Editing Pulse-width modulation (section)

==History==
The [[Corliss steam engine]] was patented in 1849. It used pulse-width modulation to control the intake valve of a steam engine cylinder. A [[centrifugal governor]] was used to provide automatic feedback.

Some machines (such as a [[sewing machine]] motor) require partial or variable power.  In the past, control (such as in a sewing machine's foot pedal) was implemented by use of a [[rheostat]] connected in series with the motor to adjust the amount of current flowing through the motor.  It was an inefficient scheme, as this also wasted power as heat in the resistor element of the rheostat, but tolerable because the total power was low. While the rheostat was one of several methods of controlling power (see [[autotransformer]]s and [[Variac]] for more info), a low cost and efficient power switching/adjustment method was yet to be found. This mechanism also needed to be able to drive motors for fans, pumps and [[robotic]] [[servomechanism]]s, and needed to be compact enough to interface with lamp dimmers.  PWM emerged as a solution for this complex problem.

PWM telecommunications systems were invented just prior to the start of [[World War II]], but at that time [[time-division multiplexing]] was already in use and there were only experimental PWM systems. This changed with the introduction of the [[cavity magnetron]] in 1940, which could produce pulses of [[microwave]] frequency energy but could not vary its frequency or precisely control its amplitude. A PWM encoder was used to trigger a magnetron in the [[British Army]]'s [[Wireless Set Number 10]], which provided long-distance telephone relay, up to {{convert|80|km}}.<ref>{{cite magazine
 |magazine=Wireless World
 |date=December 1945
 |url=https://worldradiohistory.com/UK/Wireless-World/40s/Wireless-World-1945-12.pdf
 |title=Pulse-Width Modulation: The Basic Principles Described
 |pages=361–362
 |ref=CITEREFWW1945a
}}</ref>

The [[Philips|Philips, N. V.]] company designed an optical scanning system ([https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_53/Philips_Tech_Review/PTechReview-08-1946-097.pdf published] in 1946) for [[Optical sound|variable area]] film [[soundtrack]] which produced the PWM. It was intended to reduce noise when playing back a film soundtrack. The proposed system had a threshold between "white" and "black" parts of soundtrack.<ref>{{Cite journal |last=Westmijze |first=W. K. |date=1946 |title=A New Method of Counteracting Noise in Sound Film Reproduction |url=https://ieeexplore.ieee.org/document/7252228/authors#authors |archive-url=https://web.archive.org/web/20220906215112/https://ieeexplore.ieee.org/document/7252228/authors#authors |url-status=dead |archive-date=September 6, 2022 |journal=Journal of the Society of Motion Picture Engineers |volume=47 |issue=5 |pages=426–440 |doi=10.5594/J12769 |issn=0097-5834 |via=IEEE}}</ref>

One early application of PWM was in the [[Sinclair Radionics|Sinclair]] X10, a 10&nbsp;W audio amplifier available in kit form in the 1960s. At around the same time, PWM started to be used in AC motor control.<ref>{{cite journal  | last =Schönung   | first =A. |author2=Stemmler, H.  | title =Geregelter Drehstrom-Umkehrantrieb mit gesteuertem Umrichter nach dem Unterschwingungsverfahren | journal =BBC Mitteilungen | volume = 51 | issue = 8/9 | pages =555–577 | date =August 1964 }}</ref>

Of note, for about a century, some variable-speed electric motors have had decent efficiency, but they were somewhat more complex than constant-speed motors, and sometimes required bulky external electrical apparatus, such as a bank of variable power resistors or rotating converters such as the [[Ward Leonard control|Ward Leonard drive]].