Compact Disc Digital Audio: Difference between revisions

Revision as of 00:19, 10 February 2025

Revision as of 03:15, 16 February 2025

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Some early CDs were mastered with [[pre-emphasis]], an artificial boost of high audio frequencies. The pre-emphasis improves the apparent signal-to-noise ratio by making better use of the channel's dynamic range. On playback, the player applies a de-emphasis filter to restore the frequency response curve to an overall flat one. Pre-emphasis time constants are 50&nbsp;μs and 15&nbsp;μs (9.49&nbsp;dB boost at 20&nbsp;kHz), and a binary flag in the disc [[subcode]] instructs the player to apply de-emphasis filtering if appropriate. Playback of such discs in a computer or [[ripping]] to [[WAV]] files typically does not take into account the pre-emphasis, so such files play back with an incorrect frequency response.{{citation needed|date=January 2022}} [[FFmpeg]] has a filter to remove (or apply) the pre-emphasis in order to create standard WAV files, or to create CDs with pre-emphasis.<ref>{{cite web |title=FFmpeg Filters Documentation |url=https://ffmpeg.org/ffmpeg-filters.html#aemphasis |website=FFmpeg |publisher=FFmpeg project |access-date=6 January 2024 |archive-date=28 March 2017 |archive-url=https://web.archive.org/web/20170328021020/https://ffmpeg.org/ffmpeg-filters.html#aemphasis |url-status=live }}</ref>

Four-channel, or [[quadraphonic]], supportedsupport was originally intended to be included in CD-DA.<ref>{{Cite book|url = https://books.google.com/books?id=jqwgBQAAQBAJ&q=Four-channel+Compact+Disc+Digital+Audio&pg=PA108|title = Digital Audio and Compact Disc Technology|isbn = 978-1-4831-4039-1|last1 = Baert|first1 = Luc|last2 = Theunissen|first2 = Luc|last3 = Vergult|first3 = Guido|date = 2013-10-22| publisher=Newnes |access-date = 27 August 2023|archive-date = 27 August 2023|archive-url = https://web.archive.org/web/20230827104057/https://books.google.com/books?id=jqwgBQAAQBAJ&q=Four-channel+Compact+Disc+Digital+Audio&pg=PA108|url-status = live}}</ref> The ''Red Book'' specification briefly mentioned a four-channel mode in its June 1980,<ref name="RedBook1980">"Red Book" Audio CD specification, June 1980, Sony/Philips</ref> September 1983,<ref name="RedBook1983">"Red Book" Audio CD specification, September 1983, Sony/Philips</ref> and November 1991<ref name="RedBook1991">"Red Book" Audio CD specification, November 1991, Sony/Philips</ref> editions. On the first page, it lays out the "Main parameters" of the CD system, including: "Number of channels: 2 and/or 4 simultaneously[*] sampled." The footnote says, "In the case of more than two channels the encoder and decoder diagrams have to be adapted."

In reality, however, the underspecified "four-channel" mode was dropped from the CD standard when it was adopted by the [[International Electrotechnical Commission]] and became IEC 908:1987,<ref name="IEC1987">{{citation |url=https://webstore.iec.ch/publication/18347 |title=IEC 60908:1987 Compact disc digital audio system |access-date=6 May 2015 |archive-date=4 September 2015 |archive-url=https://web.archive.org/web/20150904041217/https://webstore.iec.ch/publication/18347 |url-status=live }}</ref> and later IEC 60908:1999.<ref name="IEC1999">{{citation |url=https://webstore.iec.ch/preview/info_iec60908%7Bed2.0%7Db.pdf |title=IEC 60908:1999 Audio recording – Compact disc digital audio system |access-date=6 May 2015 |archive-date=16 October 2015 |archive-url=https://web.archive.org/web/20151016064048/https://webstore.iec.ch/preview/info_iec60908%7Bed2.0%7Db.pdf |url-status=live }}</ref> Since the behavior of the "four-channel" or "Broadcasting use" bit was never specified by either CD standard, no mass-marketed discs have attempted to use the Red Book's four-channel mode, and no players have purported to implement it.