Tuesday, May 11, 2010

DVD Capacity

DVD-1[21]SS SL1181.461.36
DVD-2SS DL1282.662.47
DVD-3DS SL2282.922.72
DVD-4DS DL2485.324.95
DVD-5SS SL11124.704.38
DVD-9SS DL12128.547.95
DVD-10DS SL22129.408.75
DVD-14[22]DS DL/SL231213.2412.33
DVD-18DS DL241217.0815.90

Capacity and nomenclature of (re)writable discs
DVD-RSS SL (1.0)11123.953.68
DVD-RSS SL (2.0)11124.704.37
DVD-RWSS SL11124.704.37
DVD+RSS SL11124.704.37
DVD+RWSS SL11124.704.37
DVD-RDS DL22129.408.75
DVD-RWDS DL22129.408.75
DVD+RDS DL22129.408.75
DVD+RWDS DL22129.408.75
DVD-RAMSS SL1181.461.36*
DVD-RAMDS DL2282.652.47*
DVD-RAMSS SL (1.0)11122.582.40
DVD-RAMSS SL (2.0)11124.704.37
DVD-RAMDS DL (1.0)22125.164.80
DVD-RAMDS DL (2.0)22129.408.75*

The basic types of DVD (12 cm diameter, single-sided or homogeneous double-sided) are referred to by a rough approximation of their capacity in gigabytes. In draft versions of the specification, DVD-5 indeed held five gigabytes, but some parameters were changed later on as explained above, so the capacity decreased. Other formats, those with 8 cm diameter and hybrid variants, acquired similar numeric names with even larger deviation.

The 12 cm type is a standard DVD, and the 8 cm variety is known as a MiniDVD. These are the same sizes as a standard CD and a mini-CD, respectively. The capacity by surface (MiB/cm2) varies from 6.92 MiB/cm2 in the DVD-1 to 18.0 MiB/cm2 in the DVD-18.

As with hard disk drives, in the DVD realm, gigabyte and the symbol GB are usually used in the SIsense (i.e., 109, or 1,000,000,000 bytes). For distinction, gibibyte (with symbol GiB) is used (i.e., 230, or 1,073,741,824 bytes). Most computer operating systems display file sizes in gibibytes, mebibytes, and kibibytes, labeled as gigabyte, megabyte, and kilobyte, respectively.

Size comparison: a 12 cm DVD+RW and a 19 cm pencil.

Each DVD sector contains 2,418 bytes of data, 2,048 bytes of which are user data. There is a small difference in storage space between + and - (hyphen) formats:

Capacity differences of writable DVD formats
DVD−R SL2,298,4964,707,319,8084,707.3204,489.2504.7074.384
DVD+R SL2,295,1044,700,372,9924,700.3734,482.6254.7004.378
DVD−R DL4,171,7128,543,666,1768,543.6668,147.8758.5447.957
DVD+R DL4,173,8248,547,991,5528,547.9928,152.0008.5487.961


Internal mechanism of a DVD-ROM Drive. See text for details.
DVD-RW Drive operating with the protective cover removed.

DVD uses 650 nm wavelength laser diode light as opposed to 780 nm for CD. This permits a smaller pit to be etched on the media surface compared to CDs (0.74 µm for DVD versus 1.6 µm for CD), allowing for a DVD's increased storage capacity.

In comparison, Blu-ray, the successor to the DVD format, uses a wavelength of 405 nm, and one dual-layer disc has a 50 GB storage capacity.

Writing speeds for DVD were 1×, that is, 1350 kB/s (1,318 KiB/s), in the first drives and media models. More recent models, at 18× or 20×, have 18 or 20 times that speed. Note that for CD drives, 1× means 150 KiB/s (153.6 kB/s), approximately one ninth as fast.[21]

DVD drive speeds
Drive speedData rate~Write time (min)[23]

Internal mechanism of a drive

This mechanism is shown right side up; the disc is above it. The laser and optical system "looks at" the underside of the disc.

Referring to the photo, just to the right of image center is the disc spin motor, a gray cylinder, with its gray centering hub and black resilient drive ring on top. A clamp (not in the photo, retained in the drive's cover), pulled down by a magnet, clamps the disc when this mechanism rises, after the disc tray stops moving inward. This motor has an external rotor – every part of it that you can see spins.

The gray metal chassis is shock-mounted at its four corners to reduce sensitivity to external shocks, and to reduce drive noise when running fast. The soft shock mount grommets are just below the brass-colored washers at the four corners (the left one is obscured). Running through those grommets are screws to fasten them to the black plastic frame that's underneath.

Two parallel precision guide rods that run between upper left and lower right in the photo carry the "sled", the moving optical read-write head. As shown, this "sled" is close to, or at the position where it reads or writes at the edge of the disc.

A dark gray disc with two holes on opposite sides has a blue lens surrounded by silver-colored metal. This is the lens that's closest to the disc; it serves to both read and write by focusing the laser light to a very small spot. It's likely that this disc rotates half a turn to position a different set of optics (the other "hole") for CDs vs. DVDs.

Under the disc is an ingenious actuator comprising permanent magnets and coils that move the lens up and down to maintain focus on the data layer. As well, the actuator moves the lens slightly toward and away from the spin-motor spindle to keep the spot on track. Both focus and tracking are relatively quite fast and very precise. The same actuator rotates the lens mount half.a turn as described.

To select tracks (or files) as well as advancing the "sled" during continuous read or write operations, a stepping motor rotates a coarse-pitch leadscrew to move the "sled" throughout its total travel range. The motor, itself, is the gray cylinder just to the left of the most-distant shock mount; its shaft is parallel to the support rods. The leadscrew, itself, is the rod with evenly-spaced darker details; these are the helical groove that engages a pin on the "sled".

The irregular orange material is flexible etched copper foil supported by thin sheet plastic; these are "flexible printed circuits" that connect everything to the electronics (which is not shown).

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