CX (audio)

CX is a noise reduction system for recorded analog audio. It was developed by CBS Laboratories (a division of CBS) in the late 1970s as a competitor to other noise reduction (NR) systems such as Dolby and dbx, and was officially introduced in 1981.[1][2][3] The name CX was derived from "Compatible eXpansion", a feature of the technique.[3] The CX integrated circuit U2141B was developed by AEG-Telefunken, Germany, in 1982, by Ernst F. Schröder, Dietrich Höppner and Kurt Hintzmann,[1] the same team who also designed the High Com noise reduction system,[1][4] a broadband compander with up to 20 dB of noise reduction.[5][6] Hitachi also offered dedicated CX chips named HA12043 and HA12044 in 1983.[4][5][7] Telefunken also used, in their RS 120 CX[8] a dual trans-conductance amplifier 13700D made by JRC (today NRJ), coupled with a pair of quad J-FET OpAmp chips TL084.[9]

The CX logo, present on LPs and laserdiscs utilizing CX noise reduction

Use on vinyl LP records

CX was originally designed by CBS as a noise-reduction technology for vinyl LP records,[10][3] similar to the earlier dbx disc or High-Com II systems. CX-encoded records required a special CX expander connected to a stereo system, in order to fully reproduce the CX encoded sound on the LP. However, CX-encoded records could also be played without a decoder, with a resulting (claimed acceptable) amount of dynamic range compression.[6][3]

The project was led by CBS Records group vice president Bob Jamieson and carried out by a team led by Daniel W. Gravereaux[1][6] and Louis A. Abbagnaro.[6] The label predicted that CX encoding would become standard on all new LP releases but this did not happen. CBS struggled to gain support for the system from other record companies. The process was controversial among CBS executives[3] and unpopular with some artists. Classical guitarist Liona Boyd demanded that the CX encoded version of one of her albums be withdrawn because of perceived shortcomings, even though Jamieson claimed that CBS had the technical means to overcome such objections.[11]

Approximately 70 CX encoded LP titles were released by CBS up to 1983 in the United States.[3] Gasparo also released a number of CX encoded records.[3] In Europe, many CX discs were manufactured in the Netherlands with the catalog number prefix "CBSCX". The albums in this series also came in standard, non-CX encoded versions.

Use on LaserDisc and CED VideoDisc

While the implementation of CX with LPs was quite unsuccessful and short-lived, CX would later see success as the noise reduction used for the stereo analog audio tracks on the LaserDisc format. It was also used for the audio tracks on discs of the RCA SelectaVision CED Videodisc system.

All LaserDisc (and stereophonic CED) players manufactured since 1981, when the CX equipped LD-1100 was introduced, had CX noise reduction capability as a standard feature. Pioneer also released a stand-alone CX adapter for use with their VP-1000, Magnavox's VH-8000/8005 and the industrial players that were all released before CX was adopted. The first CX encoded LaserDisc was Olivia Newton-John's Olivia Physical released by MCA Videodisc.

CX decoders made for LPs could not be used with LDs because the CX companding specifications for LaserDisc were changed, from 20 dB of noise reduction to 14 dB, along with moving the 'threshold' where compression/expansion changes from 2:1 to 1:1 from -40 dB to -28 dB - other minor changes to the decoding time constants were made as well. In addition, some of the LaserDisc's FM audio encoding specifications were modified too, allowing more headroom and better high frequency response at high levels. These changes were made because, at the time of CX's adoption on LaserDisc (1981), the vast majority of program sources used for mastering, such as 35 mm optical and magnetic film soundtracks, as well as the 2-inch IVC-9000 and the 1-inch C-Type video tape formats used for LaserDisc mastering, had signal-to-noise ratios low enough that undecoded playback would accentuate their noise to unacceptable levels.

By reducing the total amount of noise reduction and modifying other aspects of the CX system to better match LD's FM audio shortcomings, undecoded playback sound quality was maintained and vastly improved decoded sound was achieved at the same time. The possibility of audible pumping or breathing artifacts during CX decoded playback were reduced as well.[12]

While CX greatly improved the audio quality of LaserDisc's FM audio tracks, its primary reason for adoption was to decrease the amount of interference between the right channel's FM audio carrier and the video carrier's first chroma sideband. Without CX, strict filtering during mastering and playback as well as keeping color saturation below 75% on the master were required to keep any interference below -35 dB, which ensured that no beats or other artifacts were visible in the demodulated image. Although CX improved the picture quality, it was not normally used on discs with mono audio. Pioneer Video, the main manufacturer of LD's at the time, required the studios to request CX and, since most did not know that CX improved the video quality of the finished discs or the audio of mono titles, CX was rarely requested. Due to this lack of knowledge about CX at the studios, there were many stereo titles released without CX encoding and, in fact, CX didn't become standard on all LaserDisc titles until the late 1980s. A look at Pioneer's catalogs as late as 1987 shows that the majority of titles did not have CX encoded analog sound - most were not digital either.

