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:-

DiscPurpose
CD-Audio (CD-A)Sound or Music *
CD-Read Only Memory (CD-ROM)Computer Data †
CD-Video (CD-V)Video Recording ‡
CD-Interactive (CD-I)Multimedia
CD-Television (CD-TV)Multimedia •

* Can include CD-ROM data, sometimes containing MP3 version of audio tracks

Can be formatted to suit a specific operating system or used for MP3 audio

Better quality available using DVD-Video disc

Interim and outdated version of CD-I

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:-

DiscTypeMaximum Capacity
CD-A +RO63 minutes 74 minutes 80 minutes 90 minutes
CD-ROM +RO500 MB 650 MB 700 MB 800 MB
CD-Recordable (CD-R) *WORM500 MB 650 MB 700 MB 800 MB 1.3 GB •
CD-ReWritable (CD-RW) *RW500 MB 650 MB 700 MB 800 MB 1.3 GB •

+ Pressed disc

* Can be used to store material in CD-A, CD-ROM or other formats

1.3 GB on double density (DD) discs, which can only be used in a DD drive

Compatibility between discs and drives is as follows:-

DriveCD-ROMCD-RCR-RW
Disc   
CD-ROM
CD-R?
CR-RW?

  Drive accepts disc

?  Some drive models accept disc

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.

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 () 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 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 () 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 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 can record onto CD-RW at and onto CD-R at , 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 from DVD, records at onto DVD-R, reads at 24× from CD, records at onto CD-R and at onto CD-RW.

  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 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.

  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.

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:-

Size (in)Size (mm)CD-A (min)CD-ROM (MB)
4.712063 *500 *
4.712074650
4.712080700
4.712090800
3.18023172 or 185
* Modern discs usually have a higher capacity

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.

Single and Multi-session Recording

In a single-session drive, all the data and a single table of contents (TOC) is recorded in one pass. But in a multi-session drive, data can be recorded at different times and unwanted sessions hidden later. This allows data can be transferred singly, or in groups, with any number of TOCs on each disc.

Making a Bootable CD-R for the Classic Mac OS

Under normal circumstances, a CD-R disc only conveys data after the system has started. However, you may require a disc containing a system that will run at startup. In Mac OS 10.3 you can do this with the Disk Utility. However, in older systems you must launch Disk Copy and select Image ➡ Create New Image. In the Size pop-up of the next dialogue choose 663,000K (CD-ROM 12 cm, full), check that Mount Image is selected and click on Save. Now you can install the Mac OS onto the new mounted disk, together with any other files that you require. Finally, you can burn the contents of the disk onto a blank CD-R.

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.

  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:-

DriveCD-ROMCD-IPhoto CD
Disc   
CD-A√ •
CD-ROM√ +××
CD-ROM/XA?××
CD-I?
Photo CD?

  Drive accepts disc

×  Drive never accepts disc

?  Some drives accept disc

  Enhanced Music CD only gives sound

+  Disc may have to be formatted to suit a specific operating system

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:-

(2352 bytes/sector) ÷ (4 samples/byte) = 588 samples/sector

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

(44,100 samples/second) ÷ (588 samples/sector) = 75 sectors/second

A 175 of a second is therefore known as a CD frame and allows you to find the number of sectors on a 74-minute CD:-

74 minutes × 60 seconds/minute × 75 sectors/second = 333,000 sectors

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

(333,000 sectors × 2352 bytes) ÷ (1024 × 1024) = 746.9 MB

Super Audio CD (SACD)

These special discs, supported by Philips and other companies, operate normally in a traditional CD player. But, unlike normal audio CDs, they use dual layer technology, with an extra semi-transparent layer on top of the usual reflective layer. This additional layer is similar to that in a DVD and stores up to 4.7 GB of data, which is used for higher quality sound and well as optional text, graphics and video. However, such information can only be extracted by a SACD-compatible player.

The audio material in the extra layer is supplied in two forms: as a stereo recording, located towards the middle of the disc, and a six-channel mix, normally in 5:1 form, which is placed further out from the centre. The coding of the audio data is simpler than in CD-A, employing 1-bit Direct Stream Digital (DSD) samples at a sampling frequency of 2.8224 MHz, resulting in much better quality. It offers a dynamic range of 120 dB, compared with the 96 dB provided by CD-A.

Alternative Formats

Although standard CD-A discs normally contain CDDA data, it’s also possible to include tracks in the form of AIFF, MP3 or WAV files, although these are frequently incompatible with a ‘traditional’ player.

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.

Copy-Protected Audio CDs

Some irresponsible record companies have produced ‘copy-protected’ CDs since the year 2000. These contain modified data that prevents playback in a computer drive, stopping the user from copying any audio content, even if perfectly legitimate.

