Compact Disc (CD)

The original Compact Disc standards, as devised by Philips, included 3.1 inch (80 mm), 4.7 inch (120 mm) and 11 inch (280 mm) versions of the following:-

Most pre-recorded CD-A or CD-ROM discs are in 3.1 or 4.7 inch form, since other sizes can be unsuitable for a modern drive. As with other optical discs, CDs come in read-only (RO), write-once read-many (WORM) and read-write (RW) types, as shown in the following table:-

Compatibility between discs and drives is as follows:-

Fortunately, most DVD drives can also read all CD formats.

CD Drive Hardware

A CD drive usually has a tray or slot for receiving the disc. You can insert any kind of disc into a drive, assuming the disc is compatible with the mechanism, although there’s no guarantee it will read a particular type of CD.

• A slot-loading mechanism, as found in modern machines, only accepts a standard 3.1 inch or 4.7 inch disc. Other ‘novelty’ sizes of disc don’t work and can damage the drive.
• Excessive noise sometimes occurs if the disc’s screen-printed label results in an uneven weight distribution, although this rarely causes any harm to the drive or the disc.
• A CD jukebox gives access to data stored on several CD-ROMs, which is particularly useful when used with a network server, giving all users access to common material and software. One kind of jukebox consists of a single CD-ROM drive and a large hard disk drive in a single enclosure. The data from every CD-ROM inserted into the device is automatically transferred onto the drive, retaining access to the data without needing a multiple-CD mechanism.
• To gain access to several discs, without having to reload individual CDs or use separate drives, you can employ a CD-ROM auto-changer. This type of external drive usually accommodates up to six discs.

Speed

All CDs are played from the centre of the disc. As the drive’s laser head moves towards the centre of the disc, the disc’s rotational speed, also known as its angular velocity, increases automatically. In the oldest drives, which run at one-times (1×) speed, the disc begins rotating at 500 rev/min, slowing to 200 rev/min when the middle of the disc is reached. This means that the speed of the disc in relation to the laser doesn’t change whilst the CD plays. This constant linear velocity (CLV) recording system ensures that all data is transferred at a fixed rate, giving consistent results across the entire disc.

Early 1× drives provide a seek time of 300 milliseconds (ms) to 1.5 seconds and a data transfer rate of 150 KB per second (KB/s), both of which aren’t very exciting. Fortunately, modern devices use higher rotational speeds, as employed in a four-times (4×) speed drive, which gives 600 KB/s, or in a 12× drive, which provides 1.8 MB/s.

12× is the highest possible practical speed. However, in a so-called 24× drive the CD spins continuously at this maximum, with data extracted at variable rates, occasionally reaching 3.6 MB/s, which is useful for multimedia work involving real-time audio and video. The author assumes that 32×, 36×, 44× and 52× drives use higher speeds or a look-ahead buffer to increase the effective speed during intermittent use. The seek time for a modern CD-ROM drive is an acceptable 90 ms.

Most types of CD-R drive can’t record as fast as they can play: such a drive that records at 4× speed and plays at 24× is known as a 4× 24× drive. Similarly, some CD-RW drives can only play CDs at 12× or 16×, whilst older CD-RW drives that read at 6× can record onto CD-RW at 2× and onto CD-R at 4×, and are known as a 2× 4× 6× drive. In comparison, a typical DVD drive plays at 12×, has a transfer rate of 15.8 MB/s, an access time of 110 ms and usually contains a 512 KB buffer.

Matching Discs and Drives

Although some drives don’t accept all types of CDs and DVDs, Apple’s SuperDrive mechanism can record onto CD-RW or DVD-R and provides support for DVD-Video, CD-A, CD-I, CD Bridge, CD Extended, CD Mixed Mode and Photo CD. It reads at 6× from DVD, records at 2× onto DVD-R, reads at 24× from CD, records at 8× onto CD-R and at 4× onto CD-RW.