For the CED VideoDisc, since stereo was not added to the format until its second year on the market, RCA made CX a mandatory part of CED's stereo system - a disc could not be released in stereo without CX encoding - and the companding specifications were unchanged from those of the LP system due to the CED system's much higher noise levels than the LaserDisc format. Although RCA improved the plastic/carbon formulation used to make discs, which lowered disc noise levels by 3 dB, and modified the mastering system, the CED format still required the full 20 dB of noise reduction that was achieved with the unmodified LP system. The names given by CBS to the two different versions of CX were CX-20 and CX-14.[12][13]

The theory of operation is described in U.S. Patent 4,376,916.

Use in FM radio broadcasting

CX was used in FMX, a commercially unsuccessful noise reduction system developed in the 1980s for FM broadcasting in the United States. FMX was intended to improve fringe area reception of FM stereo by adding a CX-encoded version of the L−R (left-minus-right, or difference) signal modulated in quadrature with the conventional stereo subcarrier. About 50 stations utilized the system, but few FMX-equipped receivers were manufactured, and after FMX was accused in 1989 of actually degrading reception rather than improving it, support dried up and the system was abandoned.

Partial list of CX encoded LPs released by CBS

[6]

See also

References
  1. Schröder, Ernst F. "The Story of HIGH COM". Archived from the original on 2016-04-16. Retrieved 2016-04-16.
  2. Handbook CX Low Cost Expander Model E-1016 (revised ed.). Stamford, CT, USA: CBS Technology Center. 1981-08-02.
  3. Copeland, Peter (February 2009) [September 2008]. Redlich, Gert (ed.). "Manual of Analoque Sound Restoration Techniques". London, UK: The British Library Sound Archive (www.bl.uk.). Chapter 9. Reciprocal noise reduction. Archived from the original on 2017-11-05. Retrieved 2017-11-05 – via Deutsches Hifi-Museum, Wiesbaden, Germany. (NB. Strongly biased from a British perspective, but nevertheless very knowledgeable.)
  4. "CBS Stresses CX Commitment - Seeks more Hardware Firms' Support for System". Billboard Magazine: 4, 79. 1983-01-29.
  5. "ICs for Consumer Applications - Audio Application - Other Application". Hitachi Quick Reference Guide to Integrated Circuits and Discrete Semiconductor Devices - Preferred European Type-Selection. Hitachi. 1984. pp. 5, 46. Retrieved 2017-11-09. […] Type No.: HA12043, Outline: DP-18, Functions: CX NR-System for MCA System, Remarks: NR Effects 14 dB […] Type No.: HA12044, Outline: DP-18, Functions: CX NR-System, Remarks: NR Effects 20 dB […]
  6. "Das CX Rauschunterdrückungs-Verfahren für Schallplatten". AUDIO (in German). November 1982. […] Drei Jahre entwickelten die Elektronik-Ingenieure Louis A. Abbagnaro und Daniel W. Gravereaux (Columbia Broadcasting Corporation) am CX-Verfahren. CX bedeutet soviel wie Compatible Expansion […] und ist ein Rauschunterdrückungsverfahren für Schallplatten. Durch eine spezielle Komprimierung und anschließende Expansion der Musik bei der Wiedergabe von der Schallplatte soll dieses Verfahren die Dynamik einer Schallplatte um 20 Dezibel auf einen Wert von insgesamt 80 Dezibel erhöhen. Plattenrauschen, Knistern und Knacken sowie Rumpelgeräusche sollen mit diesem Verfahren unhörbar werden. […] Die wichtigste Bedingung bei der Entwicklung von CX war neben der Geräuschreduzierung und Dynamikerweiterung die Kompatibilität. […] Die CX-Platten sollen ohne gravierende Qualitätseinbußen auch ohne den CX-Decoder abspielbar sein. Doch […] CX-Lizenznehmer Teldec schreibt für das Abspielen von Klassik-CX-Platten unbedingt die Verwendung eines CX-Decoders vor. […] Grund […] Nur mit Hilfe eines CX-Decoders ist die Natürlichkeit der Musikwiedergabe gesichert. Das ist besonders bei klassischer Musik wichtig. Beim Anhören ohne Decoder werden […] vor allem die Höhen unwirklich überbetont und hart, außerdem wirkt auch die Dynamik, bedingt durch die fehlende Expansion, eingeengt. […] Bei Popmusik-Platten fällt indes dieser Effekt meist