Physical layout: CDs and DVDs

This page covers the disc as a physical object: what it is made of, how the data track is shaped, how the surface is divided by radius, and how recordable, rewritable and dual-layer discs differ from pressed ones. Later pages build on this. On-disk encoding explains how bytes are turned into the pits described here.

1. The common shape #

CDs and DVDs share the same outer form factor, which is why one drive can read both:

Everything else differs: where the data sits inside that 1.2 mm, how small the features are, and which laser reads them.

2. How data is stored: one long spiral #

2.1 The spiral track #

A disc does not have concentric rings like a hard disk. It has one continuous spiral of data that starts near the centre and winds outward to the edge. (The second layer of some DVDs runs the other way, described in section 6.) A drive follows the spiral with a laser, much as a record player's needle follows a groove, but nothing touches the surface.

The spacing between neighbouring turns of the spiral is the track pitch:

• CD: 1.6 µm nominal (about 1.5 µm on 80-minute discs). A CD's spiral makes about 22,000 turns and is about 5–6 km long.
• DVD: 0.74 µm, a little under half the CD pitch. That alone gives roughly twice the number of turns in the same area.

The word "track" is overloaded. Physically it means one turn of this spiral. In the CD format's logical structure it means a song or a data section (see Tracks). These pages always say which one they mean.

2.2 Pits and lands #

On a factory-made ("pressed" or embossed) disc, the spiral is a line of tiny depressions called pits, separated by flat stretches called lands. They are moulded into the polycarbonate from a metal stamper, and the surface is then coated with a thin reflective layer (usually aluminium).

The laser reads from the bottom, through the plastic. From that side a pit looks like a bump that rises toward the laser. The bump height is about a quarter of the laser's wavelength inside the plastic. Light reflected from the top of the bump and light from the surrounding land therefore return half a wavelength out of step, and partly cancel each other. A pit therefore reflects noticeably less light than a land. The drive's photodiode sees the reflected intensity rise and fall as pits pass under the beam.

An important detail: a pit does not mean "1" and a land does not mean "0". What carries information is a change: the leading or trailing edge of a pit. Each edge is a channel bit 1; every channel-bit period without an edge is a 0. This scheme is called NRZI (non-return-to-zero, inverted). The length of each pit and land, measured in channel-bit periods (T), is what encodes the data. On a CD every pit and land is between 3T and 11T long. The modulation code (EFM on CD, EFMPlus on DVD) guarantees this. See encoding.

(Exact numbers vary with the scanning velocity a particular disc was mastered at. The standards allow a range.)

2.3 The laser and the lens #

The size of the focused spot is proportional to wavelength / NA. DVD's shorter wavelength and larger NA shrink the spot enough to read the smaller pits and tighter pitch. A higher NA also makes focus more sensitive to disc tilt. That is why DVD moved the data layer to the middle of the disc (0.6 mm deep) instead of reading through the full 1.2 mm.

Reading through a thick layer of plastic has a useful side effect. At the bottom surface the beam is still wide (about 0.8 mm across on a CD), so a speck of dust or a fine scratch there is out of focus and blocks only a small part of the light. Scratches on the label side of a CD are much more dangerous, because the data layer sits directly under a thin lacquer coat there.

3. Rotation: constant linear velocity #

CDs and DVDs are designed to be read at constant linear velocity (CLV): the track passes under the laser at the same speed in metres per second wherever the laser is. A turn near the centre is shorter than one near the edge, so the disc must spin faster when the laser is near the centre and slower when it is near the edge.

Two consequences matter for disc images:

1. Sector size in "degrees of rotation" changes with radius. Near the centre one revolution holds about 9–10 CD sectors; at the edge it holds about 21–22. Alcohol 120%'s DPM (Data Position Measurement) table records this curve for a specific disc (see MDS/MDF). Some copy protections check it to tell a pressed original from a copy, whose curve differs slightly.
2. The scanning velocity is not exactly the same for every disc. It is fixed when the disc is mastered and allowed to vary within the range above. The same playing time can therefore occupy a slightly different radius on two different discs.

