Re: MIDI time signature

[Jürgen Reuter]

   Hi David,

   in case you or someone else is going to refactor the time signature
   code, please also consider the discussion that we already had about
   this topic in May 2002.  See here:

   https://lists.gnu.org/archive/html/lilypond-devel/2002-05/msg00088.html

   The suggested syntax as of then would have known the difference between
   6/8 (=3+3 8th) versus 3/4 (=2+2+2 8th) thanks to the prolatio property.

   Best wishes,
   Jürgen


   On Sun, Jun 6, 2021 at 11:32 PM David Kastrup <dak@gnu.org> wrote:

     Ok, I have a problem here.  Our MIDI time signature codes are wrong.
     The problem that I have in fixing this is that part of the
     information
     in a MIDI time signature is actually (typically) specified in \tempo
     instead.
     Here is how this works: a MIDI time signature contains numerator
     (fine),
     denominator (coded like our duration-log), midi clocks (of which
     there
     are 24 per quarter note) per beat, and 8 (32nd notes per quarter
     which
     we keep constant).
     The midi clocks per beat is basically what we could get from the
     closest
     \tempo ... = duration specification, by taking duration in terms of
     whole notes and dividing by 96.
     This value is hardcoded as 18, corresponding to 8. (3 16th notes).
     That
     does not make a whole lot of sense.
     The sane way would be to take the information from the currently
     valid
     \tempo (if specified) or otherwise apply a time signature specific
     default (for 6/8, we don't want 6 beats per measure but 2, for
     example).
     I'd argue that if we have
     \tempo 60
     \time 6/8
     then we should likely interpret this (regarding the MIDI playing
     speed)
     as
     \tempo 60 = 4.
     \time 6/8
     which would produce a different MIDI file rendition speed than what
     we
     have now, because that would be the most reasonable interpretation
     for
     the _visual_ rendition of
     \tempo 60
     \time 6/8
     In other words: looks like a long-standing mess in some need of
     cleaning
     up.  Another possible information source may be the beat grouping
     information for a time signature (used for beaming patterns).  That
     one
     will turn out to be a puzzler for things like
     \time 3,2 5/4
     However, we could try to pick the gcd from the beat groupings and
     use
     that as a multiplier for the beat length (typically the time
     signature
     denominator).  That would be tempo-independent and probably correct
     more
     often than not.
     And tempo-independent would leave us with a quite smaller head-ache.
     --
     David Kastrup



   https://lists.gnu.org/archive/html/lilypond-devel/2002-05/msg00088.html