Monday, June 11, 2012

What is the EFM on CD?

EFM (Eight-to-Fourteen Modulation)
These "pit" and "land" is the signal after "8-14 coded modulation" (EFM, Eight to Fourteen Modulation), plus three interval code could access code to form inside the burning to disc. This treatment can guarantee the accuracy of reading the signal.

When the laser head of CD-ROM drive reads the data, the "pit" and "land" reflection coefficient is not the same, but they are read as "0" and only the "pit", "land" conversions is read as "1". The length of Pit" and "land" decides the number of"0". 8-bit data have two or more of the "1", CD-ROM accesses can not be achieved. To this end, two consecutive "1" between the use of at least two "0", up to 10 "0" separated them. EFM is to extract the 14-bit data and 256 does not appear for two consecutive "1" of data (access code) and the 8-bit data correspond. In order to avoid the adjacent two data channels being two yards, "one" and therefore the channel between the plus three yards combined yards (DVD using two combined yards), the formation of 17-channel code. This channel code can be used directly for burning CDs. Reading the disc, it will be 17 yards through the channel decoding system to be converted to 8-bit binary data. 

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