Editing Bell Labs (section)

===1920s===
In 1924, Bell Labs physicist [[Walter A. Shewhart]] proposed the [[control chart]] as a method to determine when a process was in a state of statistical control. Shewhart's methods were the basis for [[statistical process control]] (SPC): the use of statistically based tools and techniques to manage and improve processes. This was the origin of the modern quality control movement, including [[Six Sigma]].

In 1926, the laboratories invented an early [[Sound film|synchronous-sound motion picture]] system, in competition with [[Movietone sound system|Fox Movietone]] and [[Phonofilm|DeForest Phonofilm]].<ref>{{Cite encyclopedia |url=https://www.britannica.com/eb/article-9015241 |archive-url=https://web.archive.org/web/20060503182230/https://www.britannica.com/eb/article-9015241 |archive-date=May 3, 2006 |title=Bell Laboratories |encyclopedia=[[Encyclopædia Britannica]]}}</ref>

In 1927, a Bell team headed by [[Herbert E. Ives]] successfully transmitted long-distance 128-line television images of [[Secretary of Commerce]] [[Herbert Hoover]] from Washington to New York. In 1928 the [[thermal noise]] in a resistor was first measured by [[John Bertrand Johnson|John B. Johnson]], for which [[Harry Nyquist]] provided the theoretical analysis; this is now termed ''Johnson-Nyquist noise''. During the 1920s, the [[one-time pad]] [[cipher]] was invented by [[Gilbert Vernam]] and [[Joseph Mauborgne]] at the laboratories. Bell Labs' [[Claude Shannon]] later proved that it is unbreakable.

In 1928, [[Harold Stephen Black|Harold Black]] invented the negative feedback system commonly used in amplifiers. Later, [[Harry Nyquist]] analyzed Black's design rule for negative feedback. This work was published in 1932 and became known as the [[Nyquist stability criterion|Nyquist criterion]].