Optical Media and Disc Images

Optical media, sector formats, filesystems, and image files are separate layers. A mountable .iso can reproduce a data volume while omitting audio tracks, session boundaries, subchannels, and recording properties. Image format must therefore follow the intended task: file distribution, booting, emulation, or preservation.

Tracks, Marks, and Reading #

CD, DVD, and Blu-ray Disc (BD) store their program area along a spiral track. A focused laser follows that spiral from the inner program area outward, and the optical pickup derives a signal from changes in reflected light. Rotation commonly uses constant linear velocity or a zoned strategy because the circumference grows toward the rim.

Pressed read-only discs mold a relief pattern into polycarbonate and add a reflective layer. “Pit” and “land” describe physical levels, not literal zero and one cells. Transitions and run lengths produce the channel signal, which the drive decodes and error-corrects. A CD uses Eight-to-Fourteen Modulation and Cross-Interleaved Reed–Solomon Code; sector-level bytes appear only after those lower layers have been decoded.

Physical dimensions differ by family. A standard CD is a 1.2 mm polycarbonate disc read with a near-infrared laser and has a 1.6 µm track pitch. DVD raises density through a shorter-wavelength red laser, smaller marks and pitch, and optional second layers. BD uses a blue-violet laser and approximately 0.32 µm track pitch. The shared spiral concept does not make their optics, channel codes, or sector formats interchangeable.

Pressed, Recordable, and Rewritable Media #

Manufacturing and recording method determine how marks are formed:

Pressed read-only media replicate the mastered relief into molded substrates. Electroforming turns a developed glass master into a metal stamper; molding, metallization, bonding, lacquer, and labeling produce finished discs.
Recordable media are write-once. CD-R and conventional recordable DVD formats alter an organic dye, while common BD-R designs use inorganic recording layers. The written contrast imitates the readout behavior expected by a compatible drive.
Rewritable media use a reversible recording layer, commonly a phase-change alloy. Heating switches regions between states with different reflectivity.

Recordable discs include a preformed groove that guides the recorder and carries format-specific addressing or timing information. Rewritability does not imply disk- like atomic updates: packet-writing software, defect management, filesystem rules, and media finalization still constrain changes. Material color alone cannot identify a formulation or predict archival life.

Tracks and Sessions #

A CD track is one contiguous stream with a declared type, such as CD Digital Audio (CD-DA) or a CD-ROM data mode. A disc can contain multiple tracks. Mixed-mode and CD Extra layouts combine audio and data under specific ordering rules; track control flags tell a player how to interpret them.

A session groups one or more tracks written in a recording operation and adds lead-in and lead-out structures. Multisession readers use session metadata to locate tracks and choose a filesystem view. Appending a session can expose a new directory tree that references data from earlier sessions; it does not rewrite already recorded sectors. A file-only copy loses those relationships.

Specification Families and Sectors #

Color-named Rainbow Books define CD-family formats:

Red Book specifies CD-DA.
Yellow Book specifies CD-ROM modes.
Orange Book covers recordable and rewritable CD.
Green, White, and Blue Books define CD-i, Video CD, and Enhanced CD applications.

ECMA-130 and ISO/IEC 10149 standardize CD-ROM recording and sector organization, while IEC 60908 covers CD-DA. ISO 9660 defines a separate volume and file structure over CD-ROM sectors. DVD Forum specifications and the Blu-ray Disc Association’s books define their respective media; DVD and BD are not Rainbow Book formats.

A conventional decoded CD sector contains 2,352 bytes. CD-DA uses all 2,352 bytes for audio samples. A CD-ROM Mode 1 sector exposes 2,048 user-data bytes and spends the remainder on synchronization, addressing, error detection, and additional error correction. Mode 2 forms divide the bytes differently. Consequently, “sector size” must state whether it means channel data, raw decoded sectors, or filesystem-visible user data.

Filesystem and Boot Layers #

Optical filesystems consume user-data sectors; they do not define the optical marks or all track metadata.

ISO 9660 and Extensions #

ISO 9660, derived from the High Sierra format, defines a portable read-only volume and directory structure. Its base interchange levels restrict names and directory organization. Rock Ridge records POSIX-oriented names, links, permissions, and device information in System Use fields. Joliet adds a Microsoft-defined Unicode directory tree. Readers must explicitly support the chosen extension.

UDF #

The Optical Storage Technology Association’s Universal Disk Format (UDF) specializes the volume and file structures defined by ECMA-167. UDF supports media and update models needed by DVD, BD, packet writing, and write-once recording. Revision and media-profile compatibility matter because a reader can support UDF without supporting every revision or feature.

El Torito and Hybrid Views #

El Torito adds firmware-discoverable boot records and a boot catalog to a CD-ROM volume. Catalog entries identify floppy-emulation, hard-disk-emulation, or no-emulation boot images. Bootability therefore depends on records, catalog values, image bytes, and placement—not only on files visible in the mounted directory.

A hybrid disc can share file data among ISO 9660, Joliet, Rock Ridge, and UDF descriptors so different systems select different directory views. Some installer images also combine optical boot metadata with an MBR or GPT layout for USB booting. Rebuilding one visible filesystem can discard the other views and boot paths.

Image Representations #

An image captures a selected disc layer:

An ISO image conventionally stores 2,048-byte logical sectors from a data track. It can contain ISO 9660, UDF, a hybrid volume, or another block layout; the file extension does not identify the filesystem.
A raw CD track image commonly stores 2,352-byte sectors. Raw sectors preserve more mode-specific bytes but still do not describe track boundaries, pregaps, control flags, or sessions by themselves.
A BIN/CUE set pairs one or more binary track files with a cue sheet describing track types, order, indexes, and gaps. Preservation requires both parts.
Container formats such as NRG can store track data plus vendor-specific layout metadata. Tool support determines which metadata survives conversion.

Subchannel data is another stream associated with CD frames. Some copy-protection, index, CD-Text, and timing use cases require selected subchannels, but ordinary ISO or BIN/CUE workflows may omit them. No extension alone proves completeness; imaging documentation must state sector mode, subchannel handling, drive behavior, and read-error policy.

Inspection, Mounting, and Extraction #

Inspection should precede mounting because a mount exposes one filesystem view and can conceal other tracks. libcdio tools inspect CD and ISO 9660 structures:

cd-info --no-device-info image.cue
iso-info -i installer.iso
iso-read -i installer.iso -e README.txt -o README.txt
isosize --sectors installer.iso

Installed versions differ, so scripts should verify each command’s manual rather than assume options. Extraction with a format-aware reader preserves the source as read-only and avoids kernel mounting requirements.

Linux can mount a compatible logical image through a loop device:

sudo mount -o loop,ro installer.iso /mnt/iso

macOS can attach a mountable image with:

hdiutil attach -readonly installer.iso

Mounting proves that one filesystem parser accepted one view; it does not validate audio, sessions, boot paths, raw error-correction bytes, or subchannels.

Integrity and Reproducibility #

A defensible imaging workflow records:

disc identity, visible condition, and drive model and firmware;
imaging tool, version, command, sector mode, and retry policy;
track, session, and subchannel metadata;
cryptographic hashes for every image and companion metadata file;
read errors, recovered sectors, and whether repeated reads agree.

Hash agreement protects the captured files after acquisition but cannot prove that the drive returned every physical feature. Logical images are appropriate when the object of interest is a mountable software distribution. Raw tracks with complete layout metadata are required when track semantics, audio, sessions, or emulator behavior matter.