Kauffmann's prelude on "Ich ruf zu dir Herr Jesu Christ" is a bicinium. The choral melody is stated in half notes and accompanied by the rapidly moving figure of eight notes, based on the first phrase of the chorale melody. The two 16' stops add a nice character and punch to the accompanying voice.
The recording was done with the Hauptwerk software and the sampleset, made by Voxus, of the Müller organ in the Sint Bavokerk, Haarlem (http://www.voxusorgans.com/en/product/haarlem).
Score available here: http://partitura.org/index.php/georg-friedrich-kauffmann-ich-ruf-zu-dir-herr-jesu-christ