CD Versus DVD

The CD and DVD are nearly identical in physical appearance but differ considerably under the covers in terms of storage capacity as well as recording and playback speeds. Here are the details.

CD

The CD, or Compact Disc, was introduced by the Philips company in 1979 as a spin-off of laserdisc technology for the storage and play back of sound recordings. Its design drew upon recent advances in optical digital audio disc technology by Philips, Sony and other companies. Audio CDs and audio CD players were first commercially manufactured in 1982.

Over time, the CD's capabilities have been extended to encompass digital data storage, including computer files and videos. The first computer readable CD, known as CD-ROM (read-only memory), was released in 1985. These CD's could only be read, not written.

Two commonly used CD formats today are CD-R, a recordable format that can be written to once, and CD-RW, rewritable media that can be written to hundreds of times. Less commonly encountered formats include Video Compact Discs (VCD), Super Video Compact Discs (SVCD), PhotoCD, PictureCD, CD-i, and Enhanced CD.



DVD

The DVD, or Digital Video Disc, was invented by Philips, Sony, Toshiba and Panasonic in 1995 as an upgrade to CDs for computers, as well as Video CD's, VHS tapes and laserdiscs in the entertainment industry. The advent of the DVD has had a major impact in the market place, leading the movie industry to drop VHS tapes and the video game industry to drop CDs in favor of DVD media.

Like CD's, DVD's come in a number of flavors. A DVD-ROM can only be read and not written nor erased. DVD-R and DVD+R discs are recordable but only once. Rewritable DVDs (DVD-RW, DVD+RW, and DVD-RAM) can be recorded and erased multiple times.

A DVD is able to store far more data than a CD due to higher density data packing, less overhead (repeated information used for error correction), multilayering (1 or 2 layers per side versus 1 layer for CD), and use of both sides of the disc.

Storage Capacities

A standard size CD has a diameter of 4.7 inches and can hold up to 80 minutes of uncompressed audio or 700 MB of digital data. CD data is stored as indentations called "pits" that are encoded in a spiral track moulded into the disc. The areas between pits are "lands". A CD drive reads the disc via a 780 nm wavelength laser that detects variations in reflectivity between the pits and lands. Pits absorb light (‘off’ bits) and lands reflect light (‘on’ bits).

A single-sided, single-layer DVD can hold 4.7 GB of data, more than 6 CDs. Single-sided, dual-layer DVDs have a storage capacity of 8.5 GB and double-sided, dual-layer DVDs have a capacity of 17 GB. Approximately 3 hours of standard definition video can be stored on a DVD. A CD can hold up to 60 minutes of standard quality SVCD-format video, which has about two-thirds the resolution of DVD.

Like a CD, a DVD stores data as a series of pits arranged into tracks. However, the distance between the pits and adjacent tracks is shorter on a DVD. A shorter wavelength laser (650 nm ) is used to read and write the more densely packed data.

Cost

Blank CDs have historically been less expensive than DVDs, especially for archival quality media, but with DVDs now outselling CDs, they are typically the less expensive option.

Other Considerations

DVD players can play DVDs as well as CDs. A CD player can only play CDs and not DVDs.

Due to faster rotational speeds of DVD media, the rate at which data can be read is higher compared to a CD. This is advantageous for more demanding files such as video that require higher data rates.