Compact Disc Digital Audio: Difference between revisions

Revision as of 02:48, 13 July 2025

Revision as of 17:47, 14 July 2025

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There was a long debate over the use of 16-bit (Sony) or 14-bit (Philips) [[Quantization (signal processing)|quantization]], and 44,056 or 44,100 samples/s (Sony) or approximately 44,000 samples/s (Philips). When the Sony/Philips task force designed the Compact Disc, Philips had already developed a 14-bit [[D/A converter]] (DAC), but Sony insisted on 16-bit. In the end Sony won, so 16 bits and 44.1 kilosamples per second prevailed. Philips found a way to produce 16-bit quality using its 14-bit DAC by using four times [[oversampling]].<ref name=Immink />

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 [[CD ripper|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]], support 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."

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Unlike on a [[DVD]] or CD-ROM, there are no [[computer file|files]] on a ''Red Book'' audio CD; there is only one continuous stream of [[LPCM]] audio data, and a parallel, smaller set of 8 [[subcode]] data streams. Computer [[operating system]]s, however, may provide access to an audio CD as if it contains files. For example, [[Windows]] represents the CD's table of contents as a set of [[Compact Disc Audio track]] (CDA) files, each file containing indexing information, not audio data. By contrast however, [[Finder (software)|Finder]] on [[macOS]] presents the CD's content as an actual set of files, with the [[Audio Interchange File Format|AIFF]]-extension, which can be copied directly, randomly and individually by track as if it were actual files. In reality, macOS performs rips as needed in the background, transparent to the user. The copied tracks are fully playable and editable on the user's computer.

In a process called [[CD ripper|ripping]], digital audio extraction software can be used to read CD-DA audio data and store it in files. Common [[audio file format]]s for this purpose include WAV and AIFF, which simply preface the LPCM data with a short [[header (computing)|header]]; [[FLAC]], [[Apple Lossless|ALAC]], and [[Windows Media Audio Lossless]], which compress the LPCM data in ways that conserve space yet allow it to be restored without any changes; and various [[lossy]], [[perceptual audio coder|perceptual coding]] formats like [[MP3]], [[Advanced Audio Coding|AAC]], and [[Opus (audio format)|Opus]], compress the audio data to a greater degree in ways that irreversibly change the audio, but that exploit features of human hearing to make the changes difficult to discern.<!--[[User:Kvng/RTH]]-->

== Format variations ==