Modern drives often read at constant angular velocity (CAV) or zoned CLV for speed. The disc spins at a fixed rpm and the data rate rises toward the edge. This is how a "52×" drive works: 52× is the speed reached at the outer edge only. It does not change the disc format.

4. Layout by radius: CD #

ECMA-130 (the CD-ROM standard) and the Red Book divide the disc into concentric zones. Starting from the centre:

 radius (mm)   0     7.5        ~13–16.5         ~23       25                           ~58  60
               |-----|----------|----------------|---------|----------------------------|----|
               hole  (no data)   clamping area    lead-in   program area                 lead-out
                                                  (TOC)     (tracks: audio and/or data)

On a multisession disc this lead-in / program area / lead-out pattern repeats once per session, further out each time (see Sessions).

4.1 Capacity #

The program area holds at most 99 tracks and, in theory, up to MSF 99:59:74. In practice capacity depends on how tightly the spiral is packed:

90- and 99-minute discs exist, but go beyond the standards. MSF minutes above 90 collide with the range used to encode negative (lead-in) addresses, which causes compatibility problems (see LBA).

4.2 Cross-section of a pressed CD #

  label (printed ink)                       ← top / label side
  protective lacquer (~10–30 µm)
  reflective layer (aluminium, ~50–100 nm)
  ─ ─ pits moulded into the top surface of the substrate ─ ─
  polycarbonate substrate (1.2 mm)
                                            ← laser enters here (bottom / read side)

The data layer is just under the label. That is why CDs are most vulnerable from the top.

5. Recordable and rewritable CDs (Orange Book) #

A CD-R or CD-RW has no moulded pits. Instead it has a moulded pregroove: a shallow spiral groove that the writing laser follows. The drive records marks along this groove that, when read back, imitate the reflectivity pattern of pits and lands.

5.1 The wobble and ATIP #

The pregroove is not a perfect spiral. It wobbles slightly from side to side at a nominal 22.05 kHz (at 1× speed). A blank disc has no sector headers or subchannel to read yet, so the wobble is the only way the drive can find its position. The wobble is frequency-modulated with a slow data stream called ATIP (Absolute Time In Pregroove). ATIP carries:

• the address (MSF time) of each point along the groove, so the drive knows where it is on a blank disc;
• in the lead-in area, special ATIP frames with disc information: the start time of the lead-in, the last possible start time of the lead-out (the disc's capacity), the recommended writing power, the disc type (CD-R or CD-RW), and so on. The lead-in start time doubles as a manufacturer code. Tools like Aaru map it to the dye manufacturer.

When a disc is written, part of the ATIP information is copied into the TOC (for example into the C0 pointer, see Sessions).

5.2 Extra zones inside the lead-in #

Recordable discs have two working areas inside the lead-in radius that pressed discs lack:

• PCA (Power Calibration Area). The drive burns test patterns here to calibrate laser power before writing.
• PMA (Program Memory Area). A temporary table of contents. It records tracks written so far before a session is closed and a real lead-in/TOC is written. An open disc's PMA can be read with MMC's READ TOC/PMA/ATIP command (format 3).

5.3 Materials #

CD-RW reflects much less light (roughly 15–25% versus 70%+ for a pressed CD), so older drives and players could not read it. The "MultiRead" logo marked drives that could.

6. Layout: DVD #

6.1 Construction #

A DVD is two 0.6 mm polycarbonate discs bonded together. The data sits in the middle of the sandwich, 0.6 mm from the read surface. That makes four basic combinations:

("GB" here means 10⁹ bytes. 4.70 GB ≈ 4.38 GiB.)

On a dual-layer disc, layer 0 (L0) is the one nearer the laser. Its reflective coating is only semi-transparent (a thin gold or silicon film), so the laser can focus through it onto layer 1 (L1) behind it. L1 has a normal, fully reflective coating. A transparent spacer of about 40–70 µm separates them. To switch layers the drive just refocuses.

