Compact Disc Digital Audio

Compact Disc Digital Audio (CDDA or CD-DA), also known as Audio CD, is the standard format for audio compact discs. The standard is defined in the Red Book, one of a series of "Rainbow Books" (named for their binding colors) that contain the technical specifications for all CD formats.

Compact Disc Digital Audio
CDDAlogo
Media typeOptical disc
Encoding2 channels of LPCM audio, each signed 16-bit values sampled at 44100 Hz
Capacityup to 74–80 minutes (up to 24 minutes for mini 8 cm CD)
Read mechanismSemiconductor laser (780 nm wavelength)
StandardIEC 60908
Developed bySony & Philips
UsageAudio storage
Released1980

Standard

The Red Book specifies the physical parameters and properties of the CD, the optical "stylus" parameters, deviations and error rate, modulation system (eight-to-fourteen modulation, EFM) and error correction facility (cross-interleaved Reed–Solomon coding, CIRC), and the eight subcode channels. These parameters are common to all compact discs and used by all logical formats, such as CD-ROM. The standard also specifies the form of digital audio encoding: 2-channel signed 16-bit Linear PCM sampled at 44,100 Hz. Although rarely used, the specification allows for discs to be mastered with a form of emphasis.

The first edition of the Red Book was released in 1980 by Philips and Sony;[1][2] it was adopted by the Digital Audio Disc Committee and ratified by the International Electrotechnical Commission Technical Committee 100, as an International Standard in 1987 with the reference IEC 60908.[3] The second edition of IEC 60908 was published in 1999[4] and it cancels and replaces the first edition, amendment 1 (1992) and the corrigendum to amendment 1. The IEC 60908 however does not contain all the information for extensions that is available in the Red Book, such as the details for CD-Text, CD+G and CD+EG.[5][6]

The standard is not freely available and must be licensed. It is available from Philips and the IEC. As of 2013, Philips outsources licensing of the standard to Adminius, which charges US$100 for the Red Book, plus US$50 each for the Subcode Channels R-W and CD Text Mode annexes.[7]

Audio format

The audio contained in a CD-DA consists of two-channel signed 16-bit Linear PCM sampled at 44,100 Hz.

Sample rate

The sampling rate is adapted from that attained when recording digital audio on a PAL (or NTSC) videotape with a PCM adaptor, an earlier way of storing digital audio.[8] An audio CD can represent frequencies up to 22.05 kHz, the Nyquist frequency of the 44.1 kHz sample rate.

The selection of the sample rate was based primarily on the need to reproduce the audible frequency range of 20–20,000 Hz (20 kHz). The Nyquist–Shannon sampling theorem states that a sampling rate of more than twice the maximum frequency of the signal to be recorded is needed, resulting in a required rate of at least 40 kHz. The exact sampling rate of 44.1 kHz was inherited from a method of converting digital audio into an analog video signal for storage on U-matic video tape, which was the most affordable way to transfer data from the recording studio to the CD manufacturer at the time the CD specification was being developed. The device that converts an analog audio signal into PCM audio, which in turn is changed into an analog video signal is called a PCM adaptor. This technology could store six samples (three samples per stereo channel) in a single horizontal line. 60 field/s black and white video (not 59.94 color) was required, and in NTSC countries (USA/Japan) that video signal has 245 usable lines per field, which works out to be (245 * 60 * 3) = 44,100 samples/s/stereo channel. Similarly, PAL has 294 lines and 50 fields, which gives 44,100 samples/s/stereo channel. This system could store 14-bit samples with some error correction, or 16-bit samples with almost no error correction.

There was a long debate over the use of 14-bit (Philips) or 16-bit (Sony) 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, 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.[9]

Pre-emphasis

Some CDs are 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 50µs and 15µs (9.49 dB boost at 20 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 wave files typically does not take into account the pre-emphasis, so such files play back with a distorted frequency response.

