "C0." Unfortunately, some companies and software writers use notation which looks like Scientific Pitch Notation yet which really isn't, because they refer to Middle C as "C3" (e.g., Yamaha does this) or "C5" (e.g., I think Cakewalk does this). So it's important to go by the MIDI Note numbers rather than by whatever note names are listed for the keys, since the MIDI specifications don't allow for any such ambiguity regarding the Note values.
[Dear Diary: Today I gave myself a blinding headache by trying to make sense of keys, notes, note notations, reference pitches, tuning systems, octaves, scales, chords, and the number of white keys versus the number of black keys. Why do octave numbers change from B to C instead of G to A? Why do notes go from A to G instead of A to L if there are 12 notes in an octave? Why is the interval from one key to the next a "semi-tone" instead of a "tone"? And don't get me started on MIDI channel numbers and MIDI program numbers. I mean, if somebody asks me to send him something on channel 1, should I send it on channel 0, 1, or 2? Maybe he numbers the channels from 0 to 15 and I number them from 1 to 16, or vice versa? It's no wonder there's so much strife and discord in the world if people can't even agree about such simple things! The next time I start to think about these issues, please remind me not to start thinking about them, or to at least have some aspirin handy. Well, Diary, I hear the kettle whistling, so I have to go for now. Until next time, I remain faithfully yours.-- Perplexed]
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