Ritesh’s Technical Blog

blu ray disc explained

Posted on: September 19, 2008

Blu-ray Disc

Blu-ray, also known as Blu-ray Disc (BD), is the name of an optical disc storage media format was introduced in year 2006. The format was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing large amounts of data.

The name Blu-ray Disc is derived from the blue laser (violet coloured) used to read and write this type of disc. Because of its shorter wavelength (405 nm), substantially more data can be stored on a Blu-ray Disc than on the DVD format, which uses a red (650 nm) laser. A dual layer Blu-ray Disc can store 50 GB, almost six times the capacity of a dual layer DVD.

The Blu-ray Disc format was developed by the Blu-ray Disc Association (BDA), a group of leading consumer electronics, personal computer and media manufacturers, with more than 180 member companies from all over the world. The Board of Directors currently consists of:

Apple Computer, Inc.
Dell Inc.
Hewlett Packard Company
Hitachi, Ltd.
LG Electronics Inc.
Matsushita Electric Industrial Co., Ltd.
Mitsubishi Electric Corporation
Pioneer Corporation
Royal Philips Electronics
Samsung Electronics Co., Ltd.
Sharp Corporation
Sony Corporation
Sun Microsystems, Inc.
TDK Corporation
Thomson Multimedia
Twentieth Century Fox
Walt Disney Pictures
Warner Bros. Entertainment

With Blu-ray, you can:

* record high-definition television (HDTV) without any quality loss
* instantly skip to any spot on the disc
* record one program while watching another on the disc
* create playlists
* edit or reorder programs recorded on the disc
* automatically search for an empty space on the disc to avoid recording over a program
* access the Web to download subtitles and other extra features

Discs store digitally encoded video and audio information in pits — spiral grooves that run from the center of the disc to its edges. A laser reads the other side of these pits — the bumps — to play the movie or program that is stored on the DVD. The more data that is contained on a disc, the smaller and more closely packed the pits must be. The smaller the pits (and therefore the bumps), the more precise the reading laser must be.

Unlike current DVDs, which use a red laser to read and write data, Blu-ray uses a blue laser (which is where the format gets its name). A blue laser has a shorter wavelength (405 nanometers) than a red laser (650 nanometers). The smaller beam focuses more precisely, enabling it to read information recorded in pits that are only 0.15 microns (µm) (1 micron = 10-6 meters) long — this is more than twice as small as the pits on a DVD. Plus, Blu-ray has reduced the track pitch from 0.74 microns to 0.32 microns. The smaller pits, smaller beam and shorter track pitch together enable a single-layer Blu-ray disc to hold more than 25 GB of information — about five times the amount of information that can be stored on a DVD.
Blu-ray discs are constructed very differently from DVDs.

Each Blu-ray disc is about the same thickness (1.2 millimeters) as a DVD. But the two types of discs store data differently. In a DVD, the data is sandwiched between two polycarbonate layers, each 0.6-mm thick. Having a polycarbonate layer on top of the data can cause a problem called birefringence, in which the substrate layer refracts the laser light into two separate beams. If the beam is split too widely, the disc cannot be read. Also, if the DVD surface is not exactly flat, and is therefore not exactly perpendicular to the beam, it can lead to a problem known as disc tilt, in which the laser beam is distorted. All of these issues lead to a very involved manufacturing process.

According to the Blu-ray Disc specification, 1x speed is defined as 36Mbps. However, as BD-ROM movies will require a 54Mbps data transfer rate the minimum speed we’re expecting to see is 2x (72Mbps). Blu-ray also has the potential for much higher speeds, as a result of the larger numerical aperture (NA) adopted by Blu-ray Disc. The large NA value effectively means that Blu-ray will require less recording power and lower disc rotation speed than DVD and HD-DVD to achieve the same data transfer rate. While the media itself limited the recording speed in the past, the only limiting factor for Blu-ray is the capacity of the hardware. If we assume a maximum disc rotation speed of 10,000 RPM, then 12x at the outer diameter should be possible (about 400Mbps). This is why the Blu-ray Disc Association (BDA) already has plans to raise the speed to 8x (288Mbps) or more in the future.

video codecs

MPEG-2 – enhanced for HD, also used for playback of DVDs and HDTV recordings.
MPEG-4 AVC – part of the MPEG-4 standard also known as H.264 (High Profile and Main Profile).
SMPTE VC-1 – standard based on Microsoft’s Windows Media Video (WMV) technology.