Storage capacity and playing time

The creators of the CD originally aimed at a playing time of 60 minutes with a disc diameter of 100 mm (Sony) or 115 mm (Philips).[10] Sony vice-president Norio Ohga suggested extending the capacity to 74 minutes to accommodate the recording of Wilhelm Furtwängler conducting Ludwig van Beethoven's Ninth Symphony at the 1951 Bayreuth Festival.[11][12] The additional 14-minute playing time subsequently required changing to a 120 mm disc. Kees Schouhamer Immink, Philips' chief engineer, however, denies this, claiming that the increase was motivated by technical considerations, and that even after the increase in size, the Furtwängler recording would not have fit on one of the earliest CDs.[9][10]

According to a Sunday Tribune interview,[13] the story is slightly more involved. In 1979, Philips owned PolyGram, one of the world's largest distributors of music. PolyGram had set up a large experimental CD plant in Hannover, Germany, which could produce huge numbers of CDs having a diameter of 115 mm. Sony did not yet have such a facility. If Sony had agreed on the 115-mm disc, Philips would have had a significant competitive edge in the market. The long playing time of Beethoven's Ninth Symphony imposed by Ohga was used to push Philips to accept 120 mm, so that Philips' PolyGram lost its edge on disc fabrication.[13]

The 74-minute playing time of a CD, which is longer than the 22 minutes per side[14][15] typical of long-playing (LP) vinyl albums, was often used to the CD's advantage during the early years when CDs and LPs vied for commercial sales. CDs would often be released with one or more bonus tracks, enticing consumers to buy the CD for the extra material. However, attempts to combine double LPs onto one CD occasionally resulted in the opposite situation in which the CD would instead offer fewer tracks than the LP.

Playing times beyond 74 minutes are achieved by decreasing track pitch (the distance separating the track as it spirals the disc) in violation of strict Red Book standards. However, most players can still accommodate the more closely spaced data if it is still within Red Book tolerances.[16] Current manufacturing processes allow an audio CD to contain up to 80 minutes (variable from one replication plant to another) without requiring the content creator to sign a waiver releasing the plant owner from responsibility if the CD produced is marginally or entirely unreadable by some playback equipment. In current practice, maximum CD playing time has crept higher by reducing minimum engineering tolerances.

This table shows the progression in the maximum duration of released audio CDs:

Title Artist Label Released Time
Mission of Burma (compilation) Mission of Burma Rykodisc 1988 80:08[17]
Proclamation (bass trombone recital) Douglas Yeo with Black Dyke Band Doyen DOY CD 055 1996 80:17[18]
Tchaikovsky's The Nutcracker Kirov Orchestra cond. Valery Gergiev Philips/Polygram 462 114-2 1998 81:14
Bruckner's Fifth (live) Munich Philharmonic cond. Christian Thielemann Deutsche Grammophon 477 5377 2004 82:34[19]
Chopin & Schumann Etudes Valentina Lisitsa Decca 478 7697 2014 85:16[20]
Mozart Violin Concertos (Mozart 225 Box Set, CD75) Various Artists Decca / Deutsche Grammophon 478 9864 2016 86:30[21]

Technical specifications

Data encoding

Each audio sample is a signed 16-bit two's complement integer, with sample values ranging from −32768 to +32767. The source audio data is divided into frames, containing twelve samples each (six left and right samples, alternating), for a total of 192 bits (24 bytes) of audio data per frame.

This stream of audio frames, as a whole, is then subjected to CIRC encoding, which segments and rearranges the data and expands it with parity bits in a way that allows occasional read errors to be detected and corrected. CIRC encoding also interleaves the audio frames throughout the disc over several consecutive frames so that the information will be more resistant to burst errors. Therefore, a physical frame on the disc will actually contain information from multiple logical audio frames. This process adds 64 bits of error correction data to each frame. After this, 8 bits of subcode or subchannel data are added to each of these encoded frames, which is used for control and addressing when playing the CD.

CIRC encoding plus the subcode byte generate 33-bytes long frames, called "channel-data" frames. These frames are then modulated through eight-to-fourteen modulation (EFM), where each 8-bit word is replaced with a corresponding 14-bit word designed to reduce the number of transitions between 0 and 1. This reduces the density of physical pits on the disc and provides an additional degree of error tolerance. Three "merging" bits are added before each 14-bit word for disambiguation and synchronization. In total there are 33 × (14 + 3) = 561 bits. A 27-bit word (a 24-bit pattern plus 3 merging bits) is added to the beginning of each frame to assist with synchronization, so the reading device can locate frames easily. With this, a frame ends up containing 588 bits of "channel data" (which are decoded to only 192 bits music).

The frames of channel data are finally written to disc physically in the form of pits and lands, with each pit or land representing a series of zeroes, and with the transition points—the edge of each pit—representing 1. A Red Book-compatible CD-R has pit-and-land-shaped spots on a layer of organic dye instead of actual pits and lands; a laser creates the spots by altering the reflective properties of the dye.