nicht so deutlich auf. Da viele dieser Platten keine sehr großen Dynamik-Unterschiede haben, stört die fehlende Expansion kaum. Was aber bleibt, ist die etwas überzogene Höhenwiedergabe vor allem beim Abhören über hochwertige HiFi-Anlagen. […] Das Angebot an Decodern ist zwar noch nicht sehr groß, doch sehr viele Firmen haben inzwischen CX-Lizenzen erworben. Einen hervorragenden Eindruck hinterließ der Kort-Decoder […]) beim Hörtest. Nicht nur, daß er sich mit Hilfe eines Reglers und eines Zeigerinstrumentes problemlos auf die Signalstärke der Schallplatte abgleichen läßt, er überzeugte auch klanglich. Der Telefunken Decoder RN 100 CX und auch die in den beiden Plattenspielern RS 120CX und RS 220 CX integrierten Decoder sind direkt mit einem Entzerrer-Vorverstärker gekoppelt. Moving-Coil-Systeme lassen sich damit also nicht betreiben. Klanglich gute Ergebnisse liefert auch der CMLab-Decoder aus den USA, wenn auch der Kort-Decoder klanglich leicht überlegen ist. Prototypen von CX-Decodern präsentierten unlängst aucn die deutschen Firmen Backes & Müller sowie Thorens. Wann diese Geräte in Serie gehen, steht noch nicht fest, doch eines ist sicher: Viele Unternehmen werden in Zukunft CX-Decoder, ob separat oder in Verstärkern oder Plattenspielern fest eingebaut, anbieten. Und damit kann auch die konventionelle Platte, vorausgesetzt, sie ist CX-codiert, störungsarmen Musikgenuß bieten. […] Im Prinzip arbeitet das Rauschverminderungs-System CX ähnlich wie andere, bei Cassettenrecordern eingesetzte Rauschminderungs-Systeme, beispielsweise Dolby B/C oder High Com. Nur werden hier beim Überspielen der Musik vom Masterband auf die Schneidemaschine zur Herstellung der Masterfolie die Signale durch einen speziellen Kompressor geschickt. Dieser schränkt die Dynamik der Musiksignale ein, senkt also laute Töne um einen definierten Betrag ab und hebt umgekehrt leise Signale ebenfalls definiert an. […] Bei der Wiedergabe der fertigen CX-Platte geschieht genau das Gegenteil. Ein Expander, das Gegenstück zum Kompressor, erweitert die Dynamik wieder, hebt also die lauten Töne wieder um den ursprünglichen Absenkungsbetrag an und senkt die leisen Töne um den ursprünglichen Anhebungsbetrag wieder ab. […] Beim Absenken der leisen Töne kommen Störgeräusche, wie etwa Rauschen und leise Knacker, die von der Platte herrühren, nicht ungeschoren davon. Sie werden in gleichem Maße mit abgesenkt und dadurch fast unhörbar. Die Gesamtdynamik einer CX-Platte kann, so die Erfinder, bis zu 80 Dezibel betragen. […] Die Investitionen für einen CX-Decoder betragen etwa 300 bis 500 Mark. […]
  7. "CX-dekoder - grammofoon weergave zonder ruis" [CX decoder - phonograph record playback without noise]. Elektuur - Maandblad voor Elektronika (in Dutch). Beek: Elektuur B.V. 23 (3): 3-68–3-76. March 1983. ISSN 0013-5895. #233. Retrieved 2017-11-09. (NB. This article describes a do-it-yourself CX-decoder based on the Hitachi HD12044 chip. The magazine, named Elektor, was also available in German language.)
  8. Techmoan.com. "CX Discs: Better, Worse & the Same".
  9. "NJM13600_NJM13700_E.pdf" (PDF).
  10. Bruch, Walter (1983). "Von der Tonwalze zur Bildplatte (Teil II) - Entwicklung der Magnetbandtechnik". Funkschau. 100 Jahre Ton- und Bildspeicherung (in German) (18, 19, 20, 21). Archived from the original on 2017-12-08. Retrieved 2017-12-08.
  11. "Slow Progress Seen in CBS' CX Campaign". Billboard Magazine: 3. 1982-02-20.
  12. Badger, Greg; Allen, Richard (October 1982). "The Audio Side Of The Laser Videodisc". Affiliation: Pioneer Video, Inc., Costa Mesa, CA. AES Convention: 72. Paper Number: 1935. Cite journal requires |journal= (help)CS1 maint: location (link)
  13. Roberts, John (March 1983). "CX - an approach to disc noise reduction". Studio Sound: 52–53.

Further reading
  • CBS CX Expander Calibration Record (Vinyl LP). CBS. 1981. CX-REF. Archived from the original on 2017-11-05. Retrieved 2017-11-05. 7-inch. 33-1/3 RPM. Band 1: Left Channel Calibration 1000 Hz, 3.54 cm/sec RMS. Band 2: Right Channel Calibration 1000 Hz, 3.54 cm/sec RMS.
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