Whats EFM?
The pits and lands on a CD do not directly correspond to 1s and 0s. The start and end of a pit (i.e. the pit edges) each correspond to 1s, and all other areas — both in pits and on lands — correspond to 0s. The number of zeroes between pit edges is determined through careful timing. This is an efficient approach that produces an easy to handle electrical signal (it’s NRZI — NonReturn to Zero Inverted — which converts easily to NRZ where 1s are high voltage and 0s are low voltage). The careful timing is possible because CDs are essentially self-clocking. Suppose you have a clock that ticks once per second. Plug your ears and count seconds to yourself, trying to keep the same pace as the clock. After ten seconds, unplug your ears. If you’ve drifted slightly, you can readjust to the clock without worrying that you’ve too far off.