Data structure

Basic Illustration and mesures of a CD
This image of a CD-R demonstrates some of the visible features of an audio CD, including the lead-in, program area, and lead-out. A microscopic spiral of digital information begins near the disc's center and progresses toward the edge. The end of the data region and the lead-out can actually be anywhere, depending on how much data is recorded. Data-free areas of the disc and silent portions of the spiral reflect light differently, sometimes allowing track boundaries to be seen

The audio data stream in an audio CD is continuous, but has three parts. The main portion, which is further divided into playable audio tracks, is the program area. This section is preceded by a lead-in track and followed by a lead-out track. The lead-in and lead-out tracks encode only silent audio, but all three sections contain subcode data streams.

The lead-in's subcode contains repeated copies of the disc's Table Of Contents (TOC), which provides an index of the start positions of the tracks in the program area and lead-out. The track positions are referenced by absolute timecode, relative to the start of the program area, in MSF format: minutes, seconds, and fractional seconds called frames. Each timecode frame is one seventy-fifth of a second, and corresponds to a block of 98 channel-data frames—ultimately, a block of 588 pairs of left and right audio samples. Timecode contained in the subchannel data allows the reading device to locate the region of the disc that corresponds to the timecode in the TOC. The TOC on discs is analogous to the partition table on hard drives. Nonstandard or corrupted TOC records are abused as a form of CD/DVD copy protection, in e.g. the key2Audio scheme.

Tracks

The largest entity on a CD is called a track. A CD can contain up to 99 tracks (including a data track for mixed mode discs). Each track can in turn have up to 100 indexes, though players which handle this feature are rarely found outside of pro audio, particularly radio broadcasting. The vast majority of songs are recorded under index 1, with the pre-gap being index 0. Sometimes hidden tracks are placed at the end of the last track of the disc, often using index 2 or 3. This is also the case with some discs offering "101 sound effects", with 100 and 101 being indexed as two and three on track 99. The index, if used, is occasionally put on the track listing as a decimal part of the track number, such as 99.2 or 99.3. (Information Society's Hack was one of very few CD releases to do this, following a release with an equally obscure CD+G feature.) The track and index structure of the CD were carried forward to the DVD format as title and chapter, respectively.

Tracks, in turn, are divided into timecode frames (or sectors), which are further subdivided into channel-data frames.

Frames and timecode frames

The smallest entity in a CD is a channel-data frame, which consists of 33 bytes and contains six complete 16-bit stereo samples: 24 bytes for the audio (two bytes × two channels × six samples = 24 bytes), eight CIRC error-correction bytes, and one subcode byte. As described in the "Data encoding" section, after the EFM modulation the number of bits in a frame totals 588.

On a Red Book audio CD, data is addressed using the MSF scheme, with timecodes expressed in minutes, seconds and another type of frames (mm:ss:ff), where one frame corresponds to 1/75th of a second of audio: 588 pairs of left and right samples. This timecode frame is distinct from the 33-byte channel-data frame described above, and is used for time display and positioning the reading laser. When editing and extracting CD audio, this timecode frame is the smallest addressable time interval for an audio CD; thus, track boundaries only occur on these frame boundaries. Each of these structures contains 98 channel-data frames, totaling 98 × 24 = 2,352 bytes of music. The CD is played at a speed of 75 frames (or sectors) per second, thus 44,100 samples or 176,400 bytes per second.

In the 1990s, CD-ROM and related Digital Audio Extraction (DAE) technology introduced the term sector to refer to each timecode frame, with each sector being identified by a sequential integer number starting at zero, and with tracks aligned on sector boundaries. An audio CD sector corresponds to 2,352 bytes of decoded data. The Red Book does not refer to sectors, nor does it distinguish the corresponding sections of the disc's data stream except as "frames" in the MSF addressing scheme.

The following table shows the relation between tracks, timecode frames (sectors) and channel-data frames:

Track level Track N
Timecode frame or sector level Timecode frame or sector 1 (2,352 b of data) Timecode frame or sector 2 (2,352 b of data) ...
Channel-data frame level Channel-data frame 1 (24 b of data) ... Channel-data frame 98 (24 b of data) ... ...

