Optical discs

An optical disc is a physical medium that carries a media product such as music or a film, and it plays in a matching player. The CD, DVD and Blu-ray are the main types. All three are 12 cm across, so their capacity, not their physical size, is the best clue to what each one suits.

A CD holds about 650 to 700 MB. That is enough for roughly 74 to 80 minutes of uncompressed audio, about the length of a music album, so a band can fit its whole album on one disc. Uncompressed audio takes a lot of space because none of the sound data is thrown away to shrink the file.

A DVD stores standard-definition video. A single-layer DVD holds about 4.7 GB and a dual-layer disc about 8.5 GB, so a longer film or extra features have more room. A double-sided, dual-layer DVD holds about 17 GB by using both faces and two layers, but the disc may need turning over part-way through and has no room for printed graphics, because both sides carry data.

A Blu-ray disc holds around 25 GB on one layer and up to 50 GB on two. High-definition video needs much more data for every second of picture than standard definition does, so the larger capacity lets one disc carry a full HD film at high quality.

Suppose a community garden group films a standard-definition guide to its allotments and the footage comes to 6 GB. A single-layer DVD holds 4.7 GB, so it is too small; a dual-layer DVD holds 8.5 GB and fits. A Blu-ray would fit too, but the group's laptop and the village hall both play DVDs, so the DVD is the sensible choice.