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Compact Disc Formats
- Compact Disc - Digital Audio (CD-DA)
- CD-ROM
- Mixed Mode CD
- CD-ROM/XA
- CD-i
- CD-i Ready
- Multi-session CD
- CD-Extra
- Bridge Disc
- Compact Disc - Recordable, CD-R, CD-MO and CD-E
- Mini Disc
- ISO 9660, HFS, and Joliet
- DVD - the high density format- Glossary (Short description of lots of formats and terms)
Compact Disc - Digital Audio (CD-DA)
The CD-DA was defined in 1982 in the RedBook by Philips and Sony. A CD-DA sector has 2,352 bytes of user data. The sectors are addressed by minutes, seconds and sectors. The address information is stored in the subchannel (Q). The maximum running time of a 12-cm CD is 74 minutes, an 8-cm single CD has a running time of about 21 minutes. There are two times for addressing a CD audio: by ATime, which means "Absolute Time", and is measured from the start of the disc; and by Track Relative Time, which is measured from the start of a track. 99 tracks on one disc can be accessed directly by a CD-DA player. A track is a continuous data sequence. Each track must contain at least 4 seconds (i.e. 300 sectors, since 1 second is divided into 75 sectors). A maximum of the entire CD can be used for one track. A single tune or musical sequence on CD-DA refers to one track.
On a CD-DA all 2,352 bytes of a sector are user data, thus 2,353 bytes multiplied by 75 sectors equals 176,400 bytes, which are transferred per second. This equals about 1.41 Mbit/sec. A cross-check: CD-DA works on a sampling rate of 44,1 kHz and 16 bit samples for 2 stereo channels, i.e. 44,100 x 16 x 2=1,411,200 bit/sec or 176,400 bytes audio data per second.
For each sector and 2,352 bytes of user data, 882 additional bytes are stored on the disc for the 2 layers of error detection and correction code of CIRC (784), and the Control Bytes (98). Each bit of the control byte is given a letter, "P", "Q", "R", "S", "T", "U", "V", and "W". The first bit is designated as "P", the second "Q", and so on. The data stream resulting from all first bits of the 98 control bytes is called "P" subchannel, the stream of all second bits "Q" subchannel. The third subchannel combines the bits "R", "S", "T", "U", "V" and "W" to a 6-bit word, and the data stream of these bits, resulting from the 98 control bytes, is called "R thru W" subchannel. The "P" subchannel has a flag that indicates when the audio data in a track is beginning. "Q" subchannel gives the time code, ATime and Track Relative Time. In the lead-in of the disc this subchannel contains the Table of Contents (TOC). 72 of the 98 bits of the "Q"-subchannel are used for information, the others for synchronization, control and error correction (for the subchannels). Beside sync, control and error correction bits, the "R thru W" subchannel may include user data (64 6-bit words per sector) for additional information. Only the RedBook allows this (for CD-DA), while the "YellowBook" specifies that these bits are set to 0. On an Audio CD, the "R thru W" subchannel is sometimes used for graphic or MIDI data. (MIDI stands for Musical Instrument Digital Interface and means a standard protocol for communication between electronic musical instruments and computers. MIDI files contain messages which refer to the MIDI specification and which can be interpreted by MIDI devices).
These discs are called CD+G (Compact disc plus graphics) or CD+MIDI, and can be displayed by a special player in conjunction with a TV and hi-fi set.
This subchannel can also be used for user-defined data. The CD+G and CD+MIDI titles are rare. CD+G titles can be read by CD-i players and some special Japanese consumer players. CD+G and CD+MIDI can be interpreted by Tandy's VIS player (VIS stands for Video Information System) and Commodore's CDTV player. However, these formats are rare and not very important.
Soon after the definition of the Audio CD people noticed that this storage medium for huge amounts of data could also be used for storing computer data. However, it had to be modified. Thus, in 1984 the "YellowBook" by Philips and Sony defined the CD-ROM for storing computer data.
Firstly, two new kinds of sectors were defined: Mode 1 for storing computer data and Mode 2 for compressed audio or video/graphic data.
First of all, computer data need a more precise access than the tracks of the Audio CD. On a Compact Disc, 99 tracks are to be accessed, but on a CD-ROM there may well be thousands of data files which have to be addressed. Thus, both formats, Mode 1 and Mode 2, make use of some bytes at the beginning of the sector for precise addressing. The first 12 bytes are sync bytes for sector separation. However, the sync pattern of these bytes might accidentally also occur in the user bytes. Thus, both sync bytes and length of the sector are used for identifying the sector. The next four bytes are header bytes. 3 of them are used for addressing, while the 4th is the mode byte which marks the mode used by the sectors of the track.
Mode 1 sectors have 2048 bytes of user data. The sector can be divided into logical blocks. Different logical blocks might be used on different CD-ROMs: blocks of 512, 1024 and 2048 bytes.
The logical block size cannot be larger than the sector size.
Sectors are the smallest addressable part of the CD-ROM that can be accessed independently of other addressable parts of the recorded area. However, smaller logical blocks can be accessed via a sector. The address of the header bytes indicates minutes, seconds and additional information for the blocks. The Logical Block Number (LBN) can be identified by this information. The first physical sector that can be accessed is sector 00:02:00. This sector contains the first Logical Block, LBN 0. If you have 512 byte blocks, 18,000 blocks make a minute, 300 make a second and 4 make a sector. Thus you can obtain the logical block address by a simple algorithm. However, in that case you have to subtract 600 blocks because of the starting address of the first sector 00:02:00. These 2 seconds equal 600 blocks.
