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10.  Tape cartridge

The tape format that became affectionately known as carts was introduced to the NAB in 1959 by Collins Radio.  The first mono cart machines were offered shortly thereafter, and stereo cart machines with separate cue tracks were introduced later.  Pacific Recorders, Broadcast Electronics, ATC, ITC, Fidelipac, Audicord, Harris, Radio Systems and Ampro all built versions of the technology that became ubiquitous in radio stations.  By the mid-1990s, however, computer-based systems had gained enough of a foothold in the playback of music, commercials and other audio messages that many new station builds excluded the cart technology altogether.  In the late 1990s the old manufacturers stopped making new machines, and cartridge tapes themselves became expensive: one minute of storage cost more than $5.  The days of missed cues, muffled high-frequencies and mono phase cancellation has, for all intents and purposes, finally come to an end.  Carts played a major role in radio for 40 years, and the while the technology itself has nearly entirely disappeared, the concept of single-play elements is carried on in its replacement technology.

9.  CD

In 1979, Philips and Sony decided to join forces in the development of a new digital audio medium: the compact audio disc.  Philips brought its expertise in video laserdisc technology to the project, whereas Sony contributed its CERC error-correction technology.
The first compact discs and players were introduced in the Asian market in 1982 and reached the United States  the following year.  This was the beginning of the digital audio revolution.  Many listeners heard CDs first played on the radio in 1985.
The first professional CD players were expensive, and the first CD recorders cost stations more than $13,000 in 1994.  Early recordable CD media cost $30 per disc.
This entry into portable digital audio storage and playback marked the demise of the analog LP.  While the CD is still popular today, it too is being usurped by audio files and portable media players.

8.  Satellite distribution

Satellite distribution of TV programs dates back to the early to mid-1960s.  The audio portion of the TV program was distributed via subcarriers that were loaded in to the baseband (usually at 6.2MHz and 6.8MHz) above the video signal.
It didn't take too long for someone to figure out that more subcarriers could be added to the baseband to distribute other audio channels.  Thus began satellite distribution of audio networks.
Soon thereafter came single channel per carrier (SCPC) distribution.  NPR used this in conjunction with audio companding.  Other ad-hoc networks were built up as well, many for the purpose of distributing regional programming, such as sports and statewide news networks.
In the early 1980s, satellite distribution of digital audio began with the introduction of the DATS system of non-compressed PCM audio.  This was followed by a data-compressed version in the mid-1990s called SEDAT.  The Starguide system has practically become the de-facto distribution system of audio networks today, with the exception of the NPR digital SCPC system.
The ease and quality with which network audio could now be disseminated certainly contributed to the success of mass appeal formats and talk radio. To a large degree the state of the industry today can be attributed to it.

7.  Electron tube technology

After WWI, two manufacturing groups developed practically all the vacuum tubes of the era: RCA/Westinghouse/General Electric and Bell Telephone.  This strong control of the technology of electron tubes relaxed in 1932 when patents began to expire. Companies such as Raytheon, Sylvania, Amperex and Eitel-McCollough soon announced new tube designs.
Through the 1920s and 1930s, as it was discovered by early pioneers in radio technology that higher and higher frequencies could be used for communication, electron-tube designers worked diligently in developing new tubes that could be used to generate power at these ever-increasing frequencies.
WWII spurred rapid development of power tube types, and brought on evolutionary changes that are still in use today: the “beam” tetrode; forced-air cooling; and perhaps the most important (at least for the FM band) the use of cavity type resonant circuits.
The continuing development of electron-tube technology was one of the driving forces in telecommunications from near the turn of the century all the way to the mid-1950s, when junction transistor development started to hit stride.

6.  Crystal detector

Early radiotelegraphy made use of spark-gap transmitters; early radiotelephony made use of high-frequency alternators and the newly invented triode tube.
The complementary receiver technology of the time was that of the crystal detector.  In the early 1900s researchers discovered that certain substances (such as lead sulfide or silicon) could be used in the detection of radio signals.  Greenleaf Whittier Pickard was awarded a patent in November 1906 for the silicon crystal detector.  The device itself was simple: a thin piece of wire (sometimes known as a cat's whisker) was put in contact with the crystal.  The crystal detector was placed in parallel with a parallel resonant tank circuit that was tuned for the particular frequency the user wanted.  A high-impedance crystal transducer was in turn placed in parallel with the crystal detector.  The detection of radio waves excited the crystal transducer, which in turn could be heard by the human ear.
Crystal detectors could also be used to demodulate AM.  This provided a simple means that the listening public (such as they were at the time) could hear the few nascent broadcast radio transmissions.

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