"klopfzeichen" was recorded on 21 december 1969 at rhenus-studio, gordorf, germany. liner notes on the cd reissue on the hypnotic label as well as some websites place the recording date precisely one year later in 1970. this is incorrect as it would place the recording after the initial release date. in addition kluster founder conrad schnitzler states in interviews that the recordings took place during the same period as the first tangerine dream album, "electronic meditation", which occurred in late 1969.