CD-ROM Mode contains the CIRC error detection and correction of the Audio CD. However, because computer data need a higher level of data integrity, an additional error detection and correction is set up on CIRC, called Layered EDC/ECC. This additional error detection and correction needs some bytes of the sector behind the user data: 4 bytes for error detection and 276 bytes for error correction. Between the error detection and the error correction are 8 unused bytes. These unused bytes are redefined by CD-ROM/XA and CD-i.
CD-ROM Mode 2 has no additional error detection and correction scheme, and thus all 2,336 bytes behind the sync and header bytes are user bytes.
The sectors of CD-DA, CD-ROM Mode 1 and Mode 2 are the same size, but the amount of user data that can be stored varies considerably because of the use of sync bytes, header bytes, error correction and detection. The Audio CD uses all bytes of a sector (2,352) for user data, CD-ROM Mode 1 blocks have 2,048, Mode 2 blocks 2,336 user bytes. Therefore, there are different data transmission rates for Mode 1 and Mode 2 (about 1,22 Mbit/sec for Mode 1 and about 1,4 Mbit/sec for Mode 2). Although Mode 2 has a higher data rate and more space for data, it is not used very often except in conjunction with CD-ROM/XA and CD-i (which are always Mode 2).
Mode 2 can be read by normal CD-ROM drives but special software drivers have to be used.
If high-level audio sequences are needed, CD-ROM tracks and Audio CD tracks may be mixed on a compact disc. A Compact Disc has frames, sectors and tracks. It is true that one track cannot contain different kinds of sectors, but the CD can have different kinds of tracks.
Usually, the first track of a Mixed Mode CD is a CD-ROM Mode 1 track, and the following tracks are CD-DA tracks. A well-known example for such a Mixed Mode CD is a title produced by the Voyager Company and Microsoft called Multimedia Beethoven. The Ninth Symphony. This CD has 6 Tracks, the first being a CD-ROM Mode 1 track, and the following ones Audio CD tracks as laid down in the specifications of the RedBook.
The disc contains the following tracks:
| ATIME | TRACK RELATIVE TIME | ||
| Track 1: CD-ROM Mode 1 | from | 00:00:00 | 00:00:00 |
| data | to | 01:06:00 | 01:06:00 |
| Track 2: audio | from | 01:06:00 | 00:00:00 |
| Allegro ma non troppo, | to | 17:43:00 | 16:37:00 |
| un poco maestoso | |||
| Track 3: audio | from | 17:43:00 | 00:00:00 |
| Molto vivace | to | 27:53:00 | 10:10:00 |
| Track 4: audio | from | 27:53:00 | 00:00:00 |
| Adagio molto e cantabile | to | 43:45:00 | 15:52:00 |
| Track 5: audio | from | 43:45:00 | 00:00:00 |
| Presto | to | 69:29:00 | 25:44:00 |
| Track 6: audio | from | 69:29:00 | 00:00:00 |
| Comments | to | 70:37:00 | 01:08:00 |
Tracks 2 to 5 are RedBook audio tracks identical to those on a normal Audio CD of Beethoven's Ninth Symphony.
Track 6 contains some audio recordings of comments for a game on this CD ("Ludwig�s answers"). Track 1 is a CD-ROM Mode 1 track containing an application program and some data files. Some MIDI sound sequences with examples of musical themes, parts, and variations are stored in a sub-directory.
In general, there is one important limitation for Mixed Mode CDs. A CD-ROM drive can only read one track at a time, and, when reading the RedBook audio track, no other data can be transmitted from the CD-ROM drive. There are two common methods to solve this problem. The first method is to use the CD-ROM drive as an Audio CD player, and to transfer the application program and data to the harddisk beforehand. The computer can now read the program information in the memory, and access the audio data continuously.
To do this, the computer must have enough memory and free space on the harddisk. The Beethoven CD uses this method.
The alternative is to read the program information into the computer's memory before accessing the audio data. But this method entails the audio being interrupted when the next portion of data has to be read.
The audio tracks on a Mixed Mode CD can be addressed in two different ways: by using ATime and by using Track Relative Time. Most programs use Track Relative Time, because when addressing by ATime all audio calls will require re-synchronizing if the data volume of the CD-ROM Mode 1 track has changed during the production process.
Even though the audio tracks could be played back by an Audio CD player, Mixed Mode CDs should not be used on such a device. Caution! If the CD-ROM track is read, this may damage the hifi-set (although there are also some new Audio CD players which will mute the CD-ROM tracks).
CD-ROM/XA stands for Compact Disc-Read Only Memory/eXtended Architecture.
The first release of the XA-specification was introduced by Philips, Sony and Microsoft in September 1989, the "Final System Description" in March 1991. However, that will probably not be the final description because CD-ROM/XA level 3 has to integrate MPEG motion video and some further specifications on the identification of the host operation system (GOE, Generic Operation Environments). Since CD-ROM/XA is not really a new standard but an extension of the YellowBook and the definitions for CD-ROM, the specification is sometimes called Extended YellowBook. The extensions are related to the CD-i specifications of the GreenBook. Because of that relationship, and the fact that CD-ROM/XA may form a bridge between computer systems based on XA and consumer players based on CD-i, this standard is often regarded as the missing link for publishers to produce discs running on different platforms for industrial and consumer markets.
