Audiotape was invented in 1927 by Fritz Pfleumer. He used a paper strip, upon which steel dust was fixed with varnish. This magnetizable sound carrier was far superior to wire recorders. In 1935, AEG developed the first tape recorder, and BASF developed the first plastic tape. In 1939, the AEG Magnetophon K4 was the first mass-produced tape recorder. With the ability to check the quality while recording and an AC bias (from mid 1940), it was the first time recording was of studio quality.