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Digital versatile disc-read only memory (DVD-ROM) is a read-only digital versatile disc (DVD) commonly used for storing large software applications. It is similar to a compact disk-read only memory (CD-ROM) but has a larger capacity. A DVD-ROM stores around 4.38 GB of data. A CD-ROM usually stores 650 MB of data.
A DVD-ROM permanently stores data files which cannot be changed, written over or erased. A personal computer (PC) with a DVD-ROM or a DVD-RAM drive is designed to read a DVD-ROM disc. Generally a DVD-ROM disc is not equipped to be used with a DVD drive connected to a home theater system or television. But many DVD-ROM drives can generally read a DVD movie disc.
A DVD-ROM is one of the various types of DVDs. A blank DVD is generally a DVD-R or DVD+R, which has a read-write format. The +R or -R references the format standards and is a rewritable or recordable DVD.
Compared to a CD-ROM, a DVD-ROM has the same 5 inch diameter and 1.2 millimeter (mm) thickness. But because a DVD-ROM uses a shorter wavelength laser with tighter compacted pits, the disc capacity is increased. In fact, the smallest DVD-ROM can store approximately 7 times more data than a CD-ROM.
This term is also known as digital video disc ROM.
The DVD-ROM was first introduced in 1996 by the DVD Forum, a group of ten international companies using and developing DVD and HD DVD formats for media, software and hardware. The DVD Forum consists of the founding companies plus over 220 other members. Japan produced the first DVD-ROMs in November 1996. By March 1997 it was introduced in the United States. The DVD Forum also releases all DVD specifications published in the DVD books by titles such as DVD-ROM Book or DVD-R Book.
A typical DVD-ROM can hold up to 17 GB/s of data if both sides of the disc are writable.
The DVD-ROM is comprised of two 0.6 millimeters (mm) acrylic layers bonded together. The double-sided disc consists of two recordable grooved sides. With two layers, a DVD’s laser beam only has to go through 0.06 mm to reach the recording layer. Having a thin layer allows the lens to focus the beam to a smaller spot size, which in turn writes smaller pits for more data. The data is encoded in the form of spiral pits that are merely nanometers apart. The spiral path begins at the center of the disc and coils numerous times until it reaches the outer edge. With a double-layered disc the path continues to the second layer. A double-sided disc needs to be manually turned over and the path resumes in the center.