Re: Key signatures in MIDI output

[Hans Åberg]

> On 10 May 2024, at 21:50, mskala@ansuz.sooke.bc.ca wrote:
> 
> On Fri, 10 May 2024, Hans Åberg wrote:
> 
>> Programs like ABC work so that one writes the music without accidentals,
>> and then apply a key signature to get them. It was my reading that the
>> OP asked for that.
> 
> Okay.  I didn't read it that way because the OP said he was getting
> correct output in the PDF, and if he'd misunderstood the input format in
> the way you describe, then the PDF would be wrong too.
> 
> But something else that occurred to me is that there may be a further
> misunderstanding of MIDI format in play here:  MIDI never contains
> "accidentals" at all.  It only contains note numbers.

MIDI is based on E12, the 12-equal tone system. The staff notation, as used in 
LilyPond, is originally based on Pythagorean tuning, without enharmonic 
equivalences, which must be applied for the MIDI output.

>  In my example code:
> 
> \score {
>  \new Voice {
>    \key c \major
>    c'4 d'4 e'4 f'4 |
>    \key d \minor
>    bes4 a4 g4 f4 |
>  }
>  \layout { }
>  \midi { }
> }
> 
> The MIDI output will contain roughly this information:
> 
> key change, C major
> note 60
> note 62
> note 64
> note 65
> key change, D minor
> note 58
> note 57
> note 55
> note 53
> 
> If some other software, reading the MIDI file, displays these as notes
> with or without accidentals, and does or doesn't make the key-change
> events visible, then that has very little to do with Lilypond, which only
> generates the above data.  Note 60, for instance, might be a C or a
> B-sharp.  Even if it is B-sharp, it might be displayed with or without a
> sharp sign depending on whether there is one in the displayed key
> signature.  The MIDI file does not contain that information; it is up to
> whatever software reads the MIDI file, to display it appropriately.

So to go back to staff notation from MIDI, one must know what enharmonic 
equivalences that have been applied.