Inductor–
capacitor ladder networks were used as analog delay lines in the 1920s. For example, Francis Hubbard's sonar direction finder patent filed in 1921. Hubbard referred to this as an
Artificial transmission line. In 1941, Gerald Tawney of
Sperry Gyroscope Company filed for a patent on a compact packaging of an inductor–capacitor ladder network that he explicitly referred to as a
time delay line. In 1924, Robert Mathes of
Bell Telephone Laboratories filed a broad patent covering essentially all electromechanical delay lines, but focusing on acoustic delay lines where an air column confined to a pipe served as the mechanical medium, and a telephone receiver at one end and a telephone transmitter at the other end served as the electromechanical transducers. Mathes was motivated by the problem of
echo suppression on long-distance telephone lines, and his patent clearly explained the fundamental relationship between inductor–capacitor ladder networks and mechanical elastic delay lines such as his acoustic line. In 1938, William Spencer Percival of Electrical & Musical Industries (later
EMI) applied for a patent on an acoustical delay line using
piezoelectric transducers and a liquid medium. He used water or
kerosene, with a 10 MHz carrier frequency, with multiple baffles and reflectors in the delay tank to create a long acoustic path in a relatively small tank. In 1939,
Laurens Hammond applied electromechanical delay lines to the problem of creating artificial reverberation for his
Hammond organ. Hammond used coil springs to transmit mechanical waves between
voice-coil transducers. The problem of suppressing
multipath interference in
television reception motivated Clarence Hansell of
RCA to use delay lines in his 1939 patent application. He used "delay cables" for this, relatively short pieces of coaxial cable used as delay lines, but he recognized the possibility of using
magnetostrictive or
piezoelectric delay lines. By 1943, compact delay lines with distributed capacitance and inductance were devised. Typical early designs involved winding an enamel insulated wire on an insulating core and then surrounding that with a grounded conductive jacket. Richard Nelson of
General Electric filed a patent for such a line that year. Other GE employees, John Rubel and Roy Troell, concluded that the insulated wire could be wound around a conducting core to achieve the same effect. Much of the development of delay lines during World War II was motivated by the problems encountered in
radar systems. In 1944, Madison G. Nicholson applied for a general patent on
magnetostrictive delay lines. He recommended their use for applications requiring delays or measurement of intervals in the 10 to 1000 microseconds time range. In 1945, Gordon D. Forbes and Herbert Shapiro filed a patent for the mercury delay line with
piezoelectric transducers. This delay line technology would play an important role, serving as the basis of the
delay-line memory used in several
first-generation computers. In 1946, David Arenberg filed patents covering the use of
piezoelectric transducers attached to single crystal solid delay lines. He tried using
quartz as a delay medium and reported that
anisotropy in the quartz crystals caused problems. He reported success with single crystals of
lithium bromide,
sodium chloride and
aluminum. Arlenberg developed the idea of complex 2- and 3-dimensional folding of the acoustic path in the solid medium in order to package long delays into a compact crystal. The delay lines used to decode
PAL television signals follow the outline of this patent, using
quartz glass as a medium instead of a single crystal. ==See also==