Each layer of a dual-layer disc holds a little less than a single-layer disc. The pits are made about 10% longer, and the velocity raised to 3.84 m/s, so that the weaker signal from each layer is still readable.

6.2 Zones by radius #

 radius (mm)   ~22.3–23.5        ~22.6        24                                   ~58
               |-----------------|------------|-------------------------------------|-------
               BCA (optional)     lead-in      data area                             lead-out
                                  (control     (PSN 0x30000 = LBA 0 ...)             (or middle
                                   data zone)                                         area)

DVD has no "tracks" or "sessions" in the CD sense on pressed discs. A DVD-ROM is one big run of 2048-byte sectors. (Recordable DVDs have a similar idea, called borders or sessions, see Sessions.)

6.3 Dual-layer track paths: PTP and OTP #

The two layers of a dual-layer disc can be arranged in two ways:

• Parallel Track Path (PTP). Both layers spiral from the inside out, and each has its own lead-in and lead-out. They are treated as independent. This is rare on pressed discs.
• Opposite Track Path (OTP). Layer 0 spirals from the inside out. Layer 1 spirals from the outside in. When the drive reaches the end of L0 at some radius, it refocuses onto L1 at the same radius and keeps going inward, with no long seek back to the centre. Movies and games use OTP so the switch between layers (the layer break) causes only a short pause.

OTP sector numbering is designed so that the numbers keep increasing across the switch. Layer 1's physical sector numbers are the bitwise complement (24 bits) of the layer 0 numbers at the same radius:

PSN_on_L1 = (~PSN_on_L0_at_same_radius) & 0xFFFFFF

L0's data area ends at some PSN E0, so L1's data begins at ~E0 & 0xFFFFFF, which equals 0xFFFFFF − E0. LBAs count straight through: L1's first sector has LBA = (number of sectors on L0). See LBA for a worked example.

The PFI records the layer count, track path and the end sector of layer 0. The PFI byte layout is in MDS/MDF.

6.4 Recordable DVD #

As with CD-R, recordable DVDs use a wobbled pregroove:

• DVD-R / DVD-RW (DVD Forum): wobble at about 140.6 kHz, plus land pre-pits between the grooves that carry address information.
• DVD+R / DVD+RW (DVD+RW Alliance): a higher-frequency wobble (about 817 kHz) that is phase-modulated with ADIP (Address In Pre-groove).

Both have power-calibration and recording-management areas, similar to CD-R's PCA and PMA. Their PFI disc category differs from DVD-ROM's. An image tool can see from it whether the original was pressed or burned, unless the tool rewrites it: Aaru's source notes that Alcohol 120% has been seen modifying the disc category and part version when it stores the PFI.

7. Side by side #

Sources #

• ECMA-130, Data interchange on read-only 120 mm optical data disks (CD-ROM), 2nd ed. Physical dimensions, zones, CLV, EFM. https://ecma-international.org/publications-and-standards/standards/ecma-130/
• ECMA-267, 120 mm DVD – Read-Only Disk. DVD dimensions, PSN, OTP/PTP, control data zone. https://ecma-international.org/publications-and-standards/standards/ecma-267/
• ECMA-394 (CD-R) and ECMA-395 (CD-RW), the publicly available equivalents of the Orange Book. https://ecma-international.org/publications-and-standards/standards/ecma-394/, https://ecma-international.org/publications-and-standards/standards/ecma-395/
• Wikipedia, "Compact disc" (physical parameters, capacities): https://en.wikipedia.org/wiki/Compact_disc
• Wikipedia, "DVD": https://en.wikipedia.org/wiki/DVD
• Wikipedia, "CD-R", "CD-RW", "Burst cutting area": https://en.wikipedia.org/wiki/CD-R, https://en.wikipedia.org/wiki/CD-RW, https://en.wikipedia.org/wiki/Burst_cutting_area
• Aaru, Aaru.Images/Alcohol120/Read.cs (note on Alcohol modifying PFI disc category), lib/Aaru/.