CD-ROM
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 CD-ROM
| |
| File:CD-ROM example.jpg | |
| Developed by: | Sony and Philips |
| Released: | 1982 |
| Capacity: | Normal: 74 minutes audio or 650MB data Extended: 80 minutes audio or 700MB data |
| Formats: | 3DO Apple MacIntosh CD-i Microsoft Xbox Nec PC Engine Personal Computer Sega Mega CD Sega Saturn Sony PlayStation Sony PlayStation 2 |
| Read-only? | Yes |
Contents |
Overview
CD-ROM is an abbreviation for "Compact Disc Read-Only Memory". the term is commonly used to reference many of the different Compact Disc formats but also describes the second Compact Disc format. Discs in this category are commonly referred to as WORM drives due to a significant overlap, though WORM media forms a much broader category and not all Compact Disc formats are true WORM media.
Compact Discs were originally invented by Sony and Philips in a joint venture in the early 1980s eventually being launched in 1982 in it's audio-only format known as Compact Disc Digital Audio, or CDDA. This format is described in Red Book format of the Rainbow Book series. The CD-ROM format itself was the second (Yellow Book) format to be released, appearing in 1984 with a capacity of 74 minutes audio or 650MB data. An extension to the CD-ROM format would later allow 80 minutes/700MB data. The "Read-Only Memory" part means that data can only read from the disk, data cannot be written to the disk. CD-ROM discs are created with their content during the manufacturing phase.
Other CD formats
The CD-R format was originally called Compact Disc Write-Once (CDWO). It comprises a true WORM format: the portions of the disc can be written to only once but may subsequently be read many times. Many other formats of Compact Disc have been created over the years. CDs have generically been eclipsed by the Digital Video Disc (DVD) format.
Gaming
The Turbografx CD was the first console to use a CD drive but the most popular was probably the Sony PlayStation. The CD drive was eventually replaced by the DVD drive and its associated DVD-ROM media, with the release of the Sony PlayStation 2. Subsequent consoles during that Generation also used DVD drives or a format based on the DVD: the Nintendo GameCube used small 8cm Discs based on Matsushita's Optical Disc Technology, referred to as Mini-DVD. DVD drives are backwardly compatible with Compact Discs, giving the developer the choice of the cheaper media for smaller releases.
Analogue or Digital?
The original CD standard (CDDA) describes a format specifically designed for relaying sound with the intention of ensuring the reading mechanism could very rapidly locate and begin playing the sound, to know very rapidly at how far into a given piece of audio the player is at and to correct any errors found with the disc. Many perceive the various Compact Disc formats as being a digital but the manner in which the laser reads the information from the disc is based on the original CDDA specification and falls very much into the analogue arena.
The information is stored on the disc in two ways, termed lands and pits. As the disc spins, the laser reads the amount of reflected light: the lands are reflective surface areas and the pits are non-reflective areas (they are actually just less-reflective). These lands and pits are both interpretted by the CD-ROM as binary 0. The areas of transition between a land and a pit are interpretted as binary 1. These lands and pits with their transitions form a single track spiralling from the centre to the circumference - in all about 3 miles of track. The disc spins at around 500 RPM in the centre down to around 250 RPM at the circumference in an attempt to keep consistent the speed at which the surface is read by the laser, this is not guaranteed, however.
Using the transition areas as binary 1 allows the reading engine to determine the speed of the spinning disc - the transition forms a single unit of time but that unit of time can alter depending on how close the laser is to the centre or circumference. From this transition length, the length in time of the lands and pits can be calculated to thier representative number of binary 0s.
Unfortunately, as the disc spin speed varies from the centre to the circumference, it necessary to have transitions at regular intervals so the areas of lands and pits surrounding can be correctly calculated. The CDDA standard depicts a maximum duration for each land or pit section by ensuring one transition occurs at least once every 10 bits of time, allowing the reading engine to re-align itself in time with the spin-speed of the disc. To enable these transitions to occur while carrying actual data, every byte (8-bits) of real information on the CD will have 3 bits of information either side - fourteen bits per one byte in a method of encoding called Eight-to-Fourteen Modulation (EFM). As the real data needing to be encoded has only 256 possible variations (8-bits), only 256 different EFM sequences are required and defined. The sequences are defined to make them as different as possible and allow the reading engine to determine at any time the EFM-boundary of the data within just a few bytes-worth of spin-time.
If two such EFM sequences were concatenated, the possibility remained of creating a third, phantom, EFM sequences in the middle which could be interpretted incorrectly. The CDDA standard further indicates to include an additional 3 bits per EFM sequence to ensure this possibility is avoided. Each byte of real data is therefore coded inside a total of 17 bits.
