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There have been a number of people who have queried the need for independent shift operators (
sound) and actually they are probably not necessary for standard transposing instruments, where all that is required is to specify the transposition between concert pitch and the instrument's written pitch.
In fact, abc already has independent shift operators provided by the
transpose= modifier for
V: fields (which only affects the playback) and the semi-official
%%transpose directive (which only affects the typeset score).
So, aside from trying to avoid producing a new transposition specification which has less functionality than what is already available, the following presents a use-case for providing independent shift operators.
There are a number of instruments, particularly in the folk and traditional music genre, where independent shifts are required. Typically a lot of instruments in this genre are limited in terms of the keys they can play in. For example whistles, simple system flutes and button accordions are usually diatonic and so can only play in a few keys (e.g. a D whistle or a D/G accordion is essentially limited to D & G and their relative minors, B minor and E minor). Even with instruments which are fully or nearly chromatic, such as hurdy-gurdies and many types of bagpipe, the keys may be restricted by the drone note (e.g. a set of D pipes or a D/G hurdy-gurdy is essentially limited to D major, D minor, G major and G minor.
This has two immediate consequences, both of which impact on the transpositions required:
Consequently players of such instruments require two independent transpositions:
Example: Suppose a bagpipe player wishes to play the following two pieces:
X:1 T:French bagpipe tune K:Gmin GABc defg:| X:2 T:Scottish bagpipe tune K:Amix ABcd efga:|
First they need to be put into preferred written pitch (e.g. six finger D):
X:1 T:French bagpipe tune K:Gmin score=GD GABc defg:| X:2 T:Scottish bagpipe tune K:Amix score=AD ABcd efga:|
Now suppose the bagpiper decides to play them on F pipes and wants to produce a midi file to practice with:
X:1 T:French bagpipe tune K:Gmin score=GD sound=GF GABc defg:| X:2 T:Scottish bagpipe tune K:Amix score=AD sound=AF ABcd efga:|
However, don't forget that the piper typically has several sets of pipes, and so the
sound modifiers will change for each different set of pipes.
Conclusion: independent shift operations are already in use in abc files and are very much a requirement for an update to the transposition syntax.