Sony CDs use corrupted data in an area only used by computer drives (this can ‘hang’ the computer), whilst the Cactus system changes the coding in a way that’s acceptable to the error-correction in an audio player but not in a computer drive.

Unfortunately, you may not know whether or not a disc is protected. Some carry the words Will not work on a PC/Mac and, since they don’t conform Red Book standard, may not be marked with the usual COMPACT disc DIGITAL AUDIO logo.

Ejecting Protected Discs from a Mac OS Computer

There are several ways of dealing with an audio CD that ‘hangs’ your machine. In Mac OS X, the system shouldn’t fail entirely and you can eject the offending item by clicking on the Eject or Burn CD buttons in an application such as iTunes. If this doesn’t work, simply restart the computer using the appropriate key and then press and hold down the X key as soon as you hear the startup sound. Keep holding until the startup process is complete and then try the buttons again in iTunes.

In the Classic Mac OS a protected disc causes the system to fail entirely. This means you’ll have to resort to using the drive’s Eject button or its ‘paper clip’ hole. The latter is hidden in some G4 models, requiring you to gently pull back the front cover, whilst in a modern slot-loading drive it isn’t provided at all. In any event, you can always restart the computer using the appropriate key and then press and hold down the mouse as soon as you hear the startup sound.

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 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.

  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:-

333,000 sectors × 2048 bytes ÷ (1024 × 1024) = 650.39 MB
The term ‘CD-ROM’ in this section refers to the type of data on the disc, not the actual disc. Hence CD-ROM data is found on CD-ROM, CD-R and CD-RW discs. Where confusion exists, you can use the term ‘data CD’ instead of 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.

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:-

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.

Filenames

The filenames on ISO 9660 discs are restricted in the same way as in MS-DOS: a maximum of eight characters, together with a three-character filename extension. The ISO 9660 Level 1 standard only allows letters, numbers and the _ (underscore) character to be used, although most computers also permit other symbols from MS-DOS or Windows 3.x filenames, such as:-

` ´ ~ ! @ # $ % ^ & ( ) - { }

Note, however, that the following can’t be used under any circumstances:-

* \ ; : / ?

To accommodate the longer names in modern versions of Windows, CD-ROMs are often prepared using the Joliet variation of ISO 9660. Such discs can be seen on other machines, although only the ‘short’ names provided by ISO 9660 may appear.

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.

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.

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.

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:-

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.

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 drive:-

ModeTOCSize (MB)Speed (KB/s)Levels
1 (X)No6001503
2 (A)Yes *6811712
* Not used for CD-I discs

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

CD-Interactive (CD-I)

This unusual type of CD was originally intended for special CD-I players, although it appears that such discs can also be played on a modern Mac OS computer. The single Mode 2 track contains Type 4 and Type 5 sectors (see above), but, to add to the confusion, a TOC isn’t employed. This rather strange variation perhaps explains why CD-I discs can’t be used in some drives.

  Photo CD

This special kind of XA-formatted CD holds up to 100 pictures at various levels of resolution, reaching a maximum of sixteen times better than a standard television image. The actual CD format is known as multitrack CD-ROM/XA, meaning that the disc can contain several XA tracks. The following versions of each image are stored on every disc:-

TypeResolutionNote
Base/16192 × 128 
Base/4384 × 256 
Base768 × 512TV format
4Base •1536 × 1024HDTV format
16Base •3072 × 2048 
Lossless Huffman encoding used for compression

All the images of a given type are kept in the appropriate folder on the disc. In addition, all Photo CDs come with a Slide Show application for viewing the contents of the disc.

  Video CD (VCD)

VCD discs aren’t common in Europe, although popular in parts of Asia and used for some film releases in the United States. These CDs, which conform to the White Book standard issued by Sony and Philips, normally contain Type 5 sectors (see above).

Standard VCDs contain files that have been created using MPEG-1 compression, which allows a video recording or a slideshow of still pictures to be viewed. These files should conform to the ISO 11172-1 standard, providing both an MPEG video stream and an MPEG audio stream. The Super VCD variety of disc is similar, but employs MPEG-2 compression.

A typical VCD contains the following folders:-

FolderContents
CDIOften empty
EXTLOT_X.VCD and PSD_X.VCD files (duplicates of similarly named items in VCD folder)
MPEGAVMPEG movie file with .DAT filename extension
SEGMENTOften empty
VCDENTRIES.VCD, INFO.VCD, LOT.VCD and PSD.VCD files

References

ATPM 8.07, Arnold Woodworth & Ken Gruberman, 2002

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

©Ray White 2004.