DVD-ROM (Digital Versatile Disc)(read-only memory) is essentially a CD-ROM disc with enhanced capabilities. The main differences lie within its audio, video, and storage capabilities. Other differences include file system and compatibility. Though the physical DVD disc is the same size and thickness as a CD, DVD-ROM is far superior to CD-ROM in all the above areas.
All DVD-ROM and DVD Video/Audio Disc comply with the following format standard:
Single format for computer and TV-based applications
CD-ROM utilizes full motion video using a compression standard called MPEG-1. This allows for playback of video similar in quality to a VHS tape. In comparison, DVD-ROM offers MPEG-2 video capabilities. MPEG-2 is a more advanced compression codec allowing images 4 times the size of those in MPEG-1 to be displayed, offering enhanced resolutions. The result is video that is far clearer than MPEG-1, equivalent to laserdisk quality.
MPEG-2 also results in a faster frame-rate (rate of frames displayed per second) than previously possible using MPEG-1. This is also partially due to the fact that DVD-ROM allows up to 9.8 mbps (megabytes per second) transfer rate as compared to CD-ROM’s 1.44 mbps. This means more fluid video playback with less choppiness than MPEG-1.
DVD-ROM offers 5.1-channel Dolby Digital AC3 audio capabilities offering a new standard in sound over CD-ROM’s mere 2 channels. This allows for not only left and right channels, but also for front and back channels, hence 3d-surround sound for a more theatrical experience, given the proper sound system.
DVD-ROM is unique when compared to CD-ROM in that all DVD disks, whether video or DVD-ROM, have a standard and identical file system. All data on the disk, whether audio, video, text, or program data, is stored in files. This file system, for all DVD and DVD-ROM disks, is broken into the subdirectories of audio, video, and additional directories for other information such as program data.
In comparison, audio CD’s do not have a file system. Rather, audio CD’s are divided into tracks, with time codes stored in a subcode channel. CD-ROM has a file system, but there is no single standard. This means separate file systems for each operating system, causing incompatibility issues.
Single-sided DVD-ROM discs currently hold over seven times the data of a CD-ROM disc. Future double-sided DVD discs will hold up to 17 gigabytes. (As compared to CD-ROM’s 650 megabyte limit) Because these disks are double sided, they must be flipped in order to access data on both sides.
The increased capacity is possible by reduced pit size and track pitch along with a more efficient error correction system. DVD requires only two layers of error correction whereas CD-ROM requires 3.
This increase in storage capacity translates to more space for storing larger amounts of high fidelity video and audio. In combination with program data, this can mean the possibility of creating software that is greatly enhanced in its multimedia capabilities. A good example of this would be the next-generation Playstation 2 by Sony, which will utilizes DVD-ROM technology for its storage and audio/video advantages in its console games.
DVD-ROM titles are few in comparison to the amount DVD-Video titles at this time even though PCs with DVD-ROM drives exceed the number of DVD players in use. One of the reasons for this is partly because DVD-Video players are purchased primarily to watch DVD videos, where as a DVD-ROM drive can be used to play a DVD-Video and CD-ROM discs as well as DVD-ROMs. Another reason is that there have been many compatibility problems with certain software and hardware configurations.
Macintosh has one of the largest problems with DVD-ROMs. CD-ROMs intended for both PC and Mac platforms use both ISO 9660 and Mac HFS file systems. But DVD-ROM does not allow Mac HFS to be included. This means that DVD-ROM titles intended for the Macintosh will only run under the latest versions of MacOs, Which support UDF. This brings about a problem for people with Macs who have older systems because DVD-ROMs won’t work.
The use of both ISO9660 and UDF file systems on a DVD-ROM disc allow it to be played on platforms which do not support UDF. Windows 98 has been designed to support UDF, but Windows 95 only supports ISO 9660. This would not be a problem but the pre-mastering tools for DVD-ROM do not support the Joliet extensions necessary for long file names. Because of this filenames have to be restricted to the original DOS 8.3 (8 character plus extension) convention.
Windows 98 includes DirectShow, which provides support for DVD-Video files, but until recently many MPEG-2 hardware decoders did not include DirectShow compatible drivers. The result was that DVD-ROM titles either did not include MPEG-2 VOB files or had to include new drivers for all decoders available. This is rapidly becoming less of a problem, with sales of new hardware increasing dramatically. Therefore it is recommended that DVD-ROM titles with high quality video should make use of the DVD-Video VOB files. These files can then be made to play on a DVD-Video player, offering additional functionality to a DVD-ROM title.
All DVD-ROM drives will read all DVD pre-recorded discs (from 4.7 to 17.1 GB capacities), plus CD audio discs and CD-ROM discs. Some early DVD-ROM drives were not able to read CD-R discs, but most now will do so. MultiRead drives are capable of reading most discs, including CD-R and DVD-R. Only the latest DVD-ROM drives will read DVD-RAM and/or CD-RW discs.