Re: Contemporary music required feature #1: chromatic transposition

[Hans Aberg]

On 22 Sep 2009, at 01:09, Joseph Wakeling wrote:

> Currently Lilypond's transposition is tonal only, with the 'smart
> transpose' snippet providing a Scheme function to minimise accidental
> use.  Unfortunately this function is incompatible with quarter-tone
> notation.

Discussing with Graham Breed, I got the impression that LilyPond does  
it correctly, only that it insists on substituted values for the  
intervals.

When transposing, roughly, one transposes intervals and scale degrees  
separately. The scale degrees may be identified with the pitch values  
of the staff.

Then, on top of that, one may impose enharmonic equivalents with  
respect to some tuning system. (E12 is a favorite.)

> The attached snippet shows the problem.  A quarter-tone
> chromatic scale is displayed, first untransposed; then untransposed  
> and
> run through the naturalizeMusic function; then transposed a quarter- 
> tone
> up; then transposed up and run through naturalizeMusic; and finally,  
> run
> through naturalizeMusic and then transposed.

So here you transpose a quarter-tone and a scale degree zero. All note  
heads should remain in the same position in the staff; only  
accidentals change.

> What you see is that
>
>   (i) without naturalizeMusic, transposition fails: transposition  
> alone
>       leaves the final pitch being 'g+5/4' which has no accidental

I think is just a bug. Somehow the sharp drops out.

>   (ii) naturalizeMusic saves the transposition, but alters accidentals
>        which don't need to be altered -- e.g. g-3/4 is transformed to
>        f+1/4.

Here, the most obvious thing for a machine to do, is to merely impose  
E12 enharmonic equivalents in order to minimize the number of  
accidentals. So the problem is to figure out a rule for the changes.

  Hans