concerts to be recorded on a Magnetophon was Mozart's 39th Symphony played by the London Philharmonic Orchestra, conducted by Sir Thomas Beecham, during their 1936 concert tour. The recording was made on an AEG K2 Magnetophon running at 100 cm/s. The tape used was the early black iron oxide Fe3O4 type. When Beecham and the musicians heard the playback, they were greatly disappointed with the distortion and noise on the recording. Although the original tape is now lost, the recording survived until the 1990s and has been transferred. Some other surviving tapes show a tendency toward overmodulation. Later in 1939, the Fe3O4 oxide was replaced by the Fe2O3 type, which gave a significantly better recording quality, so much that the formula became a worldwide standard until the 1970s when chromium dioxide tapes appeared. Adding a direct-current bias to the record head gave some improvement, but in 1941, Hans Joachim von Braunmühl and Dr. Walter Weber, both engineers at the German national broadcasting organisation RRG (Reichs-Rundfunk-Gesellschaft), accidentally discovered the technique of high-frequency bias in which the simple addition of a high level (about 10X the maximum audio level) inaudible high-frequency tone resulted in a striking improvement in sound quality by effectively smoothing the magnetization of unused portions of the audio band. The discovery was made when a Magnetophone producing recordings of extraordinary quality was sent 'for repair'. The machine was found to have an oscillating DC bias amplifier. Magnetic media are inherently non-linear, but AC bias was the means whereby the magnetisation of the recording tape was made linearly proportional to the electrical signal which represents the audio component. The Magnetophon became a 'high fidelity' recording system because in so many respects, it outperformed gramophone recording (which was the 78 rpm system of the time). Many speeches, concerts, and operatic performances were recorded. Since many of the recordings survived World War II they were later issued on LPs and compact discs. One of the more remarkable series of recordings took place at the Vienna State Opera House, also known as Wiener Staatsoper, in 1944, when the German composer Richard Strauss recorded many of his famous symphonic poems, including Don Juan, Till Eulenspiegel, and Also sprach Zarathustra, with the Vienna Philharmonic Orchestra. AEG engineers made rapid strides in perfecting the system and had practical stereo recorders by 1943. Until 1945, about 250 stereophonic tape recordings were known to exist, including some Richard