• Before buying a CD drive, check which types of discs it accepts.
• Older SuperDrive mechanisms are only available in ‘tray’ form, making them unsuitable for some computers.
• Older CD-ROM drives may require you to put each disc (including audio discs) into a caddy or cartridge, so as to avoid dust and damage. You can transfer discs in and out of caddies (to reduce costs) but that defeats the object.
• Some Plasmon CD-ROM drives also accept a Phase-change Disk (PD), a 5¼ inch rewritable disk storing 650 MB. Data is recorded with a high-power laser that heats up the surface and changes its crystalline nature. 10,000 read-write operations are possible, although the formatting is unreadable on a standard CD-ROM drive.

Drive Connections

All modern computers have built-in drives that accept CDs. If you don’t have a suitable drive, you may be able to install an internal drive within the computer. If this isn’t possible, you can add an external drive, connected via a USB or FireWire port. Unfortunately, USB 1.0 runs at 1.5 to 12 MB/s, possibly restricting the actual speed to 2× or less. Although USB 2.0 is better, FireWire offers a higher sustained data rate and can easily accommodate a 12× CD-R drive or better.

• The expansion bay in a portable computer usually accepts an AT Attachment Packet Interface (ATAPI) CD drive, although this may need special software. Failing this, you can plug a suitable external drive into a spare PC Card slot.
• Don’t plug other devices into a FireWire bus whilst it’s being used by a CD-R or CD-RW drive to write to a disc. Making such a connection can cause the interface to pause, upsetting the recording process.
• Older ‘classic’ Macs may require an external drive with a SCSI connection, usually supplied with a 50-way Amphenol adaptor cable (or similar alternative) and, in some instances, an external SCSI terminator. A switch is frequently provided on the back of this kind of drive, allowing you to give the device a specific SCSI ID number.

CD Types

A CD has a continuous spiral track 5,000 metres long, giving an effective density of 15,875 tracks per inch (tpi). The disc has four layers: the polycarbonate disc itself, 1.2 mm thick, with important optical properties and onto which the track of a ‘pressed’ disc is embossed, a reflective aluminium coating, a layer of lacquer for protection and a silk-screen printed label.

• A double-sided disc consists of a ‘sandwich’ of two discs, although the author has never encountered a CD of this kind.
• The laser in a CD player is focused on the track beneath the surface of the plastic disc, so that dust and minor scratches on the surface itself shouldn’t have a significant effect on the output.

Pressed CDs

The digital codes on a mass-produced CD-A or CD-ROM consist of microscopic pits that follow a spiral track from the centre of the disc to its edge. These pits are 0.5 micrometres (µm) wide and are spaced by 1.6 µm.

The manufacture begins with a master disc, often coated in copper and embossed using a piezoelectric stylus, or covered with a light-sensitive ‘photo-resist’, exposed to a digitally-controlled laser and then chemically etched to create the pits. This disc is plated in nickel, so as to create a metal stamper, which is used to emboss the final plastic disc. The latter is then given its very thin covering of reflective aluminium, followed by a layer of lacquer, which is finally covered by the label.

CD-Audio (CD-A)

This kind of disc, commonly known as an audio CD, contains digitised recordings lasting up to 63, 74 or 80 minutes, depending on the length of the spiral on the disc, although the older 63 minute variety is now very rare. Audio CDs can be played on almost any kind of computer CD-ROM drive, as well as on a standard audio CD player.

The music tracks on an audio CD are organised sequentially along the spiral, so that by default they’re played in order. However, to gain instant access to a particular track, a table of contents (TOC) is also employed, which can also, if necessary, prevent access to unwanted material (see below)

CD-Read Only Memory (CD-ROM)

A standard CD-ROM stores over 600 MB of data, which is roughly equivalent to 1,900 floppy disks or over 250,000 pages of typed text, whilst a Photo CD can store up to 5,000 images, although this depends on the quality of the pictures.

The following table shows the capacity of standard discs:-

• A ROM card CD, which is the same size as a standard business card, accommodates between 18 MB and 35 MB of data. This format is really a novelty and can cause serious damage if inserted into a slot-loading drive.
• A CD-ROM can also store sound recordings as MP3 or Windows Media Audio (WMA/ASF) data, the latter normally protected by Windows Rights Management. Some audio CD players can also play MP3 discs, WMA discs or both.