Bit rate

The audio bit rate for a Red Book audio CD is 1,411,200 bits per second or 176,400 bytes per second; 2 channels × 44,100 samples per second per channel × 16 bits per sample. Audio data coming in from a CD is contained in sectors, each sector being 2,352 bytes, and with 75 sectors containing 1 second of audio. For comparison, the bit rate of a "1×" CD-ROM is defined as 2,048 bytes per sector × 75 sectors per second = 153,600 bytes per second. The remaining 304 bytes in a sector are used for additional data error correction.

Data access from computers

Unlike on a DVD or CD-ROM, there are no "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 systems, 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.

In a process called ripping, digital audio extraction software can be used to read CD-DA audio data and store it in files. Common audio file formats for this purpose include WAV and AIFF, which simply preface the LPCM data with a short header; FLAC, 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 coding formats like MP3 and AAC, which modify and compress the audio data in ways that irreversibly change the audio, but that exploit features of human hearing to make the changes difficult to discern.

Format variations

Recording publishers have created CDs that violate the Red Book standard. Some do so for the purpose of copy prevention, using systems like Copy Control. Some do so for extra features such as DualDisc, which includes both a CD layer and a DVD layer whereby the CD layer is much thinner, 0.9 mm, than required by the Red Book, which stipulates a nominal 1.2 mm, but at least 1.1 mm. Philips and many other companies have stated that including the Compact Disc Digital Audio logo on such non-conforming discs may constitute trademark infringement.

Super Audio CD was a standard published in 1999 that aimed to provide better audio quality in CDs, but it never became very popular. DVD Audio, an advanced version of the audio CD, emerged in 1999.[22] The format was designed to feature audio of higher fidelity. It applies a higher sampling rate and uses 650 nm lasers.

Copyright issues

There have been moves by the recording industry to make audio CDs (Compact Disc Digital Audio) unplayable on computer CD-ROM drives, to prevent the copying of music. This is done by intentionally introducing errors onto the disc that the embedded circuits on most stand-alone audio players can automatically compensate for, but which may confuse CD-ROM drives. Consumer rights advocates as of October 2001 pushed to require warning labels on compact discs that do not conform to the official Compact Disc Digital Audio standard (often called the Red Book) to inform consumers which discs do not permit full fair use of their content.

In 2005, Sony BMG Music Entertainment was criticized when a copy protection mechanism known as Extended Copy Protection (XCP) used on some of their audio CDs automatically and surreptitiously installed copy-prevention software on computers (see 2005 Sony BMG CD copy protection scandal). Such discs are not legally allowed to be called CDs or Compact Discs because they break the Red Book standard governing CDs, and Amazon.com for example describes them as "copy protected discs" rather than "compact discs" or "CDs".

See also

References

  1. ^ "How the CD was developed". BBC News. 17 August 2007. Retrieved 2007-08-17.
  2. ^ "Philips Compact Disc". Philips Historical Products. Retrieved 2016-01-24.
  3. ^ IEC 60908:1987 Compact disc digital audio system
  4. ^ IEC 60908:1999 Audio recording - Compact disc digital audio system (PDF)
  5. ^ Approved Compact Disc Logo configurations
  6. ^ Specs for Freeware Developers Archived 1 May 2012 at the Wayback Machine
  7. ^ "CD Products". lscdweb.com. Retrieved 2013-05-24.
  8. ^ 2-35] Why 44.1KHz? Why not 48KHz?
  9. ^ a b K. Schouhamer Immink (1998). "Compact Disc Story". Journal of the Audio Engineering Society. 46(5): 458–460. Retrieved 2018-02-06.
  10. ^ a b K. Schouhamer Immink (2007). "Shannon, Beethoven, and the Compact Disc". IEEE Information Theory Society Newsletter. 57: 42–46. Retrieved 2018-02-06.
  11. ^ Philips. "Beethoven's Ninth Symphony of Greater Importance than Technology". Retrieved 2007-02-09.
  12. ^ AES. "AES Oral History Project: Kees A.Schouhamer Immink". Retrieved 2008-07-29.
  13. ^ a b Cassidy, Fergus (23 October 2005). "Great Lengths". Sunday Tribune. Archived from the original (reprint) on 2007-10-12. Retrieved 2017-01-07.
  14. ^ Hoffmann, Frank; Ferstler, Howard (2005). Encyclopedia of Recorded Sound. CRC Press. p. 1289. ISBN 978-0-415-93835-8.
  15. ^ Goldmark, Peter. Maverick inventor; My Turbulent Years at CBS. New York: Saturday Review Press, 1973.
  16. ^ Andy McFadden (9 January 2010). "CD-Recordable FAQ". Retrieved 2010-12-30.
  17. ^ "Mission of Burma 1988 Rykodisc compilation information". discogs.com. Retrieved 2011-01-18. This Rykodisc release was the first compact disc to contain 80 minutes of music; 78 minutes had previously been the longest length possible to encode on a CD.
  18. ^ "Complete Program Notes for Proclamation". Retrieved 13 March 2016.
  19. ^ "BRUCKNER: Symphony No. 5 in B flat major (original version) — Munich Philharmonic/Christian Thielemann — DGG - Audiophile Audition". Audiophile Audition. Archived from the original on 2013-05-16.
  20. ^ "Valentina Lisitsa - Études - Amazon.com Music". www.amazon.com. Retrieved 2017-01-01.
  21. ^ McElhearn, Kirk (12 May 2017). "CDs Longer than 80 Minutes Are Becoming More Common". Kirkville. Retrieved 2017-05-12.
  22. ^ Taylor, Jim. "DVD FAQ". DVD Demystified. Archived from the original on 22 August 2009. Retrieved 21 August 2012.

