By no means do I assert that this is the only way that abc might represent this music, nor do I assert that this is the best way. It is merely a possible way.
For the sake of audio output, composition, and simplicity, there is no reason not simply to use the original abc notation for all pitches available on a piano. All abc audio output programs should support arbitrary octave shifts using commas and apostrophes.
On the other hand, if a particular clef is needed, or if the music is intended for an instrument which transposes between the written and played pitch (as more than half of all orchestral instruments do), or if a particular typeset layout of notes on staves with ledger lines is required, the original abc notation does not suffice.
Discussions on how to extend abc to clefs other than
treble began on the abc mailing lists in 1997-02. The
discussions resulted in the pronouncement that the simplest ASCII
notation was preserved if the pitch denoted ``c'' were
always the C pitch closest to the center of the clef, or equivalently,
if the pitch denoted ``C'' were always the C pitch just
below the staff lines. This schematic is depicted in the GIF
linked in the heading of this paragraph.
Using these staff notations it should be possible to transcribe music from any single-staff instrument. Those instruments whose parts sometimes switch from one clef to another can also be handled using the clef notation described in Henrik Norbeck's BNF of proposed abc syntax.
In addition to the information in the GIF, I propose that
rests on the lower staff be denoted ``Z'' and that
rests on the upper staff be denoted ``z''.
There are several instruments which are fundamentally different from others. These include the piano, celesta, harpsichord, harp, and organ. Such instruments have a pitch range which covers a large number of octaves, and they have the ability to play most or all of these pitches simultaneously.
Music for these instruments is traditionally set on the ``grand staff''. Usually this consists of two adjacent staves with the lower one being bass clef and the upper one being treble clef. However there are numerous exceptions to this where both staves are treble, both are bass, or one or both staves are offset by one (or two) octaves from default. And it is extremely common to see ledger lines which permit one staff to hold notes which would more naturally fall on the other.
When notating music for such instruments in abc it is certainly simplest to use two separate staves whenever possible. However, there are numerous musical idioms which cannot be expressed by voices on separate staves.
When resolving the above issue it must be noted that more than half of all instruments are transposing. This raises the question of whether pitches in abc should be notated as written or as played, and this applies even to instruments which use the treble clef. It might be a good idea to conduct a survey of sight-reading musicians to determine the answer. My own impression, which is supported by those musicians whom I have interviewed, is that abc should be notated as written, and that explicit instructions for the amount of transposition should be included for audio playback applications.
For the sake of playback applications there is no reason not simply to use the original treble clef notation of abc. Typesetting using extended octaves of the original notation is also perfectly acceptable so long as particular usage of ledger lines is not needed, but for instruments which use only the bass clef such notation requires excessive use of commas.
Similarly, for the case of composition of orchestral score it might prove easiest to work in the original treble clef notation. Later the parts for each instrument could be transposed to the proper clef and offset for typesetting.
I find that one of the biggest drawbacks of the current BNF proposal for including clef information in voices is that it permits each voice assigned to a given staff to give a redundant, and therefore contradictory, specification of the clef currently in use. I would heartily endorse a change in the specification of clefs which removed this redundancy. On the other hand, a good typesetting program would presumably check and warn about contradictions, and the redundant information does make individual voices easier to sight read.