XA-tracks can contain binary code as well as video and graphic data, text data as well as compressed audio data. Other CD formats use only one sector format in one track. XA can use two different sector formats in a track and interleave them into the bargain. Thus, one sector may be followed by a different one. The two XA-sector formats are called Form 1 and Form 2. The use of the sectors depends on the content. Form 1 sectors are used for computer data. Like CD-ROM Mode 1 sectors they have an additional Layered Error Detection and Correction Code. They also use the first 12 bytes for a sync pattern and the following 4 bytes for header data (addressing and mode description). Like CD-ROM Mode 1 each Form 1 sector contains 2,048 user bytes. However, unlike Mode 1, a subheader is added to Form 1 sectors behind the header bytes, and unlike Mode 1 there are no more unused bytes between the error detection and the error correction code.
EDC and ECC are moved together and the previously unused 8 bytes can thus be used for the subheader bytes. CD-ROM/XA Form 1 sectors contain computer data.
Form 2 sectors are arranged in the same way as Form 1 sectors, except that there is no additional Error Correction Code (ECC). At the end of the sector�s user data a field of 4 bytes is reserved. This field can be employed for quality control during the disc production process. In that case it is recommended to use the same EDC algorithm as for Form 1 sectors. Otherwise the reserved bytes are set to zero (0).
Both Form 1 and Form 2 sectors use 8 bytes for a subheader that specifies the following user data. Because different sectors with different contents can be stored in interleaved fashion, the first byte of the subheader is a file number for identifying the interleaved sectors belonging to one and the same file.
However, an interleaved file may contain different pieces of information that can be played back in combination or separately. Thus, the second byte gives a channel number for the real-time selection of this information. The Channel Numbers 0 to 15 are used for ADPCM audio sectors, channel number 0 to 31 for video or data sectors.
The next byte defines global attributes of a sector and is called the submode byte. Each bit can be set as a flag, for example, to indicate the kind of information: video, ADPCM audio, Data, to mark the last sector of a file (EOF) or a record (EOR) and to set a real-time mode. Using real-time sectors means that the timing of the data reading is more important than data integrity, so that error correction is only carried out as long as correct timing of the data is not affected. This is an important feature when using time-based data, such as audio data streams. It would be more annoying for the listener if the data stream were interrupted than if a wrong bit were transmitted. Interruptions would cause drop-outs and glottstops. Transmitting a wrong bit which remains uncorrected would probably not be noticed.
The submode byte is followed by a byte for the Coding Information which defines the details of the type of data located in the user area of the sector: the kind of ADPCM audio, whether stereo or mono, and the video resolution and coding, etc.
To avoid data losses the information in the first four bytes of the subheader is repeated in the bytes 5 to 8.
It is possible to mix CD-ROM/XA tracks on a CD with Mode 1 tracks and Audio CD tracks. In that case, the Mode 1 track should be the first track on the disc, followed by one CD-ROM/XA track and an Audio CD track.
CD-ROM/XA offers a number of advantages in comparison with CD-ROM Mode 1. These advantages become important when using time-based data in multimedia applications because computer data and compressed audio data could be read out of the same track.
How does a system based on a normal CD-ROM work? Using Mode 1 tracks some files are read and "stocked up" in the computer's memory or swapped on the harddisk before the application can start. During this time a symbol is shown, a watch, an hour-glass or similar, and it takes at least several seconds, and sometimes even minutes, before the user gets the first information.
When using CD-ROM/XA discs, interleaved files can be read in parallel at the same time. Only those parts are read which are currently needed. The user obtains his information "just in time", and long waiting periods are not necessary. Audio data can be separated by the controller when reading the disc, decompressed and played out through the audio jacks. Only the data that is needed gets on the computer bus.
Like CD-ROM Mode 1, Form 1 sectors admit a data rate of 1.2 Mbit/sec. However, by using Form 2 sectors the data rate increases to about 1.4 Mbit/sec, because the sector is read in 1/75 second and has 276 byte more of user data.
This means that more than 20 Kbytes user data per second can be transmitted.
For the production of CD-ROM/XA, special software is required which supports the interleaving of files in different sectors. On the other hand, a special controller is needed for reading the interleaved information on the disc.
CD-ROM/XA supports some ADPCM audio and some video formats of the CD-i specification, ADPCM Level B and C, as well as the video modes based on Color look-up tables (CLUTs) as described below in the chapter about CD-i.
The CD-i standard was described in 1987 by Philips and Sony in the "GreenBook" (a year before CD-ROM/XA). The sectors of CD-i are identical with CD-ROM/XA, and there are also Form 1 and Form 2 sectors. CD-i permits the interleaving of sectors and files in the same way that CD-ROM/XA does.
In many ways one could say that CD-i is a special kind of CD-ROM/XA for use in products of the consumer electronics industry. Therefore, the specification does not only describe the format of the sectors, but also the operating system called CD-RTOS that CD-i is based on. This operating system is a derivative of OS-9. CD-i players run on a 68070 microprocessor from Motorola.
