Editing Telephony (section)

===History===
The earliest end-to-end analog telephone networks to be modified and upgraded to transmission networks with [[Digital Signal 1]] (DS1/T1) carrier systems date back to the early 1960s. They were designed to support the basic 3&nbsp;kHz voice channel by sampling the bandwidth-limited analog voice signal and encoding using [[pulse-code modulation]] (PCM). Early PCM [[codec]]-[[Filter (signal processing)|filters]] were implemented as passive [[resistor]]{{ndash}}[[capacitor]]{{ndash}}[[inductor]] filter circuits, with [[analog-to-digital]] conversion (for digitizing voices) and [[digital-to-analog conversion]] (for reconstructing voices) handled by [[discrete device]]s. Early digital telephony was impractical due to the low performance and high costs of early PCM codec-filters.<ref name="Gibson26">{{cite book |last1=Floyd |first1=Michael D. |last2=Hillman |first2=Garth D. |chapter=Pulse-Code Modulation Codec-Filters |title=The Communications Handbook |edition=2nd |date=8 October 2018 |orig-year=1st pub. 2000 |pages=26-1, 26-2, 26-3 |publisher=[[CRC Press]] |isbn=9781420041163 |chapter-url=https://books.google.com/books?id=Tokk5bZxB0MC&pg=SA26-PA1}}</ref><ref name="Allstot">{{cite book |last1=Allstot |first1=David J. |chapter=Switched Capacitor Filters |editor-last1=Maloberti |editor-first1=Franco |editor-last2=Davies |editor-first2=Anthony C. |title=A Short History of Circuits and Systems: From Green, Mobile, Pervasive Networking to Big Data Computing |date=2016 |publisher=[[IEEE Circuits and Systems Society]] |isbn=9788793609860 |pages=105–110 |url=https://ieee-cas.org/sites/default/files/a_short_history_of_circuits_and_systems-_ebook-_web.pdf |access-date=2019-11-28 |archive-date=2021-09-30 |archive-url=https://web.archive.org/web/20210930151716/https://ieee-cas.org/sites/default/files/a_short_history_of_circuits_and_systems-_ebook-_web.pdf |url-status=dead }}</ref>

Practical [[digital telecommunication]] was enabled by the invention of the [[metal–oxide–semiconductor field-effect transistor]] (MOSFET),<ref name="Colinge2005">{{cite book |last1=Colinge |first1=Jean-Pierre |last2=Colinge |first2=C. A. |title=Physics of Semiconductor Devices |date=2005 |publisher=[[Springer Science & Business Media]] |isbn=9780387285238 |page=165 |url=https://books.google.com/books?id=-o5bAG5pR3AC&pg=PA165}}</ref> which led to the rapid development and wide adoption of PCM digital telephony.<ref name="Allstot"/> In 1957, Frosch and Derick were able to manufacture the first silicon dioxide field effect transistors at Bell Labs, the first transistors in which drain and source were adjacent at the surface.<ref>{{Cite journal |last1=Frosch |first1=C. J. |last2=Derick |first2=L |date=1957 |title=Surface Protection and Selective Masking during Diffusion in Silicon |url=https://iopscience.iop.org/article/10.1149/1.2428650 |journal=Journal of the Electrochemical Society |language=en |volume=104 |issue=9 |pages=547 |doi=10.1149/1.2428650|url-access=subscription }}</ref> Subsequently, a team demonstrated a working [[MOSFET]] at Bell Labs 1960.<ref>{{Cite journal |last=KAHNG |first=D. |date=1961 |title=Silicon-Silicon Dioxide Surface Device |url=https://doi.org/10.1142/9789814503464_0076 |journal=Technical Memorandum of Bell Laboratories |pages=583–596 |doi=10.1142/9789814503464_0076 |isbn=978-981-02-0209-5|url-access=subscription }}</ref><ref>{{Cite book |last=Lojek |first=Bo |title=History of Semiconductor Engineering |date=2007 |publisher=Springer-Verlag Berlin Heidelberg |isbn=978-3-540-34258-8 |location=Berlin, Heidelberg |page=321}}</ref> MOS technology was initially overlooked by Bell because they did not find it practical for analog telephone applications, before it was commercialized by [[Fairchild Semiconductor|Fairchild]] and [[RCA]] for [[digital electronics]] such as [[computers]].<ref>{{cite book |last1=Maloberti |first1=Franco |last2=Davies |first2=Anthony C. |chapter=History of Electronic Devices |title=A Short History of Circuits and Systems: From Green, Mobile, Pervasive Networking to Big Data Computing |date=2016 |publisher=[[IEEE Circuits and Systems Society]] |isbn=9788793609860 |pages=59-70 (65-7) |url=https://ieee-cas.org/sites/default/files/a_short_history_of_circuits_and_systems-_ebook-_web.pdf |access-date=2019-11-28 |archive-date=2021-09-30 |archive-url=https://web.archive.org/web/20210930151716/https://ieee-cas.org/sites/default/files/a_short_history_of_circuits_and_systems-_ebook-_web.pdf |url-status=dead }}</ref><ref name="Allstot" /> 

