Here is a quote another forum on the subject that is pretty accurate. One is used by music education based on the grand clef created by the Acoustical Society of America.and another is based on American Standard Pitch Notation (ASPN). There are two major standards for octaves nomenclature/numbering.
The MIDI specification only defines Middle C as note number 60. The short answer is that MIDI notes are simply a note index and do not really define the pitch of a note. I'll finish by saying that C4 is the system that I will use in the here and now. Of course, that would bring its own issues.
The software engineer in me has the insane idea that a zero based counting system should be used for no other reason than it is a logical as opposed to normative basis for a standard. I'd really like to see a formal poll or process started by an accredited source such as The MIDI Association or the MIDI Manufacturers Association in an attempt to investigate the factors of the 'C-X" question to the ends of delivering an official recommendation. That said, unfortunately, even after 33 years, there is no recommendation with regard to octave numbering by the MMA at present.
I was lucky enough to get some feedback from and it seems that C4 is presently the unspoken semi-formal convention with regard to octave numbering, and there are many good reasons to use it. I'm pretty much leaning in the same direction but for no other reason than convenience. Thanks for your view and the linked article Pedro. Just a little uncomfortable with the guess work I guess! I am now considering running with using C4 to equate to middle C and put the rest down to inaccuracies. I think thats ok to accept as an immutable rule. They have provided published a living standard in relation to certain CC numbers for example. Then again, maybe already have done some work on this and I've missed it. Conventions are great, but a concrete spec would be handy IMO. I was just curious if there was an official standard or specification on this matter.
Closer to my present context (in the sense that I am presently writing a MIDI processor for use in Logic Pro X), it is not difficult to see similar questions arise That said, this page suggests middle C is C3, and this seems to suggest C5 (?). Here is another site that seems to suggest that C4 is an ISO standard and back referencing the first link, so to does Scientific pitch notation. According to this article, Many manufacturers consider middle C to be C4. I suppose the confusion point was in relation to different manufacturers and standards out there.