Create an ISO File & Understand CD Formats

CD Formats: An Introduction

Some CD formats have little relevance today, others even less – and still others are essential. The three most important ones:

• CD-DA – the Audio CD
• CD-ROM – the Data CD
• CD-ROM/XA – the Extended Data CD

Why are there so many formats?

In the early 80s, the Audio CD was the beginning of everything – great for music, but little else. Since computer data, videos, and multimedia were also desired on CD, the audio CD technology was quickly adapted. The physical dimensions remained the same – only the logical structure of the sectors was changed.

What is a sector?

A CD sector typically comprises 2352 bytes and lies like a pearl on the helical track, from inside to outside. Sectors make individual blocks of information locatable.

Sectors are grouped into structures: Lead-in (with the TOC) at the beginning, then the Data Area (at least 1, at most 99 tracks), and the Lead-out at the end. A CD must consist of at least one session but can contain up to 99.

CD-DA – The Audio CD Red Book

Published in 1980 by Sony and Philips as the very first CD format. CD-DA is the specification for discs filled with music and played on an audio CD player.

The Specs

• Music information in 16-bit stereo
• Sampling rate 44.1 kHz
• Tracks at least 4 seconds long
• A single session per CD
• Best burned in Disc at Once (DAO)

The reason for the single-session requirement: audio CD players can only recognize the first session.

CD+G – Karaoke CDs

CD+G is a remnant from older CD recording days – the first attempt to embed additional information into the R-to-W subchannel of an audio CD. The additional data is usually graphics (hence "CD plus Graphics"), less commonly text or MIDI files (CD+MIDI).

Today, the format has little significance – except for karaoke CDs, which are still written in CD+G.

CD-ROM – The Data CD Yellow Book

The next step after the audio CD: computer data on CD. The specs are largely based on CD-DA but are supplemented with extended error correction for sensitive data.

Cross-platform thanks to ISO 9660

A CD-ROM can be read by any operating system – provided an ISO 9660 driver is installed (e.g., MSCDEX for DOS/Windows).

Writing Methods

Data CDs can be created as TAO, DAO, single-session, or multi-session. Standard is a TAO multi-session CD. DAO single-session is only used if the CD is to serve as a master for duplication or be a 1:1 copy.

Mode 1 vs. Mode 2

• CD-ROM Mode 1: 2048 bytes user data/sector – practically the only mode
• CD-ROM Mode 2: 2336 bytes/sector with less error correction – rarely used

CD-ROM XA

A mix of CD-I and CD-ROM Mode 2, created to bring multimedia applications to the computer. The CD-I method (sectors of various sizes on a track) was transferred to the data CD.

Form 1 vs. Form 2

• Form 1: 2048 bytes/sector with high error correction – for programs and critical data
• Form 2: 2324 bytes/sector with less error correction – for graphics and sound

Important is the possible synchronicity of audio and graphic files by interleaving both sector forms on a track. Also: subheaders for additional information.

CD-Extra

The format of choice for data + audio on one disc. Unlike a Mixed-Mode CD, the data track is recorded as a second session after the audio session – thus invisible to audio CD players that only recognize the first session.

You can recognize this format by the term "Session at Once" – which is the required writing technique. Audio sessions contain 2352-byte sectors, data sessions contain 2048-byte sectors in CD-ROM/XA Form 1 with extended error correction.

CD-I Green Book & CD-I Ready

CD-I (Green Book)

The first format with sectors of different sizes (Form 1 and Form 2). CD-I became known as a multimedia spectacle for CD-I players – which are now hard to come by. But the format survives: Older game console CDs (e.g., Sony PlayStation 1) use it. CD-I also provided the template for CD-ROM/XA.

CD-I Ready

A so-called pregap disc: The normally 2-second silence before a music track (between index 0 and index 1) is greatly extended to accommodate additional data – song lyrics, artist information, graphics, videos.

Audio CD players normally jump to index 1 for the first track and ignore the data in index 0 – CD-I players, however, read index 0 first.

CD-ROM Ready

A variant of CD-I Ready that also makes the same additional information accessible to normal CD-ROM drives. Also known as Hidden Track, i-trax or Track Zero.

As with CD-I Ready, the data is placed in the pregap of the first track – thus located near the center hole, where the best access times exist.

Both formats (CD-I Ready and CD-ROM Ready) have largely fallen by the wayside in practice.

CD-Text

A normal audio CD contains no information about the artist or track title. CD-Text – developed by Philips and Sony – changes that: new generation audio players display artists, titles, and comments on the screen.

Where is the information stored?

CD-Text uses the R- and W-subchannel bytes in the lead-in area. The normal audio data and the CD's playing time remain untouched – the full playing time can be utilized.

Photo-CD Beige Book & Video-CD White Book

Both are so-called Bridge Discs – a mixture of specs from CD-ROM/XA and CD-I.

