Klopfzeichen was recorded on 21 December 1969 in Rhenus Studio,
Gedorf. It was released on Schwann, a publishing company usually associated with church music!!! The liner notes presented a totally unconventional new group: `the
ensemble Kluster is a progressive pop group. They personalise the perhaps most radical German underground music. Their playing technique is that one of Stockhausen and the
group Nuova Consonaza`. The trio played piano, guitar, cello, percussion and organ that was electronically manipulated by engineer Konrad Plank. Their collaboration with
Plank lasted for more than ten years and was extremely important for the development of the music. Technically he really has to be considered as a member of the group. The
two long pieces of very radical music that made up Klopfzeichen were untitled. It contained what seems to be free improvisation. There were no rhythms, rather a floating
continuum of creepy, unearthly and manipulated sounds. Side one ran for 24 minutes, and featured lyrics by radical German writers like Liselotte Raunes, Rudolf Otto Wiemer,
Dorothee Solle, Wilhelm Wills, Eva Zeller and Uwe Seidel read by Christa Runge. Side two ran for nearly 22 minutes and was instrumental. Why someone chose to dub them a
`pop band` on the liner notes is a mystery to me (was it a joke?), for they played a kind of weird contemporary classical music.