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DVD and Blu-ray Discs: A Closer Look
The DVD format was certainly a huge leap compared to VHS tapes, but it has major shortcomings as an entertainment medium for the high-definition era. DVD's compression scheme and disc structure were designed for standard-definition video. As TVs have grown bigger and better, the limitations of the DVD format have become more apparent. When watching DVDs on some of the better 1080p HDTVs with screens of 50" or larger, compression noise and artifacts are sometimes noticeable. Blu-ray, on the other hand, offers 1080p resolution for an incredibly smooth, detailed picture.
HD's much higher level of picture detail requires much more information. So, any high-definition format requires much higher data storage capacity (measured in gigabytes). Here's an example: a digital recorder with a 250GB hard drive can store about 200 hours of standard-definition video, but only about 30 hours of HD video. HD's superior picture quality also requires much faster data transfer rates (often called "bit rates") from the player to your TV (measured in megabits per second — Mbps). If the flow of information from a DVD player to a TV could be characterized as a babbling brook, the flow from a high-definition player would be a roaring river.
|DVD-Video||Blu-ray Disc (standard and 3D)|
|Disc capacity (gigabytes)||single-layer (4.7GB); dual-layer (8.5GB)||single-layer (25GB); dual-layer (50GB)|
|Maximum picture resolution (pixels)||720 x 480 (SDTV)||
1920 x 1080 (HDTV)
1920 x 2205 (3D HDTV)
|Maximum data transfer rate for movie playback (Megabits per second)||11Mbps||
|Video codecs||MPEG-2||AVC MPEG-4, VC-1, MPEG-2, MVC MPEG-4|
|Audio codecs||Dolby Digital, DTS||Dolby® Digital, Dolby Digital Plus, Dolby TrueHD (lossless), DTS®, DTS-HD™ High Resolution Audio, DTS-HD Master Audio (lossless)|
|Content protection||Content Scrambling System (CSS) 40-bit, region coding||Advanced Access Content System (AACS) 128-bit, BD+, ROM Mark, region coding|
As the chart makes clear, Blu-ray discs provide much greater data storage capacity and faster bit rates than standard DVD. Translation: much improved picture and sound quality.
Fitting more data on the disc
Even though high-definition video requires so much more data, high-def discs can easily hold even the longest movies on a single disc. Blu-ray discs can hold multiple hours of HD content, with plenty of room to spare for the bonus features you may have grown accustomed to with DVD. The developers of Blu-ray couldn't make the disc physically larger, so in order to significantly increase the information storage capacity, they increased the data density. The information pits got smaller, and the spacing of the pit rows got tighter (see illustration below). The discs also have a super-thin transparent protective coating, which places the data layer closer to the disc's surface and thus closer to the laser. In order to read these much smaller data pits, Blu-ray players use a blue-violet laser, which has a shorter wavelength and a smaller "beam spot" than the red laser used in DVD players. The players also spin the discs at higher speeds for even faster data transfer.
|Putting high-definition video on a disc requires much higher storage capacity than DVDs allow. Compared to DVD and HD DVD, Blu-ray discs have smaller data pits and more closely spaced pit rows.|
|Storing high-definition video requires much higher data density than standard DVDs allow. Blu-ray discs have smaller data "pits" and more closely spaced pit rows compared to DVDs and HD DVDs. Blu-ray players require a blue laser to read these smaller pits. In Blu-ray players, the laser's higher "numerical aperture" (NA) allows the beam to be focused to create a tighter spot for reading smaller pits.|
At the core of all recent digital entertainment forms is the concept of data "compression." Compression is needed to squeeze digital content so that it takes up a minimum of storage space. Compression is what made video formats like DVD and HDTV possible, as well as audio formats like Dolby Digital and MP3. The digital data is compressed for transmission or encoding on a disc, and then decompressed by your player. These compression/decompression technologies are often referred to as "codecs" for short.
MPEG-2 is the video codec used for DVDs and current HDTV content, including broadcast, cable and most satellite TV. Blu-ray also uses MPEG-2, as well as three newer, higher-efficiency codecs: AVC MPEG-4, VC-1 (based on Windows Media Video 9), and MVC MPEG-4 for 3D video. Because Blu-ray employs such high bit rates (6.75 Gbps and 54Mbps, compared to 19.2Mbps for over-the-air HDTV), the picture quality of Blu-ray discs is exceptionally clean, with fewer visible compression artifacts.
The expanded storage capacity of Blu-ray also makes it possible for these discs to offer dramatically improved sound quality. The fact that Dolby Digital sounds as good as it does is remarkable considering how aggressive the compression is for DVDs. High-definition discs have much more space available for soundtracks, and often feature new, higher-quality codecs from Dolby and DTS. One of Dolby's new formats, Dolby Digital Plus, offers up to 7.1-channel surround sound for even more enveloping audio than standard 5.1-channel Dolby Digital. There are even "lossless" options, Dolby TrueHD and DTS-HD Master Audio, which deliver the closest possible reproduction of the movie studio's original master. Many Blu-ray titles feature multichannel LPCM soundtracks — uncompressed audio that should also match the quality of the studio master.