MOS technology eventually became practical for telephone applications with the MOS [[mixed-signal integrated circuit]], which combines analog and [[digital signal processing]] on a single chip, developed by former Bell engineer [[David A. Hodges]] with Paul R. Gray at [[UC Berkeley]] in the early 1970s.<ref name="Allstot" /> In 1974, Hodges and Gray worked with R.E. Suarez to develop MOS [[switched capacitor]] (SC) circuit technology, which they used to develop a [[digital-to-analog converter]] (DAC) chip, using [[MOS capacitor]]s and MOSFET switches for data conversion.<ref name="Allstot" /> MOS [[analog-to-digital converter]] (ADC) and DAC chips were commercialized by 1974.<ref name="US46">{{cite book |title=Electronic Components |date=1974 |publisher=[[U.S. Government Printing Office]] |page=46 |url=https://books.google.com/books?id=HikuAAAAMAAJ&pg=PA46}}</ref>

MOS SC circuits led to the development of PCM codec-filter chips in the late 1970s.<ref name="Allstot"/><ref name="Gibson26"/> The [[silicon-gate]] [[CMOS]] (complementary MOS) PCM codec-filter chip, developed by Hodges and W.C. Black in 1980,<ref name="Allstot"/> has since been the industry standard for digital telephony.<ref name="Allstot"/><ref name="Gibson26"/> By the 1990s, [[telecommunication network]]s such as the [[public switched telephone network]] (PSTN) had been largely digitized with [[very-large-scale integration]] (VLSI) CMOS PCM codec-filters, widely used in [[electronic switching system]]s for [[telephone exchanges]], [[private branch exchange]]s (PBX) and [[key telephone system]]s (KTS); user-end [[modems]]; [[data transmission]] applications such as [[digital loop carrier]]s, [[pair gain]] [[multiplexers]], telephone [[ADSL loop extender|loop extenders]], [[integrated services digital network]] (ISDN) terminals, digital [[cordless telephones]] and digital [[cell phones]]; and applications such as [[speech recognition]] equipment, voice [[data storage]], [[voice mail]] and digital tapeless [[answering machines]].<ref name="Gibson26"/> The bandwidth of digital telecommunication networks has been rapidly increasing at an exponential rate, as observed by [[Edholm's law]],<ref name="Cherry">{{cite journal |last1=Cherry |first1=Steven |title=Edholm's law of bandwidth |journal=IEEE Spectrum |date=2004 |volume=41 |issue=7 |pages=58–60 |doi=10.1109/MSPEC.2004.1309810|s2cid=27580722 }}</ref> largely driven by the [[MOSFET scaling|rapid scaling]] and [[miniaturization]] of MOS technology.<ref name="Jindal">{{cite book |last1=Jindal |first1=Renuka P. |title=2009 2nd International Workshop on Electron Devices and Semiconductor Technology |chapter=From millibits to terabits per second and beyond - over 60 years of innovation |date=2009 |pages=1–6 |doi=10.1109/EDST.2009.5166093 |isbn=978-1-4244-3831-0 |s2cid=25112828 |chapter-url=https://events.vtools.ieee.org/m/195547}}</ref><ref name="Allstot"/>

