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Professional and personal files including Armstrong's correspondence with professional associations, other engineers, and friends, his research notes, circuit diagrams, lectures, articles, legal papers, and other related materials. Of his many inventions and developments, the most important are: 1) the regenerative or feedback circuit, 1912, the first amplified radio reception, 2) the superheterodyne circuit, 1918, the basis of modern radio and radar, 3) superregeneration, 1922, a very simple, high-power receiver now used in emergency mobile service, and 4) frequency modulation - FM, 1933, static-free radio reception of high fidelity. More than half the files concern his many lawsuits, primarily with Radio Corporation of America, over infringement of the Armstrong patents. Litigation continued until 1967. Other files deal with his work in the Marcellus Hartley Research Laboratory at Columbia University, 1913-1935, and with the American Expeditionary Forces in France during World War I, his Air Force contracts for communications development, Army research during World War II, the Radio Club of America, the Institute of Radio Engineers, FM development at his radio station at Alpine, N.J., the use of FM in television, his involvement in Federal Communications Commission hearings and legislation, and his work with the Zenith Radio Corporation. Also, letters to H.J. Round

Letters sent to Volkonskai︠a︡. Among the correspondents are outstanding musical figures, including Vladimir Ashkenazy, Geraldine Farrar, Emil Gilels, Vladimir Horowitz, Eugene Ormandy, and Svi︠a︡toslav Stokowski. There are also letters from the writer Ivan Bunin.

Letters to and from Nicolai Berezowsky documenting his career and covering his development as violinist, conductor, and composer. The profession is well represented in the correspondence, with 63 letters from David Diamond, 22 from Serge Koussevitsky, 72 from Nicolai Lopatnikoff, and 31 from Eugene Ormandy; others in the collection are Leon Barzin, Aaron Copland, Henry Cowell, Vladimir Golschmann, Eugene Goosens, Alex Grechaninov, Ernest Hutcheson, Douglas Moore, Paul Nordoff, Wallingford Riegger, Artur Rodzinski, Roger Sessions, Harold Spivacke, Leopold Stokowski, and Olga Samaroff Stokowski. Organizations, such as the WPA music program, League of Composers, Juilliard School of Music, and the American Society of Composers, Authors, and Publishers, are represented. There are also 15 volumes of music scores.