CD-Recordable (CD-R)

CD-R is a write-once read-many (WORM) format. The blank CD-R disc, sometimes in the form of a gold master, contains a layer of transparent dye, which is made opaque at certain points during recording, so imitating the ‘pits’ found on a pressed disc. The recording process, known as burning, cutting or toasting, may require a special computer application such as Toast (Roxio). In some instances you may not be able to use your computer until recording is finished.

• Any kind of content can be recorded, although a CD-R drive can’t erase any data that’s already on a disc.
• Older CD-ROM drives, usually of the 1× or 2× kind, as well as some 4× and 8× models, can’t read CD-R discs.
• Some older audio CD players accept CD-R discs containing music, but only if recorded at a speed of 2× or lower.
• The time taken to record a disc depends on the drive’s speed rating, as well as the speed rating of the blank disc. A 74-minute CD-R takes 18½ minutes on a 4× machine with a suitable disc, this figure being obtained by dividing 74 by 4. Some recent drives record at 52× speed, making it possible to create such a disc in less than 1½ minutes.
• A double density (DD) CD-R drive lets you record twice as much data, usually up to 1.3 GB, but only on a DD CD-R disc. Note that these discs must be played on a DD drive, not on a standard CD drive.

CD-ReWritable (CD-RW)

CD-RW is a write-many read-many (WMRM) format, onto which any kind of CD content can be ‘burnt’ (in the same way as a recordable CD), although you may have to erase the disc first to remove any existing content.

During writing, a laser is aimed at tiny particles in the disc’s substrate, making them selectively transparent or opaque. Although CD-RW discs can be erased and written to over a thousand times, a blank disc costs about four times the price of an equivalent CD-R, whilst recordings made on older drives don’t always comply with the modern Philips standard.

• Some older CD-ROM drives, as well as older CD drives in audio equipment, can’t read CD-RW discs. A drive that accepts CD-RW discs must conform with the ‘multi-read’ specification.
• As with CD-R, the time taken to record a disc is dependent on both the drive and the blank disc.
• A double density (DD) CD-RW drive lets you record twice as much data, usually up to 1.3 GB, but only on a DD CD-RW disc. Note that these discs must be played on a DD drive, not on a standard CD drive.

CD Content

All CDs contain up to three blocks of data, beginning at the middle of the disc. The lead-in block, located in a ring about five millimetres wide near the centre, contains the disc’s directory. The program block, containing the useful data or digital audio material, occupies most of the disc, working from the centre outwards. Finally, there’s the lead-out block, a band around one millimetre wide and positioned at the edge of the disc, showing that no more material exists beyond this point.

Pressed CDs, CD-R and CD-RW discs, can all be used to store CD-ROM data, CD-A music or other content. However, specialised material may be inaccessible to certain hardware or computer systems. The following table shows the various kinds of content found on CDs and their compatibility with typical CD-ROM, CD-Interactive (CD-I) and Photo CD drives:-

• Proprietary formats, such as Memorex VIS, Commodore CD-TV and Sony Multimedia (Dataman) MM/CD can’t be used in standard CD-ROM drives, despite the last being a variation of CD-ROM/XA. Special CDs, such as GD-ROM, a 1 GB version of CD-ROM used in the Sega Dreamcast games console, are also incompatible with other drives.
• A disc that can be played on a CD-ROM or CD-I drive is known as a CD Bridge disc.

The various types of content are described in the following sections.

CD-Audio (CD-A)

This, the oldest CD standard, is described in the Red Book, originally produced by Philips and Sony. Each track consists of Compact Disc Digital Audio (CDDA) data, these tracks being placed along the disc’s spiral in the required order.

Unlike more advanced CD formats, such as CD-ROM (see below), these discs don’t employ any form of filing system. This means that folders can’t be used and the name of each track doesn’t have any effect on the playing order. In addition, all audio CDs have to include a table of contents (TOC), which gives the drive immediate access to the tracks.

