DVD and CD formats explained

More than any other area of home computing, optical media remains under the tyranny of the abbreviation. It’s not easy having to wade through all the letters and symbols, but once you know which discs are which, and what they can be used for, it gets a lot easier.

There are two types of optical disc: compact discs (CDs) and digital versatile discs (DVDs). They are split into dozens of other formats, but the main difference between the two is size.

Although they look the same, the newer DVD format can store up to 18GB of data on a single disc, while CDs are limited to about 750MB. We will take a look at both major formats and their offspring, and explain which are suitable for specific types of disc drive and task.

Disc formats explained
The CD took off as an audio format in the mid-1980s, and as a computer format in the mid-1990s, with the introduction of cheap CD-R discs (the R stands for recordable).

Mass-produced computer CDs that cannot be written to are known as CD-Roms (Rom is short for read-only memory, an old computing term).

Every new computer comes with a writer that can write CD-R discs. Most discs on sale nowadays can hold up to 400 high-resolution pictures, or some 140 average-length songs.

As well as holding computer data, it is possible to make an audio CD of up to 74 or 80 minutes, which will play in most hi-fi CD players.

CD-Rs can only be written to once and cannot be erased, so the CD-RW (CD-rewritable) was the next step – a disc that could be written to several times. These discs store the same amount as a normal CD-R, but are slightly more expensive.

The vast majority of computer CD writers will create both CD-Rs and CD-RWs.

There are several secondary CD formats, such as video CD (VCD) and super video CD (SVCD), which contain audio and video, like a DVD film, but at lower quality (SVCD is slightly higher quality than VCD).

These discs can be produced on most PCs and will play on any computer and many DVD players.

Other types include CD-Text, which is an audio CD that works with some hi-fi CD players to display text about each track, and CD+G, which is an audio CD with graphics but which requires a special player. This format is often used to produce karaoke discs.

Moving to pictures
In the early 1990s, several manufacturers began work on a larger format, which became known as the digital video disc, or DVD.

As it became more suited to computer use, the ‘v’ changed to versatile, but no one noticed as the abbreviation had become commonplace.

A basic DVD holds 4.7GB of data (enough for about 2,000 high-quality digital photos), over six times more than the highest capacity CD.

Cleverly, this can be extended further. A disc can have more than one layer of data on its surface, nearly doubling its capacity to 8.5GB (enough for a very high-quality, two-hour film or about 18 hours of compressed video).

This format is called double layer, and some manufacturers call it dual layer. It can also be double-sided, so a double-sided, double-layer disc can hold 17GB of data.

However, double-sided discs are easily damaged so they are rarely used, although some shop-bought film or television series discs are double-sided and need to be physically turned over in the drawer to play the second side.

Prewritten computer discs such as shop-bought games are known as DVD-Roms because it’s not possible to overwrite the material on them.