Please note that this simply means that all Blu-ray players and recorders will have to support playback of these video codecs, it will still be up to the movie studios to decide which video codec(s) they use for their releases.

audio codecs

Linear PCM (LPCM) – up to 8 channels of uncompressed audio. (mandatory)
Dolby Digital (DD) – format used for DVDs, 5.1-channel surround sound. (mandatory)
Dolby Digital Plus (DD+) – extension of Dolby Digital, 7.1-channel surround sound. (optional)
Dolby TrueHD – lossless encoding of up to 8 channels of audio. (optional)
DTS Digital Surround – format used for DVDs, 5.1-channel surround sound. (mandatory)
DTS-HD High Resolution Audio – extension of DTS, 7.1-channel surround sound. (optional)
DTS-HD Master Audio – lossless encoding of up to 8 channels of audio. (optional)

How Blu-ray Reads Data

The Blu-ray disc overcomes DVD-reading issues by placing the data on top of a 1.1-mm-thick polycarbonate layer. Having the data on top prevents birefringence and therefore prevents readability problems. And, with the recording layer sitting closer to the objective lens of the reading mechanism, the problem of disc tilt is virtually eliminated. Because the data is closer to the surface, a hard coating is placed on the outside of the disc to protect it from scratches and fingerprints.

Blu-ray discs also differ from CDs and DVDs in the way that data is written to them.
Source: Blu-ray Disc Association

The design of the Blu-ray discs saves on manufacturing costs. Traditional DVDs are built by injection molding the two 0.6-mm discs between which the recording layer is sandwiched. The process must be done very carefully to prevent birefringence.

1. The two discs are molded.
2. The recording layer is added to one of the discs.
3. The two discs are glued together.

Blu-ray discs only do the injection-molding process on a single 1.1-mm disc, which reduces cost. That savings balances out the cost of adding the protective layer, so the end price is no more than the price of a regular DVD.

Will Blu-ray replace DVDs?

Yes, that’s the expectation. The Blu-ray format has received broad support from the major movie studios as a successor to today’s DVD format. In fact, seven of the eight major movie studios (Disney, Fox, Warner, Paramount, Sony, Lionsgate and MGM) have released titles in the Blu-ray format. Many studios have also announced that they will begin releasing new feature films on Blu-ray Disc day-and-date with DVD, as well as a continuous slate of catalog titles every month. For more information about Blu-ray movies, check out our Blu-ray movies section which offers information about new and upcoming Blu-ray releases, as well as what movies are currently available in the Blu-ray format.

However, the two formats (Blu-ray and DVD) will most likely co-exist for quite some time until HDTVs become more widespread.

Is Blu-ray the same thing as HD-DVD?

No, HD-DVD (previously known as AOD) is the name of a competing next-generation optical disc format developed by Toshiba and NEC. The format is quite different from Blu-ray, but also relies heavily on blue-laser technology to achieve a higher storage capacity. The format is being developed within the DVD Forum as a possible successor to the current DVD technology.

Variants

Mini Blu-ray Disc
BD9/BD5 Blu-ray Disc
AVCREC
Blu-ray Disc recordable

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1 Response to "blu ray disc explained"

Most humans doesn’t apperceive abundant about Blu Ray Disc’s. Mostly their ability is bankrupt in few facts: that one individual disc food 25 GB or 50 GB if it’s a bifold layer, which is about 5 or 10 times the accommodation of DVD and gives the accomplished superior video, looks like DVD disc. And what do you apperceive about Blu Ray?

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