Even as the regenerative-circuit lawsuit continued, Armstrong was working on another momentous invention. While working in the basement lab of Columbia's Philosophy Hall, he created wide-band frequency modulation radio (FM). Rather than varying the amplitude of a radio wave to create sound, Armstrong's method varied the frequency of the wave instead. FM radio broadcasts delivered a much clearer sound, free of static, than the AM radio dominant at the time. (Armstrong received a patent on wideband FM on December 26, 1933; his just rewards required much litigation at the end of his life, and beyond it.)
In 1922, John Renshaw Carson of AT&T, inventor of Single-sideband modulation (SSB modulation), had published a paper in the Proceedings of the Institute of Radio Engineers (IRE) arguing that FM did not appear to offer any particular advantage . Armstrong managed to demonstrate the advantages of FM radio despite Carson's skepticism in a now-famous paper on FM in the Proceedings of the IRE in 1936 , which was re-printed in the August 1984 issue of Proceedings of the IEEE .
Today the consensus regarding FM is that narrow band FM is not so advantageous in terms of noise reduction, but wide band FM can bring great improvement in signal to noise ratio if the signal is stronger than a certain threshold. Hence Carson was not entirely wrong, and the Carson bandwidth rule for FM is still important today. Thus, both Carson and Armstrong ultimately contributed significantly to the science and technology of radio. The threshold concept was discussed by Murray G. Crosby (inventor of Crosby system for FM Stereo) who pointed out that for wide band FM to provide better signal to noise ratio, the signal should be above a certain threshold, according to his paper published in Proceedings of the IRE in 1937 . Thus Crosby's work supplemented Armstrong's paper in 1936.
Armstrong conducted the first large scale field tests of his FM radio technology on the 85th floor of RCA's (Radio Corporation of America) Empire State Building from May 1934 until October 1935. However RCA had its eye on television broadcasting, and chose not to buy the patents for the FM technology. A June 17, 1936, presentation at the Federal Communications Commission (FCC) headquarters made headlines nationwide. He played a jazz record over conventional AM radio, then switched to an FM broadcast. "[I]f the audience of 50 engineers had shut their eyes they would have believed the jazz band was in the same room. There were no extraneous sounds," noted one reporter. He added that several engineers described the invention "as one of the most important radio developments since the first earphone crystal sets were introduced."
In 1937, Armstrong financed construction of the first FM radio station, W2XMN, a 40 kilowatt broadcaster in Alpine, New Jersey. The signal (at 42.8 MHz) could be heard clearly 100 miles (160 km) away, despite the use of less power than an AM radio station.
RCA began to lobby for a change in the law or FCC regulations that would prevent FM radios from becoming dominant. By June 1945, the RCA had pushed the FCC hard on the allocation of electromagnetic frequencies for the fledgling television industry. Although they denied wrongdoing, David Sarnoff and RCA managed to get the FCC to move the FM radio spectrum from (42-50 MHz), to (88-108 MHz), while getting new television channels allocated in the 40 MHz range.
This single FCC action rendered all Armstrong-era FM sets useless overnight, and protected RCA's AM-radio stronghold. Armstrong's radio network did not survive the frequency shift up into the high frequencies; most experts believe that FM technology was set back decades by the FCC decision. This change was strongly supported by AT&T, because loss of FM relaying stations forced radio stations to buy wired links from AT&T.
Furthermore, RCA also claimed invention of FM radio and won its own patent on the technology. A patent fight between RCA and Armstrong ensued. RCA's momentous victory in the courts left Armstrong unable to claim royalties on any FM radios sold in the United States. The undermining of the Yankee Network and his costly legal battles brought ruin to Armstrong, by then almost penniless and emotionally distraught.
Philosophy Hall, the Columbia building where Armstrong developed FM, was declared a National Historic Landmark in 2003 in recognition of that fact. Armstrong's home in Yonkers also received a similar designation, but it was withdrawn when the house was later demolished.
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