Editing AM broadcasting (section)

===Early amplitude modulation (AM) transmitter technologies===
It was immediately recognized that, much like the telegraph had preceded the invention of the telephone, the ability to make audio radio transmissions would be a significant technical advance. Despite this knowledge, it still took two decades to perfect the technology needed to make quality audio transmissions. In addition, the telephone had rarely been used for distributing entertainment, outside of a few "[[telephone newspaper]]" systems, most of which were established in Europe, beginning with the Paris [[Théâtrophone]]. With this in mind, most early [[radiotelephone]] development envisioned that the device would be more profitably developed as a "wireless telephone" for personal communication, or for providing links where regular telephone lines could not be run, rather than for the uncertain finances of broadcasting.
[[File:15JUN1920 Melba radio broadcast.jpg|thumb|[[Nellie Melba]] making a broadcast over the Marconi Chelmsford Works radio station in England on 15 June 1920]]
[[File:20111110-OC-AMW-0038 - Flickr - USDAgov (1).jpg|thumb|upright=1.2|Farmer listening to U.S. government weather and crop reports using a crystal radio in 1923. Public service government time, weather, and farm broadcasts were the first radio "broadcasts".]] 
[[File:Crystal radio advertisement.png|thumb|upright=1.3|A family listening to an early broadcast using a [[crystal radio]] receiver in 1922. Crystal sets, used before the advent of vacuum tube radios in the 1920s, could not drive [[loudspeaker]]s, so the family had to listen on earphones.]]

The person generally credited as the primary early developer of AM technology is Canadian-born inventor [[Reginald Fessenden]]. The original spark-gap radio transmitters were impractical for transmitting audio, since they produced discontinuous pulses known as "[[damped wave]]s". Fessenden realized that what was needed was a new type of radio transmitter that produced steady "undamped" (better known as "[[continuous wave]]") signals, which could then be "modulated" to reflect the sounds being transmitted.

Fessenden's basic approach was disclosed in U.S. Patent 706,737, which he applied for on May 29, 1901, and was issued the next year. It called for the use of a high-speed [[alternator]] (referred to as "an alternating-current dynamo") that generated "pure sine waves" and produced "a continuous train of radiant waves of substantially uniform strength", or, in modern terminology, a continuous-wave (CW) transmitter.<ref>[https://patentimages.storage.googleapis.com/pdfs/US706737.pdf U.S. Patent 706,737], submitted May 29, 1901, and issued August 12, 1902, to Reginald Fessenden.</ref> Fessenden began his research on audio transmissions while doing developmental work for the United States Weather Service on Cobb Island, Maryland. Because he did not yet have a continuous-wave transmitter, initially he worked with an experimental "high-frequency spark" transmitter, taking advantage of the fact that the higher the spark rate, the closer a spark-gap transmission comes to producing continuous waves. He later reported that, in the fall of 1900, he successfully transmitted speech over a distance of about 1.6 kilometers (one mile),<ref>[https://babel.hathitrust.org/cgi/pt?id=mdp.39015057240221&view=1up&seq=111 "Experiments and Results in Wireless Telephony"], by John Grant, ''The American Telephone Journal'', January 26, 1907, pp. 49–51.</ref> which appears to have been the first successful audio transmission using radio signals. However, at this time the sound was far too distorted to be commercially practical.<ref>''The Continuous Wave'' by Hugh G. J. Aitken, 1985, p. 61.</ref> For a time he continued working with more sophisticated high-frequency spark transmitters, including versions that used compressed air, which began to take on some of the characteristics of [[arc converter|arc-transmitters]].<ref>Aitken (1985), p. 62.</ref> Fessenden attempted to sell this form of radiotelephone for point-to-point communication, but was unsuccessful.<ref name=FESS>{{cite web|url=https://ewh.ieee.org/reg/7/millennium/radio/radio_wireless.html |title=Fessenden, Reginald A. ''Inventing the Wireless Telephone and the Future'' |publisher=[[IEEE]] |access-date=2017-07-22}}</ref>

====Alternator transmitter====

Fessenden's work with high-frequency spark transmissions was only a temporary measure. His ultimate plan for creating an audio-capable transmitter was to redesign an electrical [[alternator]], which normally produced alternating current of at most a few hundred ([[hertz|Hz]]), to increase its rotational speed and so generate currents of tens-of-thousands Hz, thus producing a steady continuous-wave transmission when connected to an aerial. The next step, adopted from standard wire-telephone practice, was to insert a simple [[carbon microphone]] into the transmission line, to modulate the [[carrier wave]] signal to produce AM audio transmissions. However, it would take many years of expensive development before even a prototype alternator-transmitter would be ready, and a few years beyond that for high-power versions to become available.<ref>Aitken (1985), p. 69.</ref>

