100 years 100 innovations


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20.  Cylinder recorder

While working on a device that would send telegraph messages in an automated fashion, Thomas Edison stumbled on the idea of recording and playing back the human voice.  Thus was born the cylinder recorder; the predecessor to all other recording devices and the germ of the technology used by the music business.  The device was a hand-cranked cylinder that was covered with tin foil; one diaphragm attached to a stylus was used to etch the track into the foil and another lighter stylus was used during the playback.  For the first crude versions, the tin foil itself would only last for a few plays and couldn't be removed from the cylinder.  Later the cylinders were made from wax and could be played multiple times.  From the turn of the century to the early 1920s, cylinder recordings were quite popular, only to be surpassed later by 78 rpm records. Edison stopped making the cylinders in 1929.


19.  Zenith-GE FM stereo

In 1961, the FCC selected the Zenith-GE system as the method that would be used to transmit stereo signals on the FM band.  One of the more important considerations during this transition was backward compatibility—making sure that older FM mono receivers would still be usable.  For this reason, FM stereo uses a matrix system.  Left and right channels are added together and make up the mono signal (L+R); at the same time, right is subtracted from left, thus forming a L-R signal.  When the L+R and L-R are added and subtracted, discrete left and right are recreated.
For transmission, the L-R signal is mixed with a 38kHz signal to form a double-sideband, suppressed carrier signal that is added to the baseband and occupies the spectrum from 23kHz to 53kHz.  A 19kHz pilot tone is also transmitted for synchronization of the stereo decoder on the receive side.
The development of stereo transmission on FM followed the widespread introduction of stereo LPs in 1958, and played a significant role in the popularization of the FM band.  Many early FM stations broadcasted classical music, taking advantage of the newly developed technology in attracting listeners.


18.  Perceptual audio coding

Perceptual audio coding takes advantage of the way human hearing works in the time and frequency domain by not encoding sounds that are ultimately imperceptible, and by efficiently using the minimum number of bits to encode the sounds that are perceptible.  The ultimate goal is to fit as many useful bits as possible through what is typically a data-bandwidth limited path between the program source and the far end: the listeners.  Lossy Codec is the name for a device that uses perceptual audio encoding to build the data streams that we eventually hear coming from speakers on the other end.
The invention and development of this technology has precipitated a revolution in the way listeners listen to programs and music—and arguably the largest challenge ever faced by radio broadcasters.


Courtesy of the AT&T Archives

17.  Nyquist Theorem

In an analog-to-digital conversion, one of the most important considerations is that of the sampling frequency.  The Nyquist rate is the sampling frequency; the Nyquist frequency is one half of that.  Nyquist in this case refers to Harry Nyquist, a Swedish-born scientist who worked at AT&T between 1917 and 1934.  The Nyquist-Shannon sampling theory says that any continuous signal can be completely reconstructed if (during the conversion) it is band limited to one-half of the sampling frequency.  At minimum, 15kHz of audio bandwidth would require a sampling rate of 30kHz.  A more familiar sampling rate for this audio bandwidth is 32kHz.
Put that together with the invention of PCM in 1937 and you have the foundation for the digital audio revolution that has had such a profound effect on radio, and in turn has created the media that may be radio's ultimate challenge.


16.  33 1/3 rpm record

The development of the 33 1/3 rpm record can be traced back to the beginning of motion picture sound.  Western Electric synchronized sound with pictures in 1925 using records that ran at 33 1/3 rpm.  After World War II, Columbia Records decided to heavily market the 12" 33 1/3 rpm LP (for long playing).  Eventually it became the standard for musical albums and remained that way until CDs became readily and widely available.  In 1958, the first stereo albums were introduced using the method invented by Alan Blumlein of EMI in 1931.




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