Eight-to-fourteen Modulation - Technological Classification

Technological Classification

EFM belongs to the class of DC-free run length limited (RLL) codes; these have the following two properties:

• the spectrum (power density function) of the encoded sequence vanishes at the low-frequency end and
• both the minimum and maximum number of consecutive bits of the same kind are within specified bounds.

In optical recording systems, servo mechanisms accurately follow the track in three dimensions: radial, focus, and rotational speed. Everyday handling damage, such as dust, fingerprints, and tiny scratches, not only affects retrieved data, but also disrupts the servo functions. In some cases, the servos may skip tracks or get stuck. Specific sequences of pits and lands are particularly susceptible to disc defects, and disc playability can be improved if such sequences are barred from recording. The use of EFM produces a disc that is highly resilient to handling and solves the engineering challenge in a very efficient manner.