External links

Audio bit depth

In digital audio using pulse-code modulation (PCM), bit depth is the number of bits of information in each sample, and it directly corresponds to the resolution of each sample. Examples of bit depth include Compact Disc Digital Audio, which uses 16 bits per sample, and DVD-Audio and Blu-ray Disc which can support up to 24 bits per sample.

In basic implementations, variations in bit depth primarily affect the noise level from quantization error—thus the signal-to-noise ratio (SNR) and dynamic range. However, techniques such as dithering, noise shaping and oversampling mitigate these effects without changing the bit depth. Bit depth also affects bit rate and file size.

Bit depth is only meaningful in reference to a PCM digital signal. Non-PCM formats, such as lossy compression formats, do not have associated bit depths.

CD-ROM

A CD-ROM (, compact disc read-only memory) is a pre-pressed optical compact disc that contains data. Computers can read—but not write to or erase—CD-ROMs, i.e. it is a type of read-only memory.

During the 1990s, CD-ROMs were popularly used to distribute software and data for computers and fourth generation video game consoles. Some CDs, called enhanced CDs, hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or digital video) is only usable on a computer (such as ISO 9660 format PC CD-ROMs).

The CD-ROM format was developed by Japanese company Denon in 1982. It was an extension of Compact Disc Digital Audio, and adapted the format to hold any form of digital data, with a storage capacity of 553 MiB. CD-ROM was then introduced by Denon and Sony at a Japanese computer show in 1984. The Yellow Book is the technical standard that defines the format of CD-ROMs. One of a set of color-bound books that contain the technical specifications for all CD formats, the Yellow Book, standardized by Sony and Philips in 1983, specifies a format for discs with a maximum capacity of 650 MiB.

CDDA

CDDA may refer to:

Circularly Disposed Dipole Array – a large circular antenna used by the military

Compact Disc Digital Audio (CD-DA) – an audio recording format

Company Directors Disqualification Act 1986 – a piece of UK company law, which sets out the procedures for company directors to be disqualified in certain cases of misconduct.

Common Database on Designated Areas – a database of the European Environment Agency (EEA) about nationally designated sites, nature protection sites such as national parks and nature reserves

Cataclysm: Dark Days Ahead – an open source post-apocalyptic survival roguelike video game

CDfs

CDfs is a virtual file system for Unix-like operating systems; it provides access to data and audio tracks on Compact Discs. When the CDfs driver mounts a Compact Disc, it represents each track as a file. This is consistent with the Unix convention "everything is a file".

CDfs supports the following track types:

Red Book Compact Disc Digital Audio (CD-DA): Appears as a WAV file; reading from it initiates DAE ripping.

White Book Video CD or Super Video CD video: Appears as a playable MPEG-1 file containing audio and video streams.

Yellow Book CD-ROM data:

Hierarchical File System: Appears as a mountable HFS file system disk image (sans partition table).

ISO 9660: Each session appears as a mountable ISO image file.

El Torito boot file: Appears as a single bootable disk image file.

Cactus Data Shield

Cactus Data Shield (CDS) is a form of CD/DVD copy protection for audio compact discs developed by Israeli company Midbar Technologies now owned by TiVo Corporation. It has been used extensively by EMI and BMG and their subsidiaries. CDS relies on basically two components: Erroneous Disc Navigation, and Data Corruption.

As of September 2006, all of Macrovision's CD copy protection products, including CDS, had quietly disappeared from their website. [1] The December 2006 issue of Billboard announced that EMI had decided to abandon Copy Control worldwide.

Cdparanoia

cdparanoia is a compact disc ripper for *nix and BeOS, developed by Xiph.org. It is designed to be a minimalistic, high-quality CD ripper that would be able to compensate for and adjust to poor hardware to produce a flawless rip.

libparanoia is a portable and platform independent library that was made from the important parts ripped from the Linux/gcc-only program cdparanoia. Libparanoia is part of cdrtools.

Compact Disc subcode

Subcode or subchannel data (called "control bytes" in the CD-ROM specification) refers to data contained in a compact disc (CD) in addition to digital audio or user data, which is used for control and playback of the CD. The original specification was defined in the Red Book standard for CD Digital Audio, though further specifications have extended their use (including the CD-ROM, CD Text and CD+G specifications).

Compact disc

Compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony and released in 1982. The format was originally developed to store and play only sound recordings (CD-DA) but was later adapted for storage of data (CD-ROM). Several other formats were further derived from these, including write-once audio and data storage (CD-R), rewritable media (CD-RW), Video Compact Disc (VCD), Super Video Compact Disc (SVCD), Photo CD, PictureCD, CD-i, and Enhanced Music CD. The first commercially available audio CD player, the Sony CDP-101, was released October 1982 in Japan.

Standard CDs have a diameter of 120 millimetres (4.7 in) and can hold up to about 80 minutes of uncompressed audio or about 700 MiB of data. The Mini CD has various diameters ranging from 60 to 80 millimetres (2.4 to 3.1 in); they are sometimes used for CD singles, storing up to 24 minutes of audio, or delivering device drivers.

At the time of the technology's introduction in 1982, a CD could store much more data than a personal computer hard drive, which would typically hold 10 MB. By 2010, hard drives commonly offered as much storage space as a thousand CDs, while their prices had plummeted to commodity level. In 2004, worldwide sales of audio CDs, CD-ROMs and CD-Rs reached about 30 billion discs. By 2007, 200 billion CDs had been sold worldwide.From the early 2000s CDs were increasingly being replaced by other forms of digital storage and distribution, with the result that by 2010 the number of audio CDs being sold in the U.S. had dropped about 50% from their peak; however, they remained one of the primary distribution methods for the music industry. In 2014, revenues from digital music services matched those from physical format sales for the first time.

Compressed audio optical disc

A compressed audio optical disc, MP3 CD, or MP3 CD-ROM or MP3 DVD is an optical disc (usually a CD-R, CD-RW, DVD-R or DVD-RW) that contains digital audio in the MP3 file format. Discs are written in the "Yellow Book" standard data format (used for CD-ROMs and DVD-ROMs), as opposed to the Red Book standard audio format (used for CD-DA audio CDs).

Emphasis (telecommunications)

Typically, prior to some process, such as transmission over cable, or recording to phonograph record or tape, the input frequency range most susceptible to noise is boosted. This is referred to as "pre-emphasis" – "pre-" the process the signal will undergo. Later, when the signal is received, or retrieved from recording, the reverse transformation is applied ("de-emphasis") so that the output accurately reproduces the original input. Any noise added by transmission or record/playback, to the frequency range previously boosted, is now attenuated in the de-emphasis stage.

The high-frequency signal components are emphasized to produce a more equal modulation index for the transmitted frequency spectrum, and therefore a better signal-to-noise ratio for the entire frequency range.

Emphasis is commonly used in FM broadcasting and vinyl (e.g. LP) records.

Four-channel Compact Disc Digital Audio

Compact Disc recordings contain two channels of 44.1-kHz 16-bit linear PCM audio. However, creators of the CD originally contemplated a four-channel, or quadraphonic, mode as well.The proprietary Red Book specification, as published by Sony and Philips, briefly mentions a four-channel mode in its June 1980, September 1983, and November 1991 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."

