synchronize CDROMs that contain a DTS soundtrack. This period also saw several other historic developments including the introduction of the first practical magnetic sound recording system, the magnetic wire recorder, which was based on the work of Danish inventor Valdemar Poulsen. Magnetic wire recorders were effective, but the sound quality was poor, so between the wars, they were primarily used for voice recording and marketed as business dictating machines. In 1924, a German engineer, Kurt Stille, improved the Telegraphone with an electronic amplifier. The following year, Ludwig Blattner began work that eventually produced the Blattnerphone, which used steel tape instead of wire. The BBC started using Blattnerphones in 1930 to record radio programs. In 1933, radio pioneer Guglielmo Marconi's company purchased the rights to the Blattnerphone, and newly developed Marconi-Stille recorders were installed in the BBC's Maida Vale Studios in March 1935. The tape used in Blattnerphones and Marconi-Stille recorders was the same material used to make razor blades, and not surprisingly the fearsome Marconi-Stille recorders were considered so dangerous that technicians had to operate them from another room for safety. Because of the high recording speeds required, they used enormous reels about one meter in diameter, and the thin tape frequently broke, sending jagged lengths of razor steel flying around the studio. Tape Magnetic tape recording uses an amplified electrical audio signal to generate analogous variations of the magnetic field produced by a tape head, which impresses corresponding variations of magnetization on the moving tape. In playback mode, the signal path is reversed, the tape head acting as a miniature electric generator as the varyingly magnetized tape passes over it. The original solid steel ribbon was replaced by a much more practical coated paper tape, but acetate soon replaced paper as the standard tape base. Acetate has fairly low tensile strength and if very thin it will snap easily, so it was in turn eventually superseded by polyester. This technology, the basis for almost all commercial recording from the 1950s to the 1980s, was developed in the 1930s by German audio engineers who also rediscovered the principle of AC biasing (first used in the 1920s for wire recorders), which dramatically improved the frequency response of tape recordings. The K1 Magnetophon was the first practical tape recorder, developed by AEG in Germany in 1935. The technology was further improved just after World War II by American audio engineer John T.