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Major Edwin Howard Armstrong was arguably the most important inventor of the 20th century with respect to radio communications. In addition to his earlier inventions--namely regeneration and superheterodyne--he is credited with the invention of frequency modulation.
He first got FM working in 1933, and showed it to the company that held the most licenses of his earlier inventions, RCA. Even though RCA spent a considerable amount of time and effort evaluating the methodology of FM during the 1930s, ultimately it decided not to license it.
Undeterred, Armstrong went ahead and licensed the invention to smaller companies. He designed an entire transmission system from end-to-end -- transmitter, antenna and receiver--and worked with the fledgling Yankee Network in New England to prove the technology. On Jan. 5, 1940, a special broadcast test was carried out on the Yankee network. The test originated at W2XCR in Yonkers, NY. It was picked up by W2XMN at Alpine, NJ, rebroadcast and picked up by W1XPW at Meriden, CT. It was picked up again by W1XOJ at Paxton, MA, and then finally ended at W1XOY at Mt. Washington, NH. "This test is most gratifying," Armstrong was quoted as saying.
Boston observers report that the program went in to that city with a tonal quality never heard before, and the operators atop Mount Washington reported it as clear as if it were next door. The broadcast went from Yonkers to Mount Washington without using an inch of wire.
Armstrong lobbied the FCC to set aside a segment of spectrum for FM radio (44MHz to 50MHz) and some stations (such as those in the Yankee Network) began broadcasting. During World War II, Armstrong was distracted by the war effort. Everyone used FM during the war, and Armstrong allowed the military to use his patents royalty-free, a gesture that he was not really able to afford. After the war, the FCC moved the FM band to the 88MHz to 108MHz spectrum where it lives today.
Lee de Forest was awarded more than 180 patents during his lifetime, but he is best known for what he called the audion--better known to us today as the triode. This vacuum tube device, an offshoot and improvement on John Fleming's two-element vacuum tube diode patented in 1905, was the first successful electronic amplifier and the genesis of today's electronics and telecommunications industries.
In 1910, while working for the Federal Telegraph Company in Palo Alto, CA, de Forest was able to make the triode perform as an amplifier and sold it to the company for use in long distance telephony.
In 1912 de Forest learned that he could cascade the triode amplifiers, thus creating a system that had far greater sensitivity than had hitherto been available for radio receivers. This, of course, was an essential technological development for the fledgling radio and telephony industries.
By 1916 de Forest had learned to use triodes as oscillator tubes, thus generating substantial amounts of RF current for use in transmission.
That triode tubes are one of the most essential elements in early broadcast technology is unquestionable. They were used for everything from mic preamps, to program amps in consoles, to oscillator tubes, modulator tubes and amplifier tubes in the early broadcast transmitters. Many of us still have the pleasure of using them today.
It is generally accepted that the transistor was invented at Bell Labs in 1947 (announced in 1948 after patents were applied for and the military was informed) by William Shockley, John Bardeen and Walter Brattain. Though a study of the prior art may cause the claim to be dubious, the fact is that the three shared the Nobel Prize for physics in 1956.
In 1951, Bell Labs made another announcement: the invention of the junction-type transistor, the ancestor to what we are familiar with today.
The properties of the junction transistor--small size, long life expectancy and no need for a filament--overcame many limitations of vacuum tubes. Western Electric (parent of Bell Labs) actively promoted the new technology and licensed the manufacturing rights to more than 30 companies--at $25,000 each--in 1952. This new invention allowed for the early replacement of tubes in two common electronic devices: the hearing aid and the one that we are so concerned with, the portable radio.
The first commercial all-transistor portable radio, the Regency TR-1, came out in time for Christmas in 1954. Although it didn't sell that well at first, the idea caught on with other manufacturers, and by 1957 almost five million portable radios had been sold by the likes of Admiral, Arvin, Emerson, GE, Raytheon, RCA, Westinghouse and Zenith.
The invention of the junction transistor precipitated the invention of the integrated circuit and later the microprocessor, devices around which nearly all broadcast equipment is designed and built today.
2. Electrically recorded records
In 1925 Henry C. Harrison of Western Electric's Bell Labs developed a new electrical recording system for 78 rpm records. Using condenser mics and vacuum tube amps (obviously an early proponent of high audio quality) he obtained a frequency response from about 50Hz out to 6kHz--far superior to the 250Hz to 2.5kHz response of the acoustical recording systems that were in use prior to that time.
By 1930 most record companies had adopted the electrical recording system, and during the 1930s many radio stations gained the equipment and ability to make these transcription discs themselves. Radio advertisers soon discovered that they could prerecord their ads, so that they could be played over and over on the air.
NBC allowed its affiliates to make and use these transcription discs on May 1, 1932. In 1935 the new acetate disc was introduced, and on Jan. 1, 1936, NBC announced the formation of its Reference Recording division. CBS acquired similar equipment in 1938, as did ABC when it was split from NBC in 1940.
Radio stations were perhaps the most fervent users of the transcription disc recording technology. The ability to play material back at a later time, or time and time again was obviously an operational advantage.
In the pages of Radio magazine, and indeed most other texts regarding technologies used for the transmission of broadcast signals, there is little emphasis on the actual receivers used by the listening audience. Still the case could be made that, no matter how good and sophisticated the transmission technology is, it all would be for naught if it were not for complementary receiver technology. While there were many innovations in the art of receiver design during the 1900s, perhaps no single one was as important as superheterodyne principle.
First, it is necessary to go back to the early 1920s to review a little about the AM broadcast receiver technology that was in use. A typical receiver (known as a TRF-for tuned radio frequency) consisted of a chain of tuned circuit/RF amplifier combinations that terminated in the detector stage. The detected audio from that stage was used to drive a loudspeaker amplifier.
This type of receiver architecture has several major flaws. First, the bandwidth of the tuned circuits (for a given Q) is proportional to the operating frequency. Therefore, as the user tunes up the dial, the selectivity of the receiver diminishes.
Secondly, the sensitivity of the TRF varies with frequency; it gets lower as the receive frequency goes lower.
In response to this problem, Edwin Howard Armstrong developed a new idea that he subsequently patented in 1917. Instead of trying to build the gain and selectivity over a range of frequencies (such as the entire AM broadcast band) he converted the varying input frequency to a fixed frequency--known as the or intermediate frequency (IF). The conversion takes place by mixing (or heterodyning) two signals--the signal trying to be received--and another known as the local oscillator. The arithmetic difference in frequency between the local oscillator and the signal being received is always the same (the IF). The IF amplifier is then designed with the appropriate selectivity and gain, thus eliminating the frequency-variable factors of the TRF receiver. What you had then was a better receiver providing a better experience to those that tune in to AM radio--and the ushering in of a new era in broadcast.
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