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Because the laser is tracking the spiral of data using the bumps, there can be no gaps in the data track where there are no bumps. To solve this problem data is encoded using EFM (eight-fourteen modulation). 8-bit bytes are converted to 14 bits.
Because the laser wants to be able to move between songs, there needs to be data encoded within the music telling the drive "where it is" on the disc. This problem is solved using the "subcode data". Subcode data can encode the absolute and relative position of the laser in the track and can also encode things like song titles.
Because the laser may misread a bump, there need to be error correcting codes to handle single-bit errors. To solve this problem, extra data bits allow the drive to detect single-bit errors and correct them.
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