CD-i players are placed in a hi-fi rack and are connected to the TV set and amplifier. They can play Audio CDs, as well as PhotoCDs, CD+G titles and, of course, CD-i titles using interactive structures and different media: text, audio, graphics, animated cartoons, still video and full motion video.
The CD-i standard specifies audio and video formats. There are three different levels of ADPCM audio which can be used. Level B and Level C are also supported by CD-ROM/XA. RedBook-Audio can only be stored in CD-DA tracks.
CD-i Audio Formats:
| RedBook-Audio | Level A | Level B | Level C | |
| Sampling rate | 44.1 | 37.8 | 37.8 | 18.9 |
| Mode | PCM | ADPCM | ADPCM | ADPCM |
| Bits | 16 | 8 | 4 | 4 |
| Compression rate | no compression | 2 | 4 | 8 |
| Mono/Stereo | Stereo | M/S | M/S | M/S |
| Time | 1:12 | 4:48/2:24 | 9:36/4:48 | 19:12/9:36 |
There are three video resolutions for CD-i: normal, double and high. The normal resolution for NTSC 525-line TVs is 384 x 240 (horizontally x vertically, for PAL 625-line systems 384 x 280). Double resolution has twice the horizontal resolution but normal vertical resolution. In high resolution mode both horizontal and vertical resolution are doubled. The different video formats support different purposes and contents of the image.
CD-i Video Formats
| Type | RGB555 | DYUV | DYUV+QHY |
| Resolution | normal | normal | high |
| Colors | 32.768 | 16.8 mill. | 16.8 mill. |
| Bits/pixel | 16 | 16/2 | 16 |
| Content | detailed images | "natural" images | "natural" images |
| Description | 5 bits for each R, G, B | Delta YUV | Delta YUV + Quantified High Y |
| Type | CLUT8 | CLUT7 | CLUT4 | RLE7 | RLE3 |
| Resolution | normal | normal | double | normal | double |
| Colors | 256 of | 128 of | 16 of | 128 of 16.8 | 7 of |
| 16,8 mill. | 16,8 mill. | 16,8 mill. | 16,8 mill. | 16,8 mill. | |
| Bits/pixel | 8 | 7 | 8/2 | 7 | 8/2 |
| Content | graphics, | graphics | graphics | animated | animated |
| natural images | cartoon | cartoon | |||
| Description | 8 bit Color | 7 bit Color | 4 bit Color | Run-Length | Run Length |
| LookUp Table | LookUp Table | LookUp Table | of CLUT 7 | of CLUT 4 |
For CD-ROM/XA the CLUT formats can be used. Both CD-i and CD-ROM/XA will support the MPEG standard for motion video associated audio for digital storage media up to about 1.5 Mbit/s.
A disc called CD-i Ready is (virtually) a normal Audio CD with additional features. However, these features can only be displayed by CD-i players.
The RedBook allows the producer to set indices on a CD-DA. If this is supported, the player may skip to the marked points on the track. Usually only two indices are in use: index 0 and index 1.
Index 0 is located before the pregap of audio silence at the beginning of a track, while index 1 marks the beginning of the audio on the track. The silence of the pregap lasts 2 or 3 seconds. Index 0 on track 1 is never used by Audio CD players. On the first track at the beginning of the disc, Index 1 is always skipped to. CD-i Ready uses the pregap between index 0 and index 1 and enlarges the distance (it should not be less then 182 seconds, but it can be much longer). In this enlarged pregap before track 1 a CD-i Ready hides a CD-i track which can only be recognized by a CD-i Player which skips to index 0. Normal Audio CD players will ignore the pregap and the data so that the disc will run on such a device as a normal Audio CD. CD-i players can identify the disc by the start address. If the address of the first track is less than 30 seconds, a normal audio CD is identified. Otherwise, the place where the CD-i files can be read is written in the data sector 00:02:16. The information is loaded into the player's RAM-memory before the audio starts, and can be displayed while playing the songs and music sequences. In this way CD-i Ready might be regarded as a Mixed Mode CD using CD-i and CD-DA, where the CD-i track is hidden so that it can be played on Audio CD players.
One of the first CD-i Ready discs published was the "American Songbook" featuring Louis Armstrong. While playing the songs, the words can be displayed on the TV screen; but from the CD-i track the player can display still videos and give an introduction to the life and music of Louis Armstrong and some of his contemporaries.
The term multisession CD was first used for Kodak Photo CD. Photo CDs store images from films. So in a first session the images of the first film can be stored on the Photo CD.
If there is free space left it can be used to store new images in further sessions. Every session has its own lead in, program area, and lead out. The benefit of multisession CDs is that this way to write a disc allows appending of new data at various points of time, and that all addressing of these data is done over all sessions.
Adding finalized sessions allows user to read the disc's like CD-ROMs in current CD-drives. But to write a lead in and a lead out for each session requires storage capacity (about 20 MB). So it could be better to write instead of session by session a 'Disc-at-Once'. But a session can also fill the disc's capacity. In that case the overhead for one lead in and one lead out does not differ from that by writing the 'Disc-at-Once'. But 'Disc-at-Once' mode on the other hand can also be used to write sessions on multisession CDs. This means that the session is written continuously from the lead in to the lead out with the advantage that the P and Q sub channels are available.
