CD DVDV ROM TECHNOLOGY

CD-ROM Technology

CD-ROM stands for Compact Disc Read-Only Memory, a mass storage medium utilizing an optical laser to read microscopic pits on the aluminized layer of a polycarbonate disc. The same format is used for audio Compact Discs. Because of its high storage capacity, reliability, and low cost, CD-ROM has become an increasingly popular storage media.
The storage capacity of a CD-ROM disc is approximately 650 megabytes, equivalent to over 500 high density 3.5" floppy disks or roughly 250,000 typed pages.
First generation drives (known as single speed), provided a transfer rate of approximately 150 kilobytes per second. Hardware manufacturers then introduced double speed (300 kB/sec), quad speed (600 kB/sec), and higher. Current drives operate at up to 40 times speed, although the maximum rate is only achievable over certain portions of the disc surface.
Most CD-ROM drives use either the Small Computer Systems Interface (SCSI), ATAPI enhanced IDE interface, or a vendor proprietary interface. They also typically support playing audio CDs via an external headphone jack or line level output. Most drives also allow reading the frames of data from audio CDs in digital form.
CD-ROMs are usually formatted with an ISO-9660 (formerly called High Sierra) file system. This format restricts filenames to the MS-DOS style (8+3 characters). The Rock Ridge Extensions use undefined fields in the ISO-9660 standard to support longer filenames and additional Unix style information (e.g. file ownership, symbolic links, etc.). Microsoft has defined a proprietary ISO file system extension called Joliet which supports long filenames using the 16-bit UNICODE character encoding.
PhotoCD is a standard developed by Kodak for storing photographic images as digital data on a CD-ROM. With appropriate software, you can view the images on a computer, manipulate them, or send them to a printer. Information can be added to a PhotoCD at a later date; this is known as multi-session capability.
CD recordable (CD-R) drives allow writing onto a special "gold" CD which can then be read by any CD-ROM drive. Data can only be written once, although using multi-session new data can be appended to a disc.
CD-RW (rewritable) drives can be erased and rewritten with new data. They use special discs which can be read by most recent CD-ROM drives (but not older ones or most audio CD players).
DVD-ROM expands the storage of a CD to as much as 17 gigabytes. They are commonly used as a medium for distributing full length motion pictures encoded using the MPEG-2 format. The MPEG video decoding is performed using specialized decoder software and/or hardware. DVD-RAM is a writable version of DVD.
DVD Formats
Format
Application
DVD-ROM
High-capacity computer ROM storage, capable of replacing multiple CDs. Multimedia, computer games, interactive systems, databases.
DVD-Audio
High-quality surround-sound music with increased playing time.
DVD-Video
High-quality multi-lingual movies on one disk with random-access to episodes and surround-sound audio.
DVD-RAM and DVD-R
Very large (compared to CD)direct access data storage.

DVD Configurations and Basic design
CD Players and CD-ROM drives use an infrared laser working at a wavelength of 780 nanometers. Since the wavelength is one of the parameters responsible for the beam diameter, which translates into smaller and denser bits (click here for details), the new DVD Players and DVD-ROM drives use the red laser working at 650 nm and 635 nm wavelengths.
Another parameter, important for smaller and denser bits, is the Numerical Aperture (NA) that was achieved in DVD by refining the laser assembly.
DVD's digital modulation and ECC (error correction code) were designed to support the increased storage capacity. The 8 to 16 (EFM PLUS) modulation scheme (8 to 17 is used in a CD) is very efficient and provides backward compatibility. Also, the RS-PC (Reed Solomon Product Code) error correction code is about 10 times more robust than that currently used in CD systems.
Unlike a CD, a DVD is a bonded disc, made of two 0.6 mm substrates joined together. Although they are similar in appearance, some substantial differences between the CD and DVD are revealed under the surface.
The DVD format provides several configurations of data layers, moving from 2D storage towards 3D storage. Each configuration is designed to provide additional storage capacity:
Name
Media structure
Capacity (GB)
DVD-5
Single Side / Single Layer
4.7
DVD-9
Single Side / Dual Layer
8.54
DVD-10
Double Side / Single Layer
9.4
DVD-18
Double Side / Dual Layer
17.08
DVD-R
Single or Double Side / Single Layer
3.95 / 7.9
DVD-RAM
Single or Double Side / Single Layer
2.6 / 5.2
DVD-ROM
Like a CD, a DVD-ROM is a pre-recorded disk. DVD-ROM is used to store general data, as well as video and audio information needed for multimedia applications and computer games. DVD-ROM satisfies the following requirements:
Backward compatibility with CD-ROMs
Forward compatibility with the future recordable (R) and rewritable (RW) disks
Single format for computer and TV applications
Single file system for all data types and media types
The backward compatibility of the DVD drives means that it will read both CD-ROM and CD-audio, which makes them a great replacement for CD-drives. Because of higher bit density and other advantageous features, even a 5x-speed DVD drive will read the CD at the rate equivalent to about 40x for the regular CD drive. For now, DVD drives are, in general, more expensive, and require special MPEG-2 hardware or software decoders to read the compressed data. To have the best video quality, the hardware approach is better unless the fastest processors are used.
This clearly makes DVD-ROM a computer storage of the near future, especially for databases, multimedia, games, interactive video, etc.

DVD-Video and DVD-Audio
One of the reasons for the success of DVD technology is the DVD-Video formats. DVD video application is strongly dependent on data compression, since at the bit rate of 167 Mbps (which corresponds to the video rate specified by the CCIR-601 digital video standard), the 4.7 gigabyte capacity of a standard DVD would be enough to store roughly 4 minutes of digital video. This provides for the nominal 133 minutes of playing time for DVD-5. Longer movies should use a dual-layer technology (DVD-9). The data on the first layer start at the inside of the disk and end at the outside, where the data on the second layer start thus providing uninterrupted playback.
Two types of video compression standards could be used for DVD: MPEG-1 and MPEG-2, but only MPEG-2 video data can be copy protected and region coded (MPEG stands for the Moving Picture Experts Group). Therefore, the same techniques of copy protection as are currently used for CDs are being adopted for DVD.
Like all compression algorithms (WinZip is one example), MPEG-2 analyzes repetition in the video signal, called redundancy, and tries to get rid of it. MPEG-2 is capable of 'filtering' about 97% of the data in the video signal without significantly degrading the quality of the picture. This allows recording of 133 minutes on a 4.7 GB disk at a much lower bit rate than required by the digital video standard.
DVD-Video may have up to 8 (typically, three) tracks of mono, stereo, or multi-channel surround sound, which makes it much better than the VCR. The audio encoding formats include Dolby Digital (5.1 channel surround sound), MPEG (up to 7.1 channels), LPCM (number of channels is hardware dependent), and DTS (only in addition to one of the other formats).
DVD's direct data access allows interactivity and direct access to the movie episodes or other information of the disk. DVD also allows subtitles (up to 32 sets in different languages), making any DVD really universal. On the other hand, to provide additional copy protection, most DVDs have so-called regional coding, making it impossible to play the same disk in different regions, since most DVD-Videos are made for a specific region or country and not for free world-wide use. There are 6 regions (see next table) used for DVD-Video coding: