Notes about

Compact Disc (CD) was developed by Philips in conjunction with Sony as an optical digital storage medium in 1980. Thanks to Sony, the release of the first audio CD player and mass produced audio CD took place in 1982, and in 2000 CD is the most widely used audio and removable data format by far.

Optical disc technology

CD just happens to be one of the first in a long line of optical media which are read using a laser beam which is reflected in varying intensity from the surface of a rapidly rotating metallic disc embedded in a protective plastic substrate. Other optical disc formats include the Philips Video Laserdisc (LD) for recording of analogue video signals, and more recently the minaturized Sony MiniDisc (MD) for digital audio and the much denser Toshiba Digital Video Disc (DVD) which has myriad applications, including high-definition digital video.

CD is a very adaptable format which was initially only intended to store digital audio data. Today however, CDs are also used to store computer files, multimedia content and even digital video. Additionally, while CD was initially evisaged as a read only medium which was pressed in a die, cast forever with one digital imprint, it wasn't long before ways were found to exploit heat-sensitive dyes as well as the magneto-optical properties of some other compounds to allow CDs to be recorded and rerecorded by powerful lasers and magnetic heads.

The key technology needed to realise optical discs, apart from the laser, is error correction. Without it, optical discs would be far more sensitive and prone to errors than they are known to be today. Error correction, specifically the Cross-Interleaved Reed-Solomon Code (CIRC,) allows small errors in the read signal to be seemlessly corrected by applying mathematical transformations.

Prerecorded audio CDs

Audio CDs contain audio data which has been encoded into a fixed-rate digital bitstream. The audio is sampled at a rate of exactly 44.1 kHz to a resolution of 16 bits, and two channels of such data are stored to give stereo output. This yields a bit rate of approximately 1.4 Mb/s, and such measures are often invoked to compare sound quality of digital audio media and encoding schemes. Generally, a higher bitrate means a nicer sound, although there are schemes to achieve higher quality for a given bitrate (such as Digital Theatre Systems Coherent Acoustics (DTS) encoding, which delivers maximal sound quality over 6 channels at CD bitrate.)

The sampling rate of CDs is no accident. It is believed that the highest frequency a young human can detect is around 22kHz. By setting a sampling rate just above this, the standard supposedly allows perfect reproduction of all audible frequencies (which is not necessarily the same as "all audible waveforms.") The 16 bit linear PCM samples are signed numbers lying between -32767 and +32768, which represent the divergence of the audio waveform from the time axis on a linear (as opposed to logarithmic) scale. The magic number "16" comes from the power of two (and even better, multiple of 8) which allowed an hour of recording time at 44.1 kHz. It allows a theoretical dynamic range of greater than 100 dB. This scheme has become the benchmark for high quality audio in recent times, and all modern audio media and encoding schemes compare themselves to CD quality using phrases like "near CD quality" or "better than CD quality."

The most common type of audio CD is the prerecorded Red Book CD. Initially, the only way to produce optical disc media was through mastering and pressing. First comes the manufacture a glass master, which in turn gives rise to metal stampers which is then be used to press aluminium sheets with the required pits. These sheets are subequently embedded in a plastic substrate to keep them rigid and to afford protection to the aluminium surface. The result is a prerecorded CD.

The name "Red Book" refers to the original CD specification originated by Philips, which specified how audio data should be encoded on CD. The "Red Book" defines what is understood by discs bearing the words "Compact Disc Digital Audio" (often abbreviated to CD-DA in technical documents) and the corresponding logo. "Red Book" is the widely adopted name of the specification for the simple reason that the cover of this specification was red. Since then, in keeping with tradition, the various emerging CD standards have also been commonly referred to by the differing colours of the covers of the books in which their specifications are published.

Data and hybrid CDs

The "Yellow Book" specification document (in combination with an associated document known as te "High Sierra Application Format") discusses an extension of the CD format, initially only intended to carry digital audio data, to accomodate more rigorously error-protected computer data in a hierarchical filing system. This is the format which forms the basis of all CD-ROM (Compact Disc Read Only Memory) discs in use today. It is more technically referred to as Compact Disc Digital Data (abbreviated to CD-DD.) The "Yellow Book" and associated "High Sierra" document will now simply be referred to collectively as "Yellow Book" in this document. (This is a common simplification.) Under this definition, Yellow Book comprises ISO10149 (error protection layer) and ISO9660 (hierarchical filing system format,) which are the ISO standards document describing it.

Because Yellow Book was written in the early days of microcomputers, it unfortuantely shares some of the limitations of the filing systems of those computers. The all uppercase, eight character filename with three letter extension was the most painful of these. Unfortunately, many extensions were developed to overcome this obvious of flaws in the original design, all mutually incompatible to a degree. The Microsoft Joliet extension was one attempt to overcome this limitation, and perhaps one of the more popular solutions today, though this wasn't always the case. Other solutions included Apple's HFS solution and Compact Disc Extended Architecture (CD-XA.)

Another limitation of Yellow Book was that it didn't allow for hybrid discs—that is, discs which contained both Red and Yellow Book sessions—or for discs with multiple Yellow Book sessions—an extension most useful in allowing CDs to be written in "stages." Multisession extensions given in the first part of the "Orange Book" specification overcame this limitation in a standardized way. Most computer drives and operating system device drivers now support hybrid or multisession discs, popularized under names such as "CD Extra" (formerly "CD Plus") and "Enhanced CD."

Recordable and Rewritable CDs

Orange Book defines two standards for recordable and rewritable CDs, one of which is really only used today as the basis of Sony's MiniDisc system. The first, still common format, Compact Disc Recordable (CD-WO or CD-R,) uses a heat-sensitive dye which can be "burned," but only once, with a powerful laser to form a pattern by a device known as a CD writer or burner. The second, Sony/Philips CD rewritable (CD-MO,) uses a material with magneto optical properties which can be rewritten many times by using a powerful laser in combination a magnetic field. This format should not be confused with what is currently commonly called CD Rewritable (CD-RW,) which is a purely optical media.

While CD-R is today used for audio, even in solid-state consumer recorders, CD-Rewritable (CD-RW) unfortunately it is incompatible with most audio players, being designed primarily as a medium for transporting computer data. This is expected to change as solid-state recorders become more popular, because CD-R is a difficult format to work with due to its more permanent nature.

Video and Interactive CDs

Building on the basic data formats (CD-DA, CD-DD and hybrid) and media modes (CD-R and CD-RW) are standards which further specify the usage of CDs in particular areas of application.

Notes about Audiotek CDs and numbering schemes

At the present time, Audiotek does not have the capacity to produce CDs en masse. Almost all CDs in the Audiotek collection have been produced by external parties, and all but a mere few are prerecorded. The Audiotek CD catalogue is divided into five sections, based on the format of the various media and their applications. Recordable and Rewritable CD variants, CD-R (from Orange Book Part II) and CD-RW (from later standards) respectively, are not distinguished.

The ATKR Red Book catalogue consists of CD-DA compatible discs, including those with other sessions. This implies that all CDs with a red book session have an ATKR catalogue number, however the number in the ATKD multisession catalogue is to be used by preference. The exception is backup copies which only include the audio session—in these cases the ATKR code should be used. Mix-mode discs, where data masquerades as an audio track in a Red Book session, belong to the ATKR catalogue. CD-Text is an extension to Red Book which stores track titles in the unused subcode space, and is not distinguished. Hidden Track, CD+G or CD+Graphics, CD-ROM Ready, Track Zero, i-Trax and Audio Vision discs are also primarily enhanced music discs, so belong to ATKR as well.

The ATKR catalogue lists whole second timings for tracks and CDs, and for each track the title and primary artists are given. The title of each CD is derived by the primary artists and the longest form of the title printed on the CD, and the publisher is listed. For soundtrack CDs, the designation "Soundtrack" is included in the title. Special formats are denoted by an annotation in box brackets. Additionally, the copyright year for the CD is given. No data on the mastering of tracks is listed, as this is rarely given in any detail on prerecorded CDs anyway.

The ATKY Yellow Book catalogue consists of all CD-DD discs which have no Red Book sessions and do not fall into the ATKJ or ATKG catalogues (see below.) This includes all variations and extensions on CD-DD, such as HFS, Joliet, El Torito/Bootable, Hybrid/Janus CDs as well as those CD-XAs which are computer specific. Only Yellow Book CDs which contain primarily audio or video content (i.e. files containing audio/video data) appear in the ATKY catalogue. Movie CD, distinct from CD-DV and VCD (White Book) by being specifically designed for use in personal computers, belong in the ATKY catalogue.

The ATKD Blue Book catalogue consists of all multisession CDs which have at least one Red Book session. These include CDs bearing the marks "CD-Plus," "CD+," "Enhanced CD," "ECD," "CD-Extra," "CDX," "CD-Enhanced," "CD-E" and "Stamped Multisession."

The ATKJ White Book catalogue consists of all CD-DV or VCD compatible discs (including multisession and hybrid discs with a White Book session) which contain highly compressed MPEG 1 video. NTSC and PAL VCDs are not distinguished. The ATKG Green Book catalogue consists of all CD-i and CD-XA compatible discs which contain machine-independent interactive content. DVCD or DVD/CD hybrid media are typically distinguished by receiving a categorization in the ATKX catalogue for DVD Video media.

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Last updated: Saturday 27th May 2000