Other ways to transfer data to the CD-R is to write 'track by track' or 'sector by sector'. But whereas the discs data are available after finishing the disc or the session, the data that have been written 'track by track' or 'sector by sector' can only be accessed by the particular publishing application during the process of building the disc. At the moment of finalization the ISO 9660 structures are added to make 'track by track' written discs readable for current CD drives. The wastage of capacity is less than by adding small sessions. It is 14,336 Bytes for every block written in a track.
Writing 'sector by sector' is called 'incremental packet writing'. Only newer CD-R drives support incremental writing with packets of fixed or variable length. The discs can only be written and read by these newer CD-R devices and the appropriate CD software. A playback system will need a re-director to read the disc.
The advantages of multisession discs have been used early by CD-I ready. The method described above was only one way to produce CD-I ready. The disadvantage of this method was that the user could possibly access the CD-i data by using the rewind function of the audio player. Playing back these data the hi-fi set could be damaged. So a second approach was to use two sessions, the first one for Audio tracks, a second one for CD-i data. Because Audio CD players cannot read multiple sessions they can only access the first session. There is the same problem when using Mixed Mode CDs. The first track is a data track that should not be read by Audio CD players. To avoid problems the concept of the multisession CD was transferred to the Mixed Mode CD resulting in a new product called CD-Extra. Technically CD-Extra combines normal CD audio tracks with CD-ROM/XA data tracks on a 12 cm disc, and is similar to the Mixed Mode CD.
But Audio players can only access the first session including the audio tracks, whereas multisession CD-ROM drives can read the further information in the second session that is located in the outer perimeter of the disc. Such information could be video clips of the band, album lyrics, artist biographies, liner notes, photographs, animation, text, and complete multimedia applications. Utilizing the bridge disc concept the second session may also contain CD-i data in addition. CD-Extras that run on Audio CD and CD-I players as well as on PCs and Apple Macintosh PCs are called 'Rainbow CDs' because they contain tracks according to the specifications of the 'RedBook' (Audio CD), 'YellowBook' (CD-ROM) and the 'GreenBook' (CD-I).
CD-Extra discs may contain a file called AUTORUN.INF located in the root directory. This file gives information about the various computer platforms that can execute the applications on the disc, and it is designed to start the CD-title when the disc is inserted in the CD-ROM drive.
CD-Extra is supported by Microsoft's Windows 95 and by the Apple Macintosh OS.
Bridge Discs are discs containing information which can be read by computer systems in conjunction with CD-ROM/XA drives, as well as CD-i players. Because of the identical character of the sectors and some identical ADPCM audio and video formats, Bridge Discs can store data accessible by both systems (although different application programs are necessary, depending on the different operation systems). All data tracks on Bridge Discs must be Mode 2 tracks. The specification for Bridge Discs, which is sometimes called WhiteBook, was created in October 1991. To produce Bridge Discs, special software tools are required. A well-known example of a Bridge Disc is Kodak's Photo CD, which runs on a CD-i player as well as on an XA system (and, of course, also on a Photo CD player).
Compact Disc - Recordable, CD-R, CD-MO and CD-E
All Compact Discs we have talked about in the preceding chapters were "read only discs". This means that they can only be read by the CD players and CD-ROM drives but information cannot be written and stored on these discs except by special production facilities. There are advantages in producing these Read-Only-Discs with molding machines when a great number of copies are required. In that way they can be produced very quickly and at a low price.
But there are disadvantages if the number of copies is low. In that case one copy becomes very expensive, and the production process is too awkward and time-consuming. So for a low number of copies it would be advantageous to write on a disc.
Some methods of writing on optical storage media have been developed. In general there are two kinds of discs. On WORM-discs (stands for Write Once-Read Many) information can be written once. However, this process is not reversible. MO-discs (stands for Magneto Optical) are rewritable so that information can be written, erased, and replaced.
The most well-known technology for WORM-discs was invented by the Japanese company Taiyo Yuden. The build-up of this WORM is similar to that of normal CDs. On a disc made of polycarbonate a thin layer of organic dye, cyanide or phthalocyanine, is applied. The laser beam causes the dye to change its properties of absorbing light. The next layer is a reflective golden metal coating (which is supposed to guarantee better properties than aluminum). The metal coating is covered with a protective lacquer.
The disc is preformatted. A thin spiral of 0.7 m is molded on the polycarbonate disc and "shows the way". However, there has to be some information about the velocity of the CLV disc too because the rotation speed depends on the place where the laser writes. Thus, the track has tiny defined wave forms. Their frequency has to be kept at 22.05 kHz during recording. There is also a frequency modulation which indicates the position of the laser and gives a time code information (ATIP, Absolute Time in Pregroove). At the beginning of the disc before the Lead-In there is a zone called Power Calibration Area (PCA) for alignment, and the Program Memory Area (PMA) that contains the track numbers of the recorded titles, including their absolute start and stop times. The PMA is used for partially recorded discs. But there is no user data on the disc before recording.