Photo-CD (Beige Book)

Layout like a Multisession CD-ROM. The first track contains a CD-I data track with the necessary files for CD-I or Photo-CD players. All other tracks are in Mode 2. Production requires special premastering tools.

Video-CD (White Book)

Bridge disc between CD-ROM/XA and CD-I, with additional specs for video encoding (MPEG-1). Layout like a single-session CD-ROM. First track: CD-I data track for the player, remainder: MPEG streams in Mode 2.

Cuesheet – The CD Script

A Cuesheet is a simple ASCII text file containing all information for the TOC (Table of Contents). Every CD recorder works internally with such a cuesheet – you can also write it manually.

The Most Important Commands

• FILE – Specifies the file to be written (image)
• TRACK – Track number and data type (AUDIO, MODE1/2048, etc.)
• FLAGS – Sets flags such as copy protection (DCP), four-channel audio (4CH), pre-emphasis (PRE)
• PREGAP – Length of the pause before a track (as silence)
• INDEX – Jump markers within a track (Index 01 = start of track)
• POSTGAP – Pause after a track

Minimal Cuesheet Example (Audio CD)

Allowed Data Types

• AUDIO – Wave/AIFF/MP3 with 2352 bytes/sector
• CDG – Karaoke / CD+G with 2448 bytes/sector
• MODE1/2048 – CD-ROM (user data area)
• MODE1/2352 – CD-ROM RAW
• MODE2/2336 – CD-ROM/XA Form 2 (user data)
• MODE2/2352 – CD-ROM/XA Form 2 RAW
• CDI/2336 or CDI/2352 – CD-I Form 2

PQ-Editing

PQ-Editing refers to intervening in the depths of the CD structure – but don't worry, it sounds worse than it is. Possibilities:

• Pauses – Silence between tracks (Pregaps in the P-subchannel)
• Indices – Jump markers within a track (Q-subchannel)
• ISRC codes – Owner/copyright info (only professionally relevant)
• Pre-emphasis markers – Treble boost (only useful for dull sound)
• 4CH – Four-channel audio
• Copy protection flags – hardly relevant anymore because computer burners ignore them

Prerequisites: Software and a recorder that both support DAO, plus access to the subchannels. This can be done either via a custom cuesheet or through the program menus.

Subchannel Data

Each sector of an audio CD consists of 2352 bytes of music + 784 bytes of error detection + 98 control bytes. These 98 control bytes are distributed over 98 frames – one per frame.

P to W

Each control byte is broken down into 8 bits, assigned to the letters P, Q, R, S, T, U, V, W. This creates the subchannels:

• P-subchannel – every first bit (e.g., pregap info)
• Q-subchannel – every second bit (e.g., TOC, track info)
• R-to-W subchannels – grouped together, e.g., CD-Text and karaoke graphics

The subchannels are read parallel to the user data without disturbing it. The memory used here does not deduct from the disc capacity. The only catch: Not every recorder can write to every subchannel, and not every player can evaluate them.

File Systems: ISO 9660, Joliet, UDF

ISO 9660 – The Cross-Platform Classic

The CD file system that virtually every operating system understands. Three levels with varying strict rules:

• Level 1: Very restrictive. Filenames 8.3 (8 characters + 3 extension), only uppercase A-Z, numbers 0-9, underscore. Maximum 8 directory levels. Files must be stored contiguously.
• Level 2: Filenames up to 31 characters, more special characters. Otherwise, everything like Level 1.
• Level 3: Other restrictions also loosened – files may be stored fragmented. However: hardly any burning software still offers Level 3; most directly use UDF.

Joliet

Microsoft extension for Windows: Removes restrictions on filename length, special characters, and directory depth. Joliet discs are limited to DOS, Windows, and Mac OS.

Romeo

Also a Windows extension. Filenames up to 128 characters including spaces – which causes problems under DOS. Now obsolete.

HFS

Apple extension of the ISO standard. Mac files are stored in two forks: Data Fork (actual file) and Resource Fork (file type, etc.). HFS CDs are based on ISO Level 2 and can only be read on Mac, Linux, OS/2, and SGI Irix.

RRIP

Rock Ridge Interchange Protocol – the Unix extension. Removes all restrictions on filenames and directory depth, allows symbolic links.

UDF – The DVD Successor

The Universal Disc Format was developed by the OSTA (Optical Storage Technology Association) – specifically with DVD in mind. It is intended to replace ISO 9660 as a cross-platform file system. The limitations of ISO are removed.

Also important for Packet Writing: ISO 9660 only allows file fragmentation in Level 3, UDF without further ado. For cross-platform access, UDF CDs can be enclosed with an ISO 9660 wrapper upon completion – so systems with an ISO driver read them as ISO, systems with a UDF driver read them as UDF.