CDDA tracks are sampled at 44.1 kHz with 16-bit resolution, requiring four bytes for each stereo sample. The Red Book defines Type 1 sectors for audio, each containing 2352 bytes. The number of samples in each sector is given by:-

whilst the number of sectors used for each second of recording is given by:-

A ¹⁄75 of a second is therefore known as a CD frame and allows you to find the number of sectors on a 74-minute CD:-

whilst the total size of the disc for audio purposes is given by:-

Playing CD-A Discs

With a CD-ROM built into your computer, the CD sound should appear on the machine’s audio outputs and be available as a source for other audio applications. If you have an external CD-ROM drive you can connect an amplifier to the audio sockets at the rear of the drive or plug headphones into the miniature jack socket on the front of the drive.

• You can use iTunes to play audio CDs in the Classic Mac OS or in Mac OS X. Any names assigned to discs or their tracks are stored for future use. In Classic, these names are also available in AppleCD Audio Player.
• In the Classic Mac OS, the Audio CD Access file in the Extensions folder, which is inside the System Folder, lets you use the AppleCD Audio Player application to control your drive. However, this only works if your drive is supported by the Apple CD/DVD Driver or Apple CD-ROM file, also found in the Extensions folder.

CD Text

This later addition to the CD-A standard, conveys disk and track information as text or JPEG images. These are compatible with the Interactive Text Transmission System (ITTS) but require a suitably-equipped audio player or suitable software.

Enhanced Music CDs

Also known as CD Plus or CD Extra, this kind of disc contains extra multimedia material in an area formatted as a CD-ROM. The disc is in multi-session form, as defined by the Blue Book standard, with the first session occupied by the audio tracks.

• The sound track from an Enhanced Music CD can be heard on any CD player.
• The Extended Music format can be used for restricted purposes, as in a CD+G, disc, which provides animated graphics and text for each sound track, usually for Karaoke, or in a CD+MIDI disc, which includes a MIDI sequence for playing synthesisers alongside each track, although special software may be required to extract the latter.
• The extra content of an Enhanced Music CD can be experienced in various applications, including the AppleCD Audio Player program, as supplied with the Classic Mac OS.

The additional data for an Enhanced Music CDs is often in hybrid HFS+/ISO 9660 or HFS/ISO 9660 form. In some instances, the HFS section contains a QuickTime Audio Containable (QuAC) file, which is used to create the disc’s structure. Both filing systems share a folder called CD EXTRA, containing CDPLUS and PICTURES folders, as well as an AUTORUN.INF file, which holds textual information (with CR plus LF line endings), but may only contain the string [autorun].

The CDPLUS folder contains files conforming to the Blue Book standard, including INFO.CDP, which points to one or more files with names in the form of SUB_INFO.lc, where the letters lc are replaced by an appropriate language code, such as en for English or de for German. These files in turn point to the relevant files containing MIDI musical sequences and lyrics, which have names in the form of TRACKnn.MID or LYRICSnn.lc, where nn is the appropriate track number, as well as picture files.

The image files in the PICTURES folder have names of the form NEWPIC.nnx, where nn specifies the track number (or 00 if the image applies to the entire album) and x indicates the image file format, as represented by the letter J, N, T, S, P, A, or L. This means, for example, that a picture in the common JPEG format uses the letter J.

• MPEG files can be used for moving images but must be in continuous sectors at a rate of 2352 bytes/sector.

CD-Read Only Memory (CD-ROM)

The CD-ROM format is set by the Yellow Book from Philips. Computer data is stored using several sizes of Type 2 sectors, consisting of synchronisation (12 bytes), header (4 bytes), data (2048 bytes), and error correction (288 bytes) sectors. Type 3 sectors are less common, with sectors for synchronisation (12 bytes), header (4 bytes) and data (2336 bytes).

The capacity of a CD-ROM is obtained by multiplying the total number of sectors by the data sector size, as shown here:-

• Since different sector sizes are used for CD-ROM and audio CD, the effective capacity of an audio CD is greater than that of an equivalent CD-ROM.