A WORM that corresponds to the CD standard was defined by Philips and Sony in November 1990 in the Orange Book. Thus, a CD-R or CD-WO can be read by a standard CD-ROM drive or an Audio CD player. However, because a CD-R can only be written once, the Orange Book permits the presence of multiple sessions on the CD, each having its own Lead-In, Program area, and Lead-out. So after storing the data and finishing the first session, another session with other data may be recorded later. In each Lead-In a TOC (Table of Contents) is written, the last Lead-In contains the updates for the whole disc. Since fall 1992 most of the CD drives on the market are able to read multiple sessions.
One of the most well-known CD-Rs is Kodak's Photo CD. If you want different films to be stored in different sessions on a Photo CD, you need such a multiple session CD-ROM/XA drive to read them.
However, there is another way to write on a disc. Magneto-optical discs (MO) have an alloy of terbium ferrite, and cobalt (terbium is a metallic element belonging to the lanthanide series). The MO method also changes the characteristics of some points on the disc's surface so that the reading laser beam is reflected there in a different way. The reading of an MO disc is based on the Kerr effect which means that linear polarized light is deflected when it is influenced by a magnetic field, and the plane of polarization is twisted. Thus, by writing on the disc a powered-up laser beam is focused on a very tiny spot where the alloy is heated up to a temperature where the ferromagnetic properties of the aligned elementary particles are lost. This temperature is called the Curie point. An electromagnet is positioned on the other side of the disc changing the polarity depending on the data to be written. Therefore at the point where the disc was heated up to the Curie point the polarity of the elementary particles is changed by magnetic influence. When reading the disc with a low-energy laser, the laser beam's polarization direction is changed by matching these points, and the changes are evaluated by a photo sensor.
Many different MO formats have been introduced in the market which are not compatible. To standardize these MO formats the "Orange Book" laid down the specifications of the CD-MO. However, because of the different techniques of optical storing, CD-MOs cannot be read by normal CD-ROM drives or CD-players. Although prototype devices have been shown at exhibitions that can record music sequences on CD-MOs as well as play normal Audio CDs this type of disc will not be launched to market. Sony's MiniDiscs are based on MO but they are not compatible with CDs.
In future rewritable CDs will be based on phase change technique. Although current CD-ROM drives cannot read CD-E media, new drives will be able to read them with only a minor modification in electronics (firmware) that can be easily implemented by the drive manufacturers. While the reflectivity of CD-Recordables and CD-ROM media is defined to be at least between 65 to 70 %, CD-E media's reflectivity will be between 15 to 20 %. When reading these media the threshold value to distinguish between reflecting and non-reflecting zones has to be lowered.
The information layer of the disc is based on an alloy of silver, indium, antimony, and tellurium (Ag-In-Sb-Te). In it's initial phase the information layer is in an amorphous state. By heating up small zones of the surface an high energy laser changes the amorphous material into a crystalline state. To read data on the disc, a low-intensity laser beam is reflected by these zones and scattered by the amorphous areas between the zones. Reflected and non-reflected light is registrated by a photo diode. The crystalline state can be changed back to an amorphous state by the writing laser when generating a different temperature.
The capacity of CD-E is about 680 MB. Although Matsushita has demonstrated phase-change media with more than 1 million rewriting cycles, this technique suffers from limited cyclability. Depending on the complexity of the alloy data can be rewritten a limited number of times, improving the alloy increases the costs of manufacturing. To keep the costs at a reasonable level CD-E media will be produced allowing 10.000 rewriting cycles. But CD-E does not only require new hardware. To allow erasing and writing of files, changes in the logical structure of the ISO 9660 have to be specified and new software drivers or additions to MSCDEX are necessary.
The Mini Disc, launched on the market by Sony in 1992, is sometimes designated as a CD but it is not a CD. It is an MO disc which does not conform to any CD standard, not even to the MO specification of the "OrangeBook". This disc is 2.5 inches in diameter and covered by a plastic package, like 3.5-inch floppy disks. It is used for audio recording and playing in consumer electronics devices, and this disc is likely to be used in future as a data storage medium especially for multimedia purposes.
In the preceding chapters CD formats have been described. However, when a CD is used as a data storage medium, a file system for file and data access is needed permitting the organization of the files in directories and subdirectories. At the beginning of CD-ROM technology there was no standard file system for a CD-ROM which permitted file access under different operation systems. Therefore, producers created their own, modified existing file systems, or transferred these existing systems like MS-DOS or Macintosh's HFS (Hierarchical File System) onto CD-ROM. In that case the discs could only be read by the special target system, and when using those special CD-ROM file systems it could happen that the user had to switch off and restart his computer in order to read another CD-ROM.
However, the need for a cross-platform file system was soon recognized. So representatives of the computer and the consumer electronics industry formed a group which was called "High Sierra Group" (HSG), named after the hotel in Nevada where the group met first in November 1985. Members of the group came from Microsoft, Apple, Digital Equipment, 3 M, Hitachi and other companies. In 1986 the HSG proposal was laid down and given to the ISO (International Organization for Standardization). In 1988 international standard ISO 9660 concerning "Information processing - Volume and file structure of CD-ROM for information interchange" was released. There were only very few differences between ISO 9660 and the preceding HSG proposal.
