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The Authoritative BD FAQ: IV. Disc Capacity
Posted Jun 1, 2006 Print Version     Page 1of 1
  

IV. Disc Capacity
     a. What capacities of BD discs are available?
     b. How much information can actually be stored on a BD disc?

     c. How far might BD disc capacity increase in the future?

What capacities of BD discs are available?
As with CD and DVD formats, BD-ROM, BD-R and BD-RE discs are marketed in 8 cm (80 mm) and 12 cm (120 mm) diameter sizes. Most applications call for 12 cm discs while the 8 cm type will most likely find a home in portable consumer electronics devices such as digital video camcorders.

In practice, 12 cm BD discs come in 25 GB single-layer (SL) and 50 GB dual-layer (DL) capacities and 8 cm in 7.8 GB (SL) and 15.6 GB (DL). Early BD-RE discs (Japan-only version 1.0 and 1.01) were 23.3 GB (SL).

Strictly speaking, BD specifications describe a range of capacities achieved by changing the fixed rotational speed of the disc (5.28 m/s, 4.917 m/s, 4.554 m/s). For 12 cm, these are 23.3 GB, 25 GB, 27 GB (SL) and 46.6 GB, 50 GB, 54 GB (DL) and for 8 cm, 7.8 GB (SL) and 15.6 GB (DL). These alternatives accommodate past limitations, while permitting future improvements, in mastering, replication and recording technologies. For example, 23.3 GB was the practical limit when BD-RE was first introduced in 2003 but is now obsolete while 27 GB is not yet feasible to produce. In many ways the planned development path for BD parallels the history of prerecorded and writable CDs where discs eventually evolved, as technology permitted, from 63 (550 MB) to 74 (650 MB) and then to 80 (700 MB) minutes in length.

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How much information can actually be stored on a BD disc?
As with DVD, manufacturers quote BD disc capacities in decimal (base 10) rather than binary (base 2) notation so a 25 GB disc stores roughly 25 billion bytes [25,025,315,816 bytes ÷ 1000 = 25,025,315.816 KB ÷ 1000 = 25,025.316 MB ÷ 1000 = 25.025 GB]. Expressed in binary notation (as is typical with most operating systems) the same disc has a capacity of roughly 23.3 GB [25,025,315,816 bytes ÷ 1024 = 24,438,784.977 KB ÷ 1024 = 23,866 MB ÷ 1024 = 23.307 GB].

Discs store information in multitude of sectors that each house 2KB (2,048 bytes) of user data. 32 sectors are then grouped together into 64KB (65,536 byte) partitions called clusters (see chart). A disc contains a fixed (BD-R, BD-RE) or maximum (BD-ROM) number of clusters in a special area of the disc reserved to hold user information (Data Zone) so disc capacity can be precisely calculated by multiplying the size of the Data Zone by the amount of information stored in each cluster. For example, a 25 GB disc: 381,856 clusters x 65,536 bytes per cluster = 25,025,315,816 bytes. This rounds to 25 GB (decimal notation).

BD Disc Capacites
(Unformatted)

Disc Size

Status

Number of Layers

Number of Clusters in Data Zone

Gross Capacity (bytes)

Gross Capacity (GB decimal)

Gross Capacity (GB binary)

8 cm

Current

1

118,884

7,791,181,824

7.791

7.256

 

Current

2

237,768

15,582,363,648

15.582

14.512

12 cm

Obsolete

1

355,603

23,304,798,208

23.305

21.704

 

Current

1

381,856

25,025,314,816

25.025

23.307

 

Reserved/future

1

412,294

27,020,099,584

27.020

25.164

 

Obsolete

2

711,206

46,609,596,416

46.610

43.409

 

Current

2

763,712

50,050,629,632

50.050

46.613

 

Reserved/future

2

824,588

54,040,199,168

54.040

50.329

Be aware that the file system (UDF, HFS, etc.) and any defect management system employed consume space otherwise available for user information. For example, spare clusters used for defect management (allocated as inner and outer spare areas) are taken directly out of the Data Zone, which is otherwise used for user information storage. In practice, a BD-RE can assign 5% and a BD-R as much as 50% of its usable capacity for defect management (see chart).

BD Disc Defect Management

Layer 0

Layer 0

Layer 1

Layer 1

Disc Type

Inner Spare Area
(ISA0)

Outer Spare Area
(OSA0)

Inner Spare Area
(ISA1)

Outer Spare Area
(OSA1)

BD-R 8 cm (SL & DL)
BD-R 12 cm (SL & DL)

4,096 clusters
4,096 clusters

0 to 65,536 clusters
0 to 196,608 clusters

0 to 16,384 clusters
0 to 16,384 clusters

0 to 6,553 clusters
0 to 196,608 clusters

BD-RE 8cm & 12 cm (SL)
BD-RE 8cm & 12 cm (DL)

4,096 clusters
4,096 clusters

0 to 16,384 clusters
0 to 8,192 clusters

NA
0 to 16,384 clusters

NA
0 to 8,192 clusters

Also note that while a 2 KB sector is the smallest element of information that can be read from a disc, the smallest that can be written is a 64 KB cluster (32 sectors). Consequently, if less than a cluster is required to store a data file, the remaining sectors in the cluster are padded with null information. This inefficiency creates "slack" or wasted space.

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How far might BD disc capacity increase in the future?
Several manufacturers have discussed the prospect of expanding BD disc capacity by adding additional data layers as well as increasing the amount of information stored per layer. For example, Sony has proposed a possible eight-layer 200 GB BD-ROM while TDK has announced that is has created 100 GB four-layer and 200 GB six-layer BD-R prototypes and is petitioning to extend specifications accordingly.

Realistically, these discs would be incompatible with existing BD players, recorders and software. If or when such complex multi-layer types could be cost effectively mass-produced or if there is sufficient market need or interest to warrant their development and introduction are, of course, additional considerations.

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