The Red Book also reserved the first bit of the so-called Q subchannel "control field" to signal the presence of four-channel audio, but did not specify a method for using four-channel in the CD system. Had it been later specified, this mode might have included four separate channels of linear PCM audio (requiring some combination of faster rotation, a lower sampling rate, or fewer bits per sample). Alternatively, the "four-channel" bit could have been used merely to indicate the presence of a matrix-encoded recording.

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, and later IEC 60908:1999. (Various national authorities have also adopted the IEC standard. E.g., it is also European Standard EN 60908:1999.)

Neither the 1987 nor the 1999 version of the IEC standard discusses the possibility of four-channel audio. Instead, the IEC document reserves the first bit of the Q subchannel "control field" to a different, although similarly cryptic, purpose—according to clause 17.5 note 2, it is for "Broadcasting use" in "non-audio applications of the Compact Disc."

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.

Hafler circuit

The Hafler circuit is a passive electronics circuit with the aim of getting derived surround sound or ambiophony from regular stereo recordings without using costly electronics. The Dynaquad system works using similar principles.Named after its early proponent audio engineer David Hafler, the circuit exploits the high amount of stereo separation in the front speakers. Using the circuit typically reduces this stereo separation by only about 2 dB.

This type of system is called 2:2:4, since the rear channels are simulated from a two channel stereo track, with no actual extra tracks encoded.

With this setup the rear speakers could be smaller, cheaper and have a smaller frequency range than the front speakers.

High-resolution audio

High-resolution audio, also known as High-definition audio or HD audio, is a marketing term used by some recorded-music retailers and high-fidelity sound reproduction equipment vendors. It refers to higher than 44.1 kHz sample rate and/or higher than 16-bit linear bit depth. It usually means 96 kHz (or even much higher), sometimes informally written as "96k", meaning a Nyquist frequency of 48 kHz. However, there also exist 44.1 kHz 24 bit recordings that are labeled HD Audio .

Sample-rate conversion

Sample-rate conversion is the process of changing the sampling rate of a discrete signal to obtain a new discrete representation of the underlying continuous signal. Application areas include image scaling and audio/visual systems, where different sampling rates may be used for engineering, economic, or historical reasons.

For example, Compact Disc Digital Audio and Digital Audio Tape systems use different sampling rates, and American television, European television, and movies all use different frame rates. Sample-rate conversion prevents changes in speed and pitch that would otherwise occur when transferring recorded material between such systems.

Within specific domains or for specific conversions, the following alternative terms for sample-rate conversion are also used: sampling-frequency conversion, resampling, upsampling, downsampling, interpolation, decimation, upscaling, downscaling. The term multi-rate digital signal processing is sometimes used to refer to systems that incorporate sample-rate conversion.

Swaggie Records

Swaggie Records was a jazz record company and label founded in 1949 by Graeme Bell in Australia. The early years of the label were defined by recordings by Australian jazz musicians. In the 1960s, it made licensing deals with American companies for vintage jazz reissues on 7-inch LPs. Similar programs followed in the 1970s (12-inch LPs) though the 1990s (CDs).

TurboDuo

The TurboDuo is a fourth-generation video game console developed by NEC Home Electronics and Hudson Soft for the North American market. The TurboDuo, released in October 1992, is the North American version of the PC Engine Duo, Japanese game console released in September 1991.

In the United States, the TurboDuo was marketed by Turbo Technologies, Inc. (or TTI) of Los Angeles, a joint venture of NEC Home Electronics and Hudson Soft. It was established to market NEC consoles in North America after NEC Home Electronics USA failed to effectively market the platform.

The TurboDuo integrates the capabilities of the TurboGrafx-16 and its CD-ROM drive (the TurboGrafx-CD) into a single, redesigned unit with an updated BIOS and 192 KB of additional RAM. TTI also offered the Super System Card via mail order, which provided the original TurboGrafx-CD with the 192 KB RAM upgrade.

The RAM increase and BIOS update afford the TurboDuo and PC Engine Duo compatibility with all CD-ROM² and Super CD-ROM² titles (Japanese and North American). Like the TurboGrafx-CD, the TurboDuo can read Compact Disc Digital Audio and CD+G discs. The TurboDuo, however, cannot read PC Engine HuCards without modification or an adapter. With a HuCard adapter and an Arcade Card Duo, the TurboDuo can also read Arcade CD-ROM² games (which were sold only in Japan).

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