The software that makes a MS-DOS computer read ISO 9660 discs is called MSCDEX.EXE (standing for Microsoft CD-ROM Extensions). This program is called from the AUTOEXEC.BAT file, but there has to be another hardware specific driver for the CD-ROM drive in the CONFIG.SYS file. MSCDEX version 1.0 can only read High Sierra, not ISO 9660 files. A CD-ROM drive for a multimedia PC requires at least MSCDEX version 2.2. A special driver is also needed for the Apple Macintosh. However, for the Macintosh family there are many discs using HFS. To permit a complete support of ISO 9660 by UNIX systems at the start of the 90s the Rock Ridge Group of representatives from the industry was formed. It laid down the RRIP (Rock Ridge Interchange Protocol). Discs conforming with the Rock Ridge standard are fully compatible with ISO 9660.
The advantage of ISO 9660 is that this standard permits the access of data on one and the same disc by different computer systems. Of course there must be different executable programs for different operating systems on the disc (for example, one program for UNIX, one for Apple and another for MS-DOS), but when running these programs the same data files may be accessed by different systems. Obviously the executable program file may be accessed but cannot be executed by the wrong operation system.
ISO 9660 is a hierarchical file system like MS-DOS and defines directories, subdirectories and paths. The depth of the directory hierarchy is not to exceed 8.
For File and Directory Identifiers capital letters from "A" to "Z", digits from "0" to 9", and the underscore "_" are to be used. The length of the name may be 8, the length of the file extension may be 3 letters. These restrictions are laid down for Interchange Level 1 which is mostly used by ISO 9660 discs. There are also the Interchange Levels 2 and 3 without these restrictions for naming the files, but level 2 and 3 discs are very rarely used.
Most of today's discs conform with ISO 9660. Also the extensions and further developments of CD-ROM described in the preceding chapters are based on the file system defined by ISO 9660, although there have to be some modifications and adaptations, for example for the multiple session CD-R.
In the last year many multimedia applications have been published as Hybrid CDs using more than one operating system, for example Apple's HFS and the ISO 9660 file system. In that case two applications with all their data could be stored on the same disc. The data files that can be interpreted under different operating systems, for example TIFF-, JPEG- and MPEG-Files, can be stored only once on the disc and be shared by the applications for the different operating systems.
Modern Operating Systems have File Systems that allow long filenames. Microsoft's Joliet File System enhances the ISO-9660 specifications and allows files names with up to 64 characters. Characters from any language are allowed. The length of the full file name including the directory path is limited to 120 characters.
New specifications for file systems like ISO 13490 have to be developed to support new features of CD writing like incremental packet writing, drive letter access.
On the other hand new standards have been proposed to support bootable CDs allowing the developer of CD-ROM titles to package the application and the environment of the operating system on the disc, so that the disc can run using all necessary information from the CD. The bootable CD-ROM Format Specification proposed by the CD/OS Association in 1995 is called 'El Torito'.
In September 1995 a proposal for a single format of the next generation high-density optical disc format was accepted. In the months before there has been a format between two proposed high-density discs: the Multimedia CD (MMCD) supported by a group of companies lead by Philips and Sony, and the Super Density Disc (SDD) that was introduced by a consortium called SD Alliance lead by Toshiba, Matsushita, and Time Warner.
A look at the technical details of DVD shows that the main features of this disc have been adopted from the SD proposal. The disc is formed by back-to back bonding of two 0.6 mm-thick 12 cm discs. The advantage of the doubled 0.6 mm disc is an improvement of the values for birefringence due to the thinner disc, and greater tilt margins, both resulting in a reduced optical aberration of the read out spot. To increase the density of the tracks and to allow smaller pits the wavelength of the read-out laser had to be changed from 780 nm at current CDs to at least 650 nm for high-density discs. On the other hand the optical system had to be modified. The value for the numerical aperture had to be improved substantially from 0.45 to 0.60. These modifications are resulting in a four to five times higher density for the information on the disc. Compared to the current CD, the header structure was changed, the subcode was dropped, and less parity bits are used. The high-density disc has a more effective error correction scheme than current CDs, and uses another modulation. CDs are based on an Eight-to-Fourteen Modulation (EFM). Due to the way in that information is stored, 8 bits are represented by 14 so-called channel bits on the CD and additional 3 merge bits, resulting in 17 channel bits that represent 8 bit of information. There are more effective ways to code the information now with rates of 8:16 and 8:15 that have been proposed for the high density from Philips and Toshiba for the high-density disc. The result of all these modifications is a capacity more than seven times higher than those of current CDs. Toshiba's first Super Density Disc (SDD) proposal of bonded 0.6 mm discs had a storage capacity of 5 GB on one information layer using 8:15 modulation. Considering the request of the computer industry to enhance the reliability in the worst case, the new DVD changed to the 8:16 method that sets aside 1 bit for reliability improvement, but decreases the capacity from 5 GB to 4.7 GB per information layer.
The DVD system is backwards compatible with existing CDs using special dual focus pick-up to read the disc. The first application is to distribute movies on CD using MPEG-2 compression technique. Solutions on recordable and erasable discs based on organic dye coating and phase-change technique arrived about 1998.
Note: Disc with "c" is always used when talking about optical or magneto optical discs, while disk with "k" is used for magnetic storage media.
Glossary's easy jump to related topic :
Bootable CD
A Bootable CD allows a computer system to start the operating systems from the CD-ROM drive. Bootable CDs for Intel PCs are created in the "El Torito" standard.