Uncompressed PCM [[digital audio]] with [[Audio bit depth|8-bit depth]] and 8{{nbsp}}[[kHz]] [[sample rate]] requires a [[bit rate]] of 64{{nbsp}}[[kbit/s]], which was impractical for early digital telecommunication networks with limited [[network bandwidth]]. A solution to this issue was [[linear predictive coding]] (LPC), a [[speech coding]] [[data compression]] algorithm that was first proposed by [[Fumitada Itakura]] of [[Nagoya University]] and Shuzo Saito of [[Nippon Telegraph and Telephone]] (NTT) in 1966. LPC was capable of [[audio data compression]] down to 2.4{{nbsp}}kbit/s, leading to the first successful real-time conversations over digital networks in the 1970s.<ref name="Gray">{{cite journal |last1=Gray |first1=Robert M. |title=A History of Realtime Digital Speech on Packet Networks: Part II of Linear Predictive Coding and the Internet Protocol |journal=Found. Trends Signal Process. |date=2010 |volume=3 |issue=4 |pages=203–303 |doi=10.1561/2000000036 |url=https://ee.stanford.edu/~gray/lpcip.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ee.stanford.edu/~gray/lpcip.pdf |archive-date=2022-10-09 |url-status=live |issn=1932-8346|doi-access=free }}</ref> LPC has since been the most widely used speech coding method.<ref>{{cite journal |last1=Gupta |first1=Shipra |title=Application of MFCC in Text Independent Speaker Recognition |journal=International Journal of Advanced Research in Computer Science and Software Engineering |date=May 2016 |volume=6 |issue=5 |pages=805–810 (806) |s2cid=212485331 |issn=2277-128X |url=https://pdfs.semanticscholar.org/2aa9/c2971342e8b0b1a0714938f39c406f258477.pdf |archive-url=https://web.archive.org/web/20191018231621/https://pdfs.semanticscholar.org/2aa9/c2971342e8b0b1a0714938f39c406f258477.pdf |url-status=dead |archive-date=2019-10-18 |access-date=18 October 2019}}</ref> Another [[audio data compression]] method, a [[discrete cosine transform]] (DCT) algorithm called the [[modified discrete cosine transform]] (MDCT), has been widely adopted for speech coding in [[voice-over-IP]] (VoIP) applications since the late 1990s.<ref name="Schnell">{{cite conference|last1=Schnell|first1=Markus|last2=Schmidt|first2=Markus|last3=Jander|first3=Manuel|last4=Albert|first4=Tobias|last5=Geiger|first5=Ralf|last6=Ruoppila|first6=Vesa|last7=Ekstrand|first7=Per|last8=Bernhard|first8=Grill|date=October 2008|title=MPEG-4 Enhanced Low Delay AAC - A New Standard for High Quality Communication|url=https://www.iis.fraunhofer.de/content/dam/iis/de/doc/ame/conference/AES-125-Convention_AAC-ELD-NewStandardForHighQualityCommunication_AES7503.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.iis.fraunhofer.de/content/dam/iis/de/doc/ame/conference/AES-125-Convention_AAC-ELD-NewStandardForHighQualityCommunication_AES7503.pdf |archive-date=2022-10-09 |url-status=live|conference=125th AES Convention|publisher=[[Audio Engineering Society]]|access-date=20 October 2019|website=[[Fraunhofer IIS]]}}</ref>

The development of transmission methods such as [[SONET]] and [[fiber optic]] transmission further advanced digital transmission. Although analog carrier systems existed that multiplexed multiple analog voice channels onto a single transmission medium, digital transmission allowed lower cost and more channels [[multiplexing|multiplexed]] on the transmission medium. Today the end instrument often remains analog but the analog signals are typically converted to [[Digital signal (signal processing)|digital signals]] at the [[serving area interface]] (SAI), [[Telephone exchange|central office]] (CO), or other aggregation point. [[Digital loop carrier]]s (DLC) and [[fiber to the x]] place the digital network ever closer to the customer premises, relegating the analog [[local loop]] to legacy status.