Fessenden worked with [[General Electric]]'s (GE) [[Ernst Alexanderson|Ernst F. W. Alexanderson]], who in August 1906 delivered an improved model which operated at a transmitting frequency of approximately 50&nbsp;kHz, although at low power. The alternator-transmitter achieved the goal of transmitting quality audio signals, but the lack of any way to amplify the signals meant they were somewhat weak. On December 21, 1906, Fessenden made an extensive demonstration of the new alternator-transmitter at Brant Rock, Massachusetts, showing its utility for point-to-point wireless telephony, including interconnecting his stations to the wire telephone network. As part of the demonstration, speech was transmitted 18 kilometers (11 miles) to a listening site at Plymouth, Massachusetts.<ref name="experiments">"Experiments and Results in Wireless Telephony" by John Grant, ''The American Telephone Journal''. [https://babel.hathitrust.org/cgi/pt?id=mdp.39015057240221;view=1up;seq=111 Part I]: January 26, 1907, pp. 49–51; [https://babel.hathitrust.org/cgi/pt?id=mdp.39015057240221&view=1up&seq=147 Part II]: February 2, 1907, pp. 68–70, 79–80.</ref>

An ''American Telephone Journal'' account of the December 21 alternator-transmitter demonstration included the statement that "It is admirably adapted to the transmission of news, music, etc. as, owing to the fact that no wires are needed, simultaneous transmission to many subscribers can be effected as easily as to a few",<ref name="experiments"/> echoing the words of a handout distributed to the demonstration witnesses, which stated "[Radio] Telephony is admirably adapted for transmitting news, stock quotations, music, race reports, etc. simultaneously over a city, on account of the fact that no wires are needed and a single apparatus can distribute to ten thousand subscribers as easily as to a few. It is proposed to erect stations for this purpose in the large cities here and abroad."<ref>[https://www.radioworld.com/columns-and-views/dec-21-1906-a-very-significant-date-in-radio/338869 "Dec. 21, 1906: A Very Significant Date in Radio"] by James E. O'Neal, December 22, 2016 (radioworld.com).</ref> However, other than two holiday transmissions reportedly made shortly after these demonstrations, Fessenden does not appear to have conducted any radio broadcasts for the general public, or to have even given additional thought about the potential of a regular broadcast service, and in a 1908 article providing a comprehensive review of the potential uses for his radiotelephone invention, he made no references to broadcasting.<ref>[https://babel.hathitrust.org/cgi/pt?id=mdp.39015068171985&view=1up&seq=638 "Wireless Telephony: G. Possibilities"] by Reginald A. Fessenden, ''Transactions of the American Institute of Electrical Engineers'', Vol. XXVII (1908), Part I, pp. 606–608.</ref>

Because there was no way to amplify electrical currents at this time, modulation was usually accomplished by a carbon [[microphone]] inserted directly in the antenna wire. This meant that the full transmitter power flowed through the microphone, and even using water cooling, the power handling ability of the microphones severely limited the power of the transmissions. Ultimately only a small number of large and powerful [[Alexanderson alternator]]s would be developed. However, they would be almost exclusively used for long-range radiotelegraph communication, and occasionally for radiotelephone experimentation, but were never used for general broadcasting.

====Arc transmitters====
Almost all of the continuous wave AM transmissions made prior to 1915 were made by versions of the [[arc converter]] transmitter, which had been initially developed by [[Valdemar Poulsen]] in 1903.<ref>[https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=789449A&KC=A&FT=D&ND=1&date=19050509&DB=&locale=en_EP# "Method of Producing Alternating Currents With a High Number of Vibrations"] U.S. patent 789,449, filed June 10, 1903, and granted May 9, 1905, to Valdemar Poulsen.</ref> Arc transmitters worked by producing a pulsating electrical arc in an enclosed hydrogen atmosphere. They were much more compact than alternator transmitters, and could operate on somewhat higher transmitting frequencies. However, they suffered from some of the same deficiencies. The lack of any means to amplify electrical currents meant that, like the alternator transmitters, modulation was usually accomplished by a microphone inserted directly in the antenna wire, which again resulted in overheating issues, even with the use of water-cooled microphones. Thus, transmitter powers tended to be limited. The arc was also somewhat unstable, which reduced audio quality. Experimenters who used arc transmitters for their radiotelephone research included [[Ernst Ruhmer]], [[Quirino Majorana]], [[Charles Herrold|Charles "Doc" Herrold]], and [[Lee de Forest]].

====Vacuum tube transmitters====
Advances in [[vacuum tube]] technology (called "valves" in British usage), especially after around 1915, revolutionized radio technology. Vacuum tube devices could be used to amplify electrical currents, which overcame the overheating issues of needing to insert microphones directly in the transmission antenna circuit. Vacuum tube transmitters also provided high-quality AM signals, and could operate on higher transmitting frequencies than alternator and arc transmitters.<ref>[https://archive.org/stream/electricalexperi07gern#page/1000/mode/1up "The Versatile Audion"] by H. Winfield Secor, ''Electrical Experimenter'', February 1920, pages 1000–1001, 1080-1083.</ref> Non-governmental radio transmissions were prohibited in many countries during World War I, but AM radiotelephony technology advanced greatly due to wartime research, and after the war the availability of tubes sparked a great increase in the number of amateur radio stations experimenting with AM transmission of news or music. Vacuum tubes remained the central technology of radio for 40 years, until [[transistor]]s began to dominate in the late 1950s, and are still used in the highest power broadcast transmitters.