A Bridge Disc is a special form of CD-XA disc containing an ISO 9660 file system. On this disc, a CD-i application program enables CD-i players to access the data on these discs. On other systems you need a special application program to handle the data. The Video CD is an example for a Bridge Disc.
Digital Audio is the familiar standard for audio data from an audio CD player, as defined in the RedBook.
The audio information is stored in frames of 1/75 second length. There are 44.100 samples per second stored. Each sample occupies two bytes (16 bit) and there are two channels (left and right) stored on the CD-DA. This gives a sector size of 44.100 * 2 * 2 / 75 = 2352 bytes per frame, which is the total size of a physical block on a CD. WinOnCD can write CD-DA disks.
The CD-Extra is a format which combines audio and data, with the data track being the final one on the CD. This enables an audio player to play the CD safely.
CD-Extra discs also contain additional information, such as song lyrics and still pictures, playable on multimedia PCs and special CD-EXTRA players.
CD-i means CD-interactive and is a multimedia playstation designed by Philips. A Video CD, being a bridge disc, contains a CD-i application enabling playback in CD-i players.
This format is now called "CD-Extra".
The Compact Disc Read Only Memory was invented by Philips and Sony for computer data. The YellowBook of the CD inventors, Philips and Sony, contains only the standard for the physical record medium.
The complete data of the CD-ROM are stored in one track, the user block size being 2048 bytes. As a physical block contains 2352, there remain 304 bytes left in the physical CD frame. These bytes are used for extended low-level error correction and additional information bytes.
The information of the CD can be stored in a variety of formats, including the ISO 9660 standard. Some computers support their native file system on a CD-ROM (i.e. Macintosh or most UNIX systems).
XA means Extended Architecture and is a standard defined by the Philips and Sony to record multimedia CDs containing audio, video and program data. CD-XA is recorded in mode 2 (2336 bytes) and uses two logical block sizes:
2048 bytes per block for data
2324 bytes per block for audio/video information
An additional difference to mode 1 (CD-ROM) is the subheader. These 8 bytes, unused in mode 1, are used in mode 2 for storing additional information for readback devices. For example, it is possible to generate an interrupt message when reading a specified sector.
The GreenBook contains the definition for CD-interactive (CD-i).
The International Standards Organization (ISO) defines standards for all areas of technology and business. For example, ISO 9002 is a quality standard for production processes.
ISO 9660 is an internationally standardized file system adapted by most operating system manufacturers. This standard is also known as ECMA 119. The use of this file system enables many systems to access files recorded conforming to this file system. The disc has to be read back on MS-DOS, Apple Macintosh, UNIX or VMS systems and must meet all the restrictions of these various file systems.
ISO 9660 is usually recorded in CD-ROM mode.
The lead-in contains the table of contents of a session, which holds information about the track layout of the current session. It is always written together with the lead-out at the end of a session. Each lead-in takes up 4500 sectors (about 9 Mbytes) on the CD.
The lead-out indicates the physical end of a session, but contains no actual data. It is always written together with the lead-in at the end of a session. The first lead-out written to a disc takes up 6750 sectors (about 13 Mbytes) on the CD, while subsequent lead-outs take up 2250 sectors (about 4 Mbytes).
A Mixed Mode CD contains one data track, and after this a number of digital audio tracks. WinOnCD can write Mixed Mode CDs.
The Microsoft MPC (Multimedia PC) standards define certain sets of hardware and software for multimedia capabilities.
A computer conforming to MPC3 must be able to play MPEG videos from a CD, for example.
A compressed method video, so named after the Motion Picture Experts Group.
MPEG is a lossy compression, i.e. not all original information is restored when the movie is played back. This allows a much more efficient compression.
A multisession CD-R is a CD-R with more than one session on it. However, multisession is more commonly used in conjunction with the ISO 9660 file system, where it describes the process of adding information to an ISO 9660 CD after its initial creation.
In the OrangeBook Philips and Sony have defined the CD-Recordable and the CD-MO (magneto optical). Multisession is also included in the OrangeBook.
The PhotoCD, introduced by Kodak, is a medium for storing picture information on a CD. The PhotoCD is a multisession Bridge Disc.
The RedBook by Philips and Sony is the basic definition of the CD. It includes only the physical characteristics of a CD and the normal Audio CD. All other books, however, use the same physical medium and low-level data format.
Each time you write to a CD is called one session. The number of sessions on a CD is important, as some formats require all data to be written in one session to comply with the CD norm. Generally, a distinction is made between singlesession and multisession CDs.
The Video CD is a special Bridge Disc, which contains MPEG-1 compressed, full-motion video. Video CDs can be played back with properly-equipped multimedia PCs (MPC3) or CD-i players, as well as with special Video CD players.
The WhiteBook is a definition document for the Video CD standard agreed upon by Philips, Sony, Matsushita, and JVC. There are two main versions, version 1.1 and version 2.0.
In the YellowBook, also known as ECMA-130, Philips and Sony defined the extensions from the RedBook audio CD to the data CD (CD-ROM).
The YellowBook defines two data "modes": mode 1 contains 2048 bytes of user data (like CD-ROM), mode 2 contains 2336 bytes of user data. The remainder of the physical block (2352) is used for error correction and sync information.