To confirm that the data had been read correctly from the disc and that scratches and manufacturing defects have not affected the data, the CDDA standard indicates for every 12 sequences of encoded bytes (i.e., every 12*(EFM+3) bits = 204 bits) a further 4 sequences are written to the disc as error correction (4*(EFM+3) bits = 68 bits). There is nothing to distinguish these error correction sequences from normal data sequences, so to prevent them being misinterpretted, the CDDA standard wraps two such data+correction sets into a frame. Each frame consists of a synchronisation header, a Control and Display byte and then two of the data+correction pairs.
The synchronisation header is formed from a specific pattern of 24 bits which does not coincide with the 256 different possibilities of the EFM sequences. The synchronisation pattern is given another 3 bit sequence at the tail end so it cannot be misinterpretted as an EFM sequence. This then forms the basic block of data - a frame - and carries just 24 bytes (192 bits) of real data in its 588 bit sequence:
| Data Type | Bit make up | Total bits |
| Sequence Header | 24+3 | 27 |
| Control and Display Byte | 14+3 | 17 |
| 12* Data bytes | 12 * (14+3) | 204 |
| 4 * Error correction | 4 * (14+3) | 68 |
| 12* Data bytes | 12 * (14+3) | 204 |
| 4 * Error correction | 4 * (14+3) | 68 |
| 588 |
So, is that it then? What of the Control and Display byte? Recall near the beginning of this section, one of the aims of CDDA is to know very rapidly at how far into a given piece of audio the player is at. The Control and Display byte carries eight seperate channels of information called the Subcoding Channels: labelled P through W, one bit each in each frame. In the CDDA specification, only channels P and Q have defined behaviour. Channel P carries a simple track seperator for the music where as Channel Q carries different information depending on the location on the disc. At the beginning of the disc is a lead-in section where Channel Q carries a table of contents for the disc including how many tracks and their starting location. Within an audio track the Channel Q carries the timing information for the audio along with which track number and type (audio or data) contained in this track.
A complete sequence of channel data is carried over 98 sequential frames and is referred to as one Subcoding Block. Each subcoding block covers 7203 bytes of on-disc data to describe a total of just 2352 bytes real audio data and associated timing and supplementary information (from the Q channel).
Formats
| Compact Disc Formats and Official Logos | ||
|---|---|---|
| Logo | Format Name | Format Specification |
| Compact Disc Digital Audio (CD-DA) | This logo must only be used on discs complying with the CD-DA specification, the IEC 908 standard and or the Philips-Sony Compact Disc Digital Audio System Description also known as the Red Book. |
| Compact Disc Read Only Memory (CD-ROM) | This logo may only be used on discs complying with the CD-ROM specifications - ISO/IEC 10149 standard and/or Philips-Sony Compact Disc Read Only Memory System Description also known as the Yellow Book. Use this logo for CD-ROM XA (XA for eXtended Architecture). |
| Compact Disc CD Extra | This logo may only be used on discs complying with the CD Extra specifications - Philips-Sony CD Extra specifications also known as the Blue Book. NOTE - May not be used for Enhanced CDs. |
| Compact Disc Graphics (CD-G) | This logo may only be used on discs complying with both the CD-DA specifications as above and which utilise and comply with the optional TV-Graphics mode as specified in the options section for Subcode Channels R-W of the Philips-Sony RED Book |
| Compact Disc Extended Graphics (CD-EG) | This logo may only be used on discs complying with both the CD-DA specifications as above and which utilise and comply with the optional Extended TV-Graphics mode as specified in the options section for Subcode Channels R-W of the Philips-Sony RED Book |
| Video Compact Disc (Video CD) | This logo may only be used on discs complying with the Video CD specifications - Philips-JVC-Matsushita-Sony Compact Disc Digital Video specifications also known as the White Book |
| Compact Disc Interactive (CD-i) | This logo may only be used on discs complying with the CD-i specifications Philips-Sony Compact Disc Interactive Full Functional Specifications also known as the Green Book. |
| Compact Disc MIDI (CD-MIDI) | This logo may only be used on discs complying with both the CD-DA specification (IEC 908 and / or the Philips-Sony Compact Disc Digital Audio System Description - Red Book) and which utilise and comply with the optional MIDI mode as specified in the options section for Subcode Channels R-W of the Red Book. |
| Photo CD (P-CD) | This logo may only be used on discs complying with the Photo CD specification - Philips-Kodak Systems Description Photo CD NOTE This logo is not to be used for CD ROMs containing photographic images unless they are in the Kodak Photo CD format. |
| Compact Disc Video (CD-V) | This logo may only be used on 12cm. video discs complying with the CD-V specifications: the IEC 908 and IEC 1104 standards and/or the Philips-Sony Compact Disc Digital Audio System Description (the RED Book) with reference to the IEC 856 and IEC 857 standards. |
| Compact Disc Digital Audio Recordable (CD-DAR) | This logo may only be used on recordable discs complying with the CD-R and CD-DA specifications: Phillips-Sony Recordable Compact Disc System Descriptions, Part II: CD-WO (Write Once) (the ORANGE Book Part II) and which are exclusively preset for audio (CD-DA) applications as specified in the IEC 908 standard and/or the Phillips-Sony Compact Disc Digital Audio System Description (the RED Book). |
| Compact Disc Recordable (CD-R) | This logo may only be used on recordable discs complying with the CD-R and CD-ROM specifications: Phillips-Sony Recordable Compact Disc System Descriptions, Part II: CD-WO (Write Once) (the ORANGE Book Part II) and which are exclusively preset for data (CD-ROM) applications as specified in the ISO/IEC 10149 standard and/or the Phillips-Sony Compact Disc Read Only Memory System Description (the YELLOW Book). |
| Compact Disc Digital Audio Text (CD-Text) | CD Text is a extension of the existing audio CD standard ("Red Book") that adds the ability to store text and graphical information like album titles, artist names, and song titles on a music CD. This information is stored in such a way that it doesn't interfere with the normal operation of any CD players or CDROM drives. |
| Not applicable | China Video Disc | The principal pre-cursor for Super Video Compact Disc developed by C-Cube Microsystems and it's partners in China. The development of the CVD specification began in 1997 and the first CVD players were released on the market in June 1998. |
| Not applicable | High Quality Video Compact Disc (HQ-VCD) | Developed by the original Video-CD consortium (Philips-Sony-Matsushita-JVC) in an effort to preserve their patents and rights by proposing an evolution of their standard Video-CD 2.0. The standard was still at the draft stage when the CVD standard came to market in June 1998. |
| Super Video Compact Disc (SVCD) | An enhancement to Video CD developed by a Chinese government-backed committee of manufacturers and researchers. Partly and effort to circumvent DVD technology royalties and partly to create pressure for lower DVD player and disc prices in China. Like HQ-VCD, the standard was still at the draft stage when the CVD standard came to market in June 1998. The Chinese government form an alliance with the Video-CD consortium as they were unaware of the CVD developments until it had arrived on the market. The final SVCD specification (HQ-VCD was merged into SVCD), set by the China National Committee of Recording Standards, was announced in September 1998. The CVD format was already widespread in China at the time of announcement and the Chinese government made great efforts to ensure new players would be compatible, producing the Chaoji VCD standard. |
| Chaoji VCD | A compatibility specification rather than a format. A Chaoji VCD player must be able to play back at least SVCD, CVD, VCD 3.0, VCD 2.0, VCD 1.1 and CD-DA discs. |
| Microsoft High Definition Compatible Digital | The patented HDCD technology was developed and perfected between 1986 and 1991 by Keith Johnson and Pflash Pflaumer, two preeminent technologists in the audio arena. In 1995, the HDCD technology was officially introduced to the market. Microsoft built on this success and extended the reach of the decoder technology by building relationships with large, established, audio integrated-circuit (IC) vendors focused on the CD player, audio/video (A/V) receiver, and DVD markets. High Definition Compatible Digital (HDCD) is a patented encode/decode process for delivering the full richness and detail of the original microphone feed on compact discs (CDs) and DVD-Audio. HDCD has been used in the recording of more than 5,000 CD titles, which include more than 250 Billboard Top 200 recordings and more than 175 GRAMMY nominations, and account for more than 300 million CDs sold. |
| Super Audio Compact Disc (SACD) | Super Audio CD was launched in early September 1999 by Sony and Philips Electronics. In addition to exceptional sound quality through the DSD system, the SACD format can accommodate more than four times the information of the current CD format. With this extra capacity, a standard Super Audio CD will provide space for 2-channel stereo data, as well as an area for up to 6-track multi-channel data, storage capacity for text and images, disc variations, copyright protection and much more. A Hybrid SACD contains two separate layers. One layer carries the normal CD information and the other layer contains the high density SACD information - a multichannel mix and/or stereo mix. A Hybrid SACD can be played on any CD compatible player. However, if played on a standard CD player, only the CD layer will play. The SACD layer can only be played on a SACD player. Purportedly supported by the Sony PlayStation 3. |
