23.4 DVD-ROM
The next generation of CD-ROMs are called "DVD-ROM". DVD originally stood for Digital Video Disc but is now more often called Digital Versatile Disc because it can be used to store computer data as well as video. DVD is a format based on the compact disc. It is 5 inches in diameter and can have two layers on each side. What this means is by changing the focus of the laser that is reading the disc slightly, a layer below the surface layer can be accessed. This increased the storage capacity, especially when you consider that DVD can be double sided as well. You can store approximately 133 minutes of video on one layer, as well as 8 soundtracks, including a 5.1 soundtrack such as Dolby Digital, and up to 32 sets of subtitles. Also, DVD gives the option of different camera angles to be stored on the disc so that the user can switch between them and also different ratings (such as PG etc.) and aspect ratios.
They can contain up to 4.7GB of storage on a single layer and on a single side. The capacity can be increased to more than 17GB by making the discs double sided and double-layered, which means the laser now reads through two substrate levels, and thus the CD contains double the information in the same vertical space. This is the new standard called DVD, agreed upon by CD manufacturers. DVDs will be able to store entire full-length movies on the space of a regularly-sized disc. Beyond that, double-layered, double-sided discs are waiting on the horizon, which will be able to store 26 times the amount of data of a conventional CD-ROM disc -- up to 17GB.
With DVD, the multimedia era comes of age. The same size as an audio CD, DVD provides high capacity digital storage of movies, music, and much more. A single-sided, single-layer DVD offers 4.7 gigabytes of storage capacity--enough to hold over 2 hours of high quality moving images and sound. And this means most films can fit on one side of a disc.
Fig. 23-19.
Thanks to MPEG2 video compression technology, the picture quality is nothing short of astonishing at 500 horizontal lines. And that's really just the start of the story. DVD is compatible with Dolby Digital, the 5.1 channel surround system. With Dolby Digital you get playback from 5 independent audio channels (front L, front R, center, rear L, rear R) plus a separate channel for a subwoofer for bass notes below 120Hz. This results in sound so realistic you'll think the action is actually taking place around you. The multi-audio* and multi-subtitle* functions allow audio information for soundtracks in up to 8 different languages and subtitles in up to 32 languages.
Fig. 23-20
23.4.6 Multiple Layers For Various Configurations
The DVD format provides multiple configurations of data layers. Each configuration providing additional data capacity. The configurations are:
• Single Side, Single Layer
• Single Side, Dual Layer
• Double Side, Single Layer
• Double Side, Dual Layer
The basic configuration of Single Side, Single Layer is 4.7 GB of data capacity. This 4.7 GB is "7 times" the data capacity of today’s music CDs and CD-ROMs.
Fig. 23-24. Single-Sided, Single Layer Disc.
The Single Side, Dual Layer configuration provides a total of 8.5 GB on one side using an additional 3.8 GB on the second layer. This is more than "13 times" the capacity of today’s music CDs and CD-ROMs.
Fig.23-25. Single-Sided, Dual Layer Disc.
The Double Side, Single Layer configuration provides a total of 9.4 GB (4.7 on each side). This configuration provides a little more capacity. However, because half of the data is on the second side, it requires either flipping the disc or having a DVD Player capable of two-sided playback
Fig.23-26. Double Sided, Single Layer Disc.
The Double Side, Dual Layer configuration provides the maximum capacity of 17GB (8.5 on each side). Again, because half the data is on the second side, it requires either flipping the disc or having a DVD Player capable of two-sided playback.<Picture>
Fig. 23-27. Double-Sided, Dual Layer Disc.
23.4.7 Studio Production Quality
Just as the original CD created a revolution in audio, DVD will raise the standard for home video picture quality. In fact, picture quality approaches "D-1," the CCIR-601 TV studio production standard. DVD delivers far and away the best color, sharpness and clarity in home video, far surpassing the LaserDisc standard. DVD also offers high resolution, with exceptional rendering of fine picture detail. Video distortion is extremely low, which reduces unwanted color "noise".
Because the recording format is component video, as opposed to NTSC composite video, the pictures are free of the well-known drawbacks of NTSC - artifacts including dot crawl and cross color distortion. And because DVD is an optical format, the picture quality doesn't degrade over time and repeated use.
23.4.8 MPEG2 Video Compression
The digital video standard specifies a video rate of 167 megabits per second. At this bit rate, the 4.7 gigabyte capacity of a standard DVD could only store roughly 4 minutes of digital video! Thus, some form of data compression is required.
DVD takes advantage of a sophisticated compression technology called MPEG2. It's a set of flexible compression standards, the second to emerge from the Moving Picture Experts Group (MPEG). Sony is an active participant in the MPEG proceedings, a primary developer of the MPEG system and a manufacturer of encoders. This expertise even extends to developing our own MPEG decoding devices. MPEG2 works by analyzing the video picture for repetition, called redundancy. In fact, over 97% of the digital data that represent a video signal is redundant, and can be compressed without visibly harming the picture quality. By eliminating redundancy, MPEG2 achieves superb pictures at far lower bit rates.
23.4.9 The BIT Rate Fluctuates
As implemented for DVD, MPEG2 encoding is a two-stage process, where the signal is first evaluated for complexity. Then, higher bit rates are assigned to complex pictures and lower bit rates to simple pictures, using an "adaptive," variable bit-rate process. The DVD format uses 4:2:0 component digital video compressed to bit rates with a range of up to 10 megabits per second. Although the "average" bit rate for digital video is often quoted as 3.5 megabits per second, the actual figure will vary according to movie length, picture complexity and the number of audio channels required.
23.4.10 Two Hours Of Movie On One Side
Thanks to MPEG2 compression, a single-layer, single-sided DVD has enough capacity to hold two hours and 13 minutes of spectacular video on a 4-3/4-inch disc! At the nominal average data rate of 3.5 megabits per second, this still leaves enough capacity for discrete 5.1-channel digital sound in three languages, plus subtitles in four additional languages! Including video, audio and subtitles, the total average data rate is 4.962 megabits per second. And because it's single-sided, DVD can store all this with no need to flip the disc over.
The following is the formula for calculating the total capacity needed on a disc if you wish to encode a specific film.
Picture Complexity ( Ave. 3.5 MB/SEC ).
+ Number of Languages X 0.384 MB/SEC.
+ Number of Subtitled Languages X 0.01MB/SEC
= Required Number of Bits For one Second.
X 60 Seconds.
= Required Number of Bits Per Minute.
X Movie Run Time in Minutes.
= Total Number of Bits required for one Movie.
¸ 8 To convert Bits to Bytes.
= Total Number of Bytes required For one Movie.
Average Bit Rates: Though bit rates may vary due to complexity of image and other variables, the following are averages you may consider in generic calculations.
Fig.23-28. Disc space Calculation for: Video + 3 Audio Tracks + 4 Subtitles.
Fig.23-29. Disc Space calculations for:
Video + 1 Audio Track + 1 Subtitle.
