Understanding DVD: Structure & Formats

The DVD Books (A–E)

Similar to the "Rainbow Books" for CDs, DVDs have five specification books. Books A–C cover read-only formats, D and E cover recordable formats.

The Physical Structure

A DVD looks deceptively similar to a CD – and shares its dimensions: 12 cm diameter, 1.2 mm thick. There are also 8 cm DVDs.

Data is stored using pits and lands, similar to a CD, but the dimensions are drastically reduced:

• Pits only 0.4 µm (CD: 0.9 µm)
• Track pitch of the helix: 0.74 µm (CD: 1.6 µm)
• Laser wavelength: 650/635 nm red (CD: 780 nm)
• Data layer: 0.6 mm thick (CD: 1.2 mm)

Layers and Sides

A DVD consists of two 0.6 mm thick halves, glued back-to-back. Each half can carry one or two data layers – hence the variety of options:

Imation vs. Matsushita Method

For two layers on one side, a partially reflective first layer is used, through which the laser can focus to the deeper layer:

• Imation: True two layers – more complex, but double capacity per side.
• Matsushita: Two sides, semi-transparently glued – simpler, but effectively a two-sided disc without requiring flipping.

Track Paths: PTP and OTP

The data track of the first layer spirals outwards, like on a CD. For the second layer, there are two options:

• PTP (Parallel Track Path): Both tracks parallel – enables, for example, camera angle changes.
• OTP (Opposite Track Path): Reversed reading direction – seamless playback for extra-long movies.

The Logical Structure

Logical structures define how files and directories are mapped to physical sectors – volumes, blocks, allocation tables, character sets, paths.

For CDs, ISO 9660 was the standard. For DVDs, ISO 9660 was too rigid from the start – the OSTA (Optical Storage Technology Association) designated UDF as its successor.

UDF — Universal Disc Format

UDF is the universal file system for DVDs: cross-platform, without the old ISO 9660 limitations. Up to 64,000 characters, long filenames, arbitrary directory depth, fragmented files.

UDF Bridge / Micro-UDF

In practice, almost every DVD is wrapped in an ISO 9660 shell (Level 3) – as a bridge disc:

• Systems with an ISO driver read it as an ISO disc
• Systems with a UDF driver read it as a UDF disc
• Both access the same files, no separate partitions

Pure UDF discs without an ISO shell only work with an installed UDF driver – various CD burning programs have packet-writing applications with (often proprietary) UDF variations.

DVD-ROM — the Basis of All DVDs

The DVD-ROM specification lays down the framework for all read-only DVDs: dimensions, file system, EFM Plus, Reed Solomon error correction. Plus:

• Reflectivity: 45–85% for the first layer (CD: ~80%), 13–35% for the second
• Minimum data rate during reading
• Storage capacities of all variants
• Error correction overhead: approx. 13%

This makes it clear: Every DVD is essentially a DVD-ROM. A DVD-Video is just a sub-unit – but one that packs a punch: the application layer regulates everything from audio codec to menu navigation.

DVD-R — Write-Once

DVD-R is the counterpart to CD-R: write-once media with an organic polymer recording layer. Once burned, compatible with most DVD players and drives.

Capacities

• 4.7 GB — 12 cm, single-sided (standard today)
• 8.5 GB — 12 cm, double-layer
• 9.4 GB — 12 cm, double-sided (rare)
• 1.4 GB — 8 cm, single-sided
• 2.8 GB — 8 cm, double-sided

Use as a video recorder only became worthwhile from 4.7 GB – smaller capacities were more of an interim solution.

DVD-RAM — The Misconception

DVD-RAM initially sounded like the solution for multiple rewrites. The reality: almost 100% incompatible with all other players. The technology is based on Phase Change plus MO and hard disk techniques; data is also written on the partition between tracks.

Capacities

• 2.6 GB single-sided (often in a caddy version)
• 5.2 GB double-sided (only without a caddy)

Companies involved: Toshiba, Panasonic, Hitachi. Conclusion: Storage space is insufficient for videos, and almost no players read the discs. DVD-RAM did not become a seriously viable standard.

DVD-RW & DVD+RW

DVD-RW

Rewritable format based on Phase Change. Developed by Pioneer, it uses the pit sizes of DVD-R – it should be compatible with existing DVD players. Capacity: 4.7 GB (Single-Layer), 8.5 GB (Double-Layer).

DVD+RW

Competing format to DVD-RAM, introduced in autumn 1999 – also based on Phase Change. Main participants: Philips, Sony, Hewlett Packard, plus Ricoh, Yamaha, Mitsubishi, and the major film studios.

Special DVD Formats

Mac/PC Hybrid DVD

The DVD variant of the well-known Hybrid CD: two partitions on one disc – Mac-specific and PC-specific. Each can only be managed by its respective operating system.

WWW-DVD

A DVD-ROM or DVD-Video with built-in browser functions: screenplays as HTML versions with direct jumps to film scenes or websites. Only available as a US import in this country, playback requires a PC with a DVD-ROM drive.

Mixed-Media (Rewritable Sandwich-Disc)

An unusual mix: a DVD with two layers – one being a DVD-RAM writable layer (rewritable multiple times), the other containing DVD-ROM data.

Alternative Formats

SACD — Super Audio CD

Sony and Philips' competing format to DVD-Audio. A Hybrid Disc with two layers: the upper layer can be played on any CD player, the lower only on DVD-SACD players. Also known as Legacy or Sandwich Disc. It did not catch on.

MMFV and AS-MO

An alternative DVD writing format introduced by NEC – similar to DVD/RW and DVD+RW, with approx. 5 GB capacity. AS-MO was supposed to achieve the same. Both probably only became relevant for video surveillance and conference recording.

DIVX (the hardware format!)

Not to be confused with the MPEG-4 codec DivX. DIVX was an attempt by the film industry in the late 90s to establish an alternative distribution method for DVD-Videos – DIVX players had a modem for film rental, which charged money per viewing. The format died in 1998.