What is the difference between magnetic media and optical media?

Answer 1

Optical media is written and read on disks with lasers, such as a CD-Roms, BluRays, and DVDs. Magnetic media is written and read with magnets (like your hard drive and a floppy disks (which are almost non-existant anymore)).

Answer 2

Both are used to encode binary data that can be read back and decoded. In order to encode binary data, you need to be able to represent two opposing states. In the case of magnetic media, clusters of particles on the ferromagnetic material are polarised (positively and negatively). In the case of optical media, a sequence of long and short grooves are "burned" into the substrate with a laser. However, in both cases, the opposing states do not represent actual binary digits because it would be impossible to differentiate two consecutive bits with the same state. Moreover, timing information must be encoded into the data stream to prevent synchronisation errors. Both problems are addressed by using the transitions from one state to the other (flux transitions) rather than the states themselves. Timing is typically achieved using an encoding such as RLL (1,7) which maps two bits of data to a three bit value which is then encoded in groups of 2 or 4 bits.