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BD

In February 2002, the leading optical storage companies formed the Blu-ray Disc Founders (BDF) and announced the initial specifications for BD, a high-capacity optical disc format. By May 2002, BD specification 1.0 was released, and in April 2003, Sony released the BDZ-S77 for the Japanese market, the first commercially available BD recorder. In January 2006, the Blu-ray Disc Association also released a 2.0 specification for BD-RE discs. Blu-ray is a fully rewritable format that enables recording up to 25GB of data or up to 11.5 hours of standard-definition video on a single-sided, single-layer 12cm diameter disc (which is the same as existing CDs and DVDs) using a 405nm blue-violet laser. Dual-layer BD-R DL recorders are also available and can record up to 50GB or 23 hours of standard-definition video. The latest BD specifications, BDXL (recordable) and BD-RE XL (rewritable), can store up to 100GB or 128GB at 2x or 4x speeds.

Although backward compatibility with DVD and CD is not a requirement of the Blu-ray specification, it is a feature drive manufacturers have included. One of the main applications for higher-capacity optical storage is recording high-definition TV, which takes an incredible amount of storage. Current DVD recorders can't store enough data to handle high-definition video. Blu-ray, on the other hand, is designed to store up to 4.5 hours of high-definition video (or more than 13 hours of standard broadcast-quality TV) on a single-layer disc, and 9 hours on dual-layer versions. As with DVD, Blu-ray uses the industry-standard MPEG-2 compression technology.

Figure 11.15 compares the track size and laser types used for CD, DVD, and standard BD media.

Figure 11.15

Figure 11.15 CD, DVD, and BD media and laser comparison.

The BD specification includes the following formats:

  • BD-ROM—Read-only for prerecorded content
  • BD-R—Recordable
  • BD-RE—Rewritable
  • BD-RE XL—Rewritable

The data transfer speed of a BD depends on the speed rating of the drive and media. The practical maximum rotational speed of an optical disc is 10,000 rpm, which limits the maximum speed of BD drives to 12x, which equates to a 54MBps transfer rate. The time to read or write an entire single- or dual-layer disc at various BD drive/media speeds is shown in Table 11.16.

Table 11.16. BD Drive/Media Speeds and Disc Read/Write Times

Drive Speed

Data Rate

Single-Layer Disc Read/Write Time

Dual- Layer Disc Read/Write Time

1x

4.5 MBps

90 minutes

180 minutes

2x

9 MBps

45 minutes

90 minutes

4x

18 MBps

23 minutes

45 minutes

6x

27 MBps

15 minutes

30 minutes

8x

36 MBps

12 minutes

23 minutes

12x

54 MBps

8 minutes

15 minutes

Standard CDs use a 780nm (infrared) laser combined with a 0.45 numerical aperture lens, whereas DVDs use a 650nm (red) laser combined with a 0.60 numerical aperture lens. Blu-ray uses a much shorter 405nm (blue-violet) laser with a 0.85 numerical aperture lens. Numerical aperture is a measurement of the light-gathering capability of a lens, as well as the focal length and relative magnification. The numerical aperture of a lens is derived by taking the sine of the maximum angle of light entering the lens. For example, the lens in a CD drive gathers light at up to a 26.7° angle, which results in a numerical aperture of SIN(26.7) = 0.45. By comparison, the lens in a DVD drive gathers light at up to a 36.9° angle, resulting in a numerical aperture of SIN(36.9) = 0.60. Blu-ray drives gather light at up to a 58.2° angle, resulting in a numerical aperture of SIN(58.2) = 0.85. Higher numerical apertures allow increasingly oblique (angled) rays of light to enter the lens and therefore produce a more highly resolved image.

The higher the aperture, the shorter the focal length and the greater the magnification. The lens in a CD drive magnifies roughly 20 times, whereas the lens in a DVD drive magnifies about 40 times. The Blu-ray lens magnifies about 60 times. This greater magnification is necessary because the distance between tracks on a BD is reduced to 0.32um, which is almost half that of a regular DVD. A comparison of BD and standard DVD is shown in Table 11.17.

Table 11.17. Comparison of BD and DVD Specifications

Disc Type

BD

DVD

Laser

405nm

650nm

Numerical aperture

0.85

0.60

Storage capacity (single layer)

25GB

4.7GB

Storage capacity (dual layer)

50GB

8.5GB

SD Video (single layer)

11.5 hours

2 hours

SD Video (dual layer)

23 hours

4 hours

HD Video (single layer)

4.5 hours

HD Video (dual layer)

9 hours

Video codecs

MPEG-4 AVC (H.264), VC-1, MPEG-2

MPEG-2

Lossless audio codecs

Linear PCM

Optional: MLP(True HD)[2-ch, 8-ch]*

Linear PCM[2-ch]

Lossy audio codecs

Dolby Digital Plus/DTS/Dolby Digital/MPEG Audio

Dolby Digital/MPEG Audio

Max. transfer rate

54.0Mbps

11.08Mbps

Content protection

AACS 128-bit

CSS 40-bit

Max. video resolution

1920x1080p (HDTV)

720x480p (SDTV)

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