'', 1938 In the 1920s,
Phonofilm and other early motion picture sound systems employed
optical recording technology, in which the audio signal was graphically recorded on photographic film. The amplitude variations comprising the signal were used to modulate a light source which was imaged onto the moving film through a narrow slit, allowing the signal to be photographed as variations in the density or width of a
sound track. The
projector used a steady light and a
photodetector to convert these variations back into an electrical signal, which was amplified and sent to
loudspeakers behind the screen. Optical sound became the standard motion picture audio system throughout the world and remains so for theatrical release prints despite attempts in the 1950s to substitute magnetic soundtracks. Currently, all release prints on
35 mm movie film include an analog optical soundtrack, usually stereo with
Dolby SR noise reduction. In addition, an optically recorded digital soundtrack in Dolby Digital or Sony SDDS form is likely to be present. An optically recorded timecode is also commonly included to 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. Mullin with backing from
Bing Crosby Enterprises. Mullin's pioneering recorders were modifications of captured German recorders. In the late 1940s, the
Ampex company produced the first tape recorders commercially available in the US. Magnetic tape brought about sweeping changes in both radio and the recording industry. Sound could be recorded, erased and re-recorded on the same tape many times, sounds could be duplicated from tape to tape with only minor loss of quality, and recordings could now be very precisely edited by physically cutting the tape and rejoining it. Within a few years of the introduction of the first commercial tape recorder—the
Ampex 200 model, launched in 1948—American musician-inventor
Les Paul had invented the first
multitrack tape recorder, ushering in another technical revolution in the recording industry. Tape made possible the first sound recordings totally created by electronic means, opening the way for the bold sonic experiments of the
Musique Concrète school and avant-garde composers like
Karlheinz Stockhausen, which in turn led to the innovative pop music recordings of artists such as
the Beatles and
the Beach Boys. The ease and accuracy of tape editing, as compared to the cumbersome disc-to-disc editing procedures previously in some limited use, together with tape's consistently high audio quality, finally convinced radio networks to routinely prerecord their entertainment programming, most of which had formerly been broadcast live. Also, for the first time, broadcasters, regulators and other interested parties were able to undertake comprehensive audio logging of each day's radio broadcasts. Innovations like multitracking and
tape echo allowed radio programs and advertisements to be produced to a high level of complexity and sophistication. The combined impact with innovations such as the endless loop
broadcast cartridge led to significant changes in the pacing and production style of radio program content and advertising. ==Stereo and hi-fi==