Using CD-ROM Discs

Most computers can show the files that are contained on almost any kind of CD-ROM, although the actual data may be useless for your operating system.

• To use a Multimedia PC (MPC) disc on a PC you must have a suitable CD-ROM drive and computer. An MPC drive with a buffer or cache of a least 64 KB is essential. Fortunately, most 4× drives have a buffer of 256 KB or more.
• Some modern CD-ROMs use the universal disk format (UDF) standard, which is supported by Mac OS 8.1 or higher. You’ll need at least a 2× drive to read such discs.
• In the Classic Mac OS you must have the Apple CD/DVD Driver or Apple CD-ROM file in the Extensions folder, inside the System Folder. Sadly, the Apple CD-ROM file supplied with Mac OS 7.6.1 or later doesn’t work with non-Apple drives, although version 5.3 of this file, as provided with Mac OS 7.6, can also be used with later Classic systems.

Disk Formatting

As with any other computer disc, the formatting of a CD-ROM must be acceptable to the machine that’s using the CD. The following systems are frequently encountered:-

• ISO 9660

  An international standard, derived from the older High Sierra (HS) convention and based on MS-DOS. CD-ROMs made to these rules work in any computer, including Mac OS machines, although long filenames aren’t supported.

• ISO 9660 (Joliet)

  A modified form of ISO 9660 used in the Windows operating system, allowing 255 Unicode characters in filenames. The long names can’t be seen on some platforms, including the Classic Mac OS, but they’re visible in Mac OS X.

• ISO 9660 (Mac Extended)

  A non-standard variation of ISO 9660 that permits up to 255 characters in a filename, although the long names aren’t recognised by a computer that doesn’t use the Mac OS.

• ISO 9660 (Rock Ridge)

  Another form of ISO 9660, allowing 255 Roman ASCII characters in a filename, and originally used on Unix computers. The long names can’t be seen on some computer platforms.

• Mac OS Standard

  Also known as the Hierarchical Filing System (HFS), this is Apple’s original disc formatting standard. HFS CD-ROMs can be viewed on any Mac but can’t be used on a Windows machine unless ISO 9660 formatting is also provided.

• Mac OS Extended

  Also called Hierarchical Filing System Plus (HFS+), this is a development of the original HFS format. Older versions of the Mac OS don’t recognise such discs, so this kind of formatting is often best avoided when creating CD-ROMs. HFS+ CD-ROMs can’t be used on a Windows machine unless ISO 9660 formatting is also provided.

Fortunately, if you have a fairly modern machine, you should be able to at least see all the files on most kinds of disc, even if you can’t use them.

• To read HFS discs on a PC you must install special software.

Hybrid Discs

A hybrid disc has two or more filing systems, although the data itself isn’t usually duplicated. However, where speed is important, two versions of some files may be provided, since Intel and other processors in Windows machines need to receive their data bytes in the opposite order to a Mac processor. So the available space on a hybrid disc can be restricted.

• You can view both the Mac and PC data on a hybrid disc by means of a Classic Mac OS computer equipped with special software, such as Joliet Volume Access (see above).

CD-ROMs and MP3 Audio

A CD-ROM containing MP3 audio files can be played on an MP3-compatible audio CD player or in a computer, but not on a ‘traditional’ CD player. Up to 10 hours of sound, usually consisting of up to 120 tracks, can be kept on a single disc.

• You can ‘burn’ MP3 files onto CD-R or CD-RW discs using iTunes (Apple) or a similar application.
• A CD-ROM containing MP3 recordings must be recorded as a single-session disc. If you try to use a multi-session disc in an MP3 CD player you’ll only hear the first session.
• MP3 files can also be recorded onto a 3.1 inch (80 mm) CD-ROM, giving 3 to 4 hours of music, and played on a portable mini CD player, although this kind of device doesn’t accept standard 4.7 inch (120 mm) discs.
• Windows Media Audio (WMA/ASF) files, usually protected by Microsoft’s Windows Rights Management may also be stored on a CD-ROM. Some audio CD players that accept MP3 CDs can also be used to play WMA discs.

