eli5 How do rewritable cds 💿 work?


I know how normal cds work, they are basically aluminum molds of data in binary. So how do rewritable ones get erased and reformatted?

In: 80

Pressed CDs are just like that, pressed. A press puts dents into the plastic disk.

Normal writable CDs contain a material that discolors when hit with a powerful laser, so from now on it will reflect differently which the laser can detect while reading.

A rewriteable CD has a (more expensive) material that changes color based on temperature. Hitting it with a certain temperature laser makes it darker, hitting it with another temperature laser makes it lighter / transparent. That way, once a CD has been written, you can “erase” it again by burning with a different temperature.

There is a metal film inside rewritable discs. It can be melted with a laser and then solidifies. If a low power laser is used, it turns into a crystalline structure that is shiny. But if a high power is used it becomes an amorphous solid that absorbs more light. Thus it is possible to create a pattern between light and dark that a reading laser can detect. The dark spots can still be melted to erase the disk.

The data layer is made of a metal alloy that has multiple solid states. When exposed to a very high temperature, it melts and resolidifies into a less reflective form. When exposed to a lower (but still high) temperature, it shifts into a crystalline form that’s more reflective. The two forms function similarly to pits and lands, though it’s different enough that a reader needs to be designed to support it.