This piece is constructed in such a way that the soprano part which has the chorale tune together with the alto and the tenor alternate suspirans figure (three sixteenth notes after the rest) most of the time. The pedal part (the bass) has a harmonic foundation and moves in eighth notes using the chordal notes.
Here is how you can implement this figure in your improvisations:
1. Open your hymnal and choose a tune. This can basically be any tune from any period but older chorales work better with this figure.
2. Harmonize a tune in four parts - soprano, alto, tenor, and bass, giving the tune to the soprano part. You can keep the same harmonization as in the hymnal or you can create the chords yourself.
3. Play soprano line (the chorale tune) and the bass line with pedals using the chordal notes.
4. Add the suspirans figure which would alternate between the three upper voices. Be very systematic at first: alto-tenor-soprano. Keep this order until you are fluent with this figure.
BONUS TIP: When there is an interval of the third or the sixth between the voice which has this figure, don't hesitate to add the same figure in another voice - your sixteenth notes would move in sweet sounding parallel intervals.
Remember that it is easier to create your piece on paper first. In fact, you could take 10-20 hymn tunes and compose chorale preludes on paper following this model.
Then you could take the hymn tunes, put them on the music rack in front of you and start improvising your chorale preludes. That's the fastest way to progress in organ improvisation.
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