guitar/bass as a transposing instrument and midi

[bb]

A transposing instrument is one in which the absolute pitches of the notes played on the instrument are shifted up or down by a certain amount, compared to how they are written in notation. \clef "treble_8" for guitar takes account of, but is not always written, as that transposing property ias a standard. Bass is a transposing instrument as well. \clef "bass_8"works as well but I found it never written in any bass notation.

Writing tabulatures  can easily be corrected with \transpose in the  \new TabStaff code section. 

That does not work the easy way for midi. The resulting midi-sound is an octave higher in tone.

i cannot find a solution to lower the pitch for midi only. Every \transpose alters the written notes as well. Eventually one has to write an aditinal special music for midi? 

Thanks for any help, regards

I add a simple test file. Is there any soultion?

\version "2.19.65"
guitar = \relative c {
  \clef "treble_8"
  r4 a b c d e f g
}
bass = \relative c, {
  %\clef "bass_8"
  \clef bass
  a b c d e f g
}
\score {
  <<
    \new Staff  { \guitar }
      \new Staff  {
        %\transpose c c' % with this \transpose midi sounds to high  but notation is correct
        \bass}
    >>
  \layout {  }
  \midi { }
}