Creating MP3 Discs

There aren’t any standards for MP3 discs, so you can have difficulties with some players, although all accept standard ISO 9660 formatting. In addition, the creation of a hybrid CD isn’t a problem, since ISO 9660 works with most portable devices, the Joliet variation of ISO 9660 provides long names for Windows machines, and HFS or HFS+ suits Mac OS computers.

• MP3 files shouldn’t be encoded at a very high rate, as this may stretch the ability of your player. Under 256 kbit/s is best: in fact, 128 kbit/s is often preferred as this also conserves disc space. In addition, plain stereo or normal stereo, not joint stereo should be selected in your MP3 encoding application, as this is less demanding on the playback device.
• Most players can display the ID3 tag information from each MP3 file. However, some only recognise ID3v1 tags, not the later ID3v2 tags, which should therefore be avoided. ID3v1 tags provide the track name, album name, artist’s name and 30 characters of text, a total of 128 bytes, whilst ID3v2 tags use variable length fields to incorporate pictures and lyrics. Although tracks can have long names, some players only show the first 20 or so characters.

Most players allow files to be placed in a folder, although not all devices like files to be in subfolders. Ideally, you should create a folder for each artist at the top level of the disc. Inside each of these you can create a folder for each album, each of which contains the actual tracks, as shown in these examples:-

• As a rule, you shouldn’t have more than 50 MP3 files in any folder or more than 255 files and folders on an entire CD. Also, avoid having more than eight ‘artists’ folders at the top level of a disc.

The tracks in each folder are usually played in alphabetical order. However, you can enforce a required order by using special numbered filenames, as shown here:-

Note that these are ‘long’ names, not the short names used in standard ISO 9660 formatting.

• If necessary, you can avoid using folders altogether by extending this numbering system to all the files on a disc, using appropriate sets of numbers to group together the tracks of each album.

Kodak Picture CD

This is a cheaper alternative to the older Kodak Photo CD system (see below), using single-session ISO 9660 CD-ROM discs to store up to forty pictures. These are all in 4Base format (see below), giving a resolution of 1536 × 1024 pixels. However, APS film images, reproduced with a resolution of 1536 × 864 pixels, are stored as standard JPEG files.

CD-ROM Extended Architecture (CD-ROM/XA)

The XA group of discs includes those formats not covered by the more common Red Book (CD-A) and Yellow Book (CD-ROM) standards. Unlike the CDs described so far, which can contain audio or data, XA discs can contain both kinds of information: they are defined by the Green Book standard, as produced by Philips.

The data on any CD is organised in one of two modes, depending on whether a table of contents (TOC) is used. The mode sets the number of error detection levels, influencing the speed and capacity of a disc, as shown here for a 1× drive:-

Mode 1 (X), as used for data on a CD-ROM, employs three levels of error detection, whilst Mode 2 (A), which is used to convey less critical audio or video material, only requires two levels of detection.

As already explained, a CD-ROM/XA contains material in both modes, which means that compressed medium-quality audio or video material can accompany CD-ROM data, thereby accommodating a multimedia presentation. The Mode 1 data is in Type 4 sectors, consisting of synchronisation (12 bytes), header (4 bytes), sub-header (8 bytes) data (2048 bytes), and error correction (280 bytes) sectors, whilst Mode 2 audio and video information is kept in Type 5 sectors, with sectors for synchronisation (12 bytes), header (4 bytes), sub-header (8 bytes) data (2324 bytes), and error correction (4 bytes).

• The TOC on an XA disc is only used for locating audio tracks.
• A CD-ROM/XA disc can’t be played on all types of drive. In some instances you may require extra computer hardware to take full advantage of the content.

The following variations of CD-ROM/XA are also available:-

References

ATPM 8.07, Arnold Woodworth ＆ Ken Gruberman, 2002

User’s Guide, Toast 5, Roxio, Inc, 2002

©Ray White 2004.