If you are into CloneCD and have tried to backup recently released games, you might have found out that the CloneCD website is reporting about "EFM encoding" and the supported drive list, lists drives with sheeps for correct or almost correct EFM encoding.
But most of you will probably not know what EFM encoding actually is, and for those that would like to know, one of our Moderators (Mr Belvedere) has posted a thread on our forum that links to an article that explains it.
EFM stands for Eight to Fourteen Modulation and is part of the error correction technology used on CDs. It's one of the reasons you can make small scratches on CDs without losing data. The technology might be a little complicated for the average user, but for those who would like to learn more, the article is a must read !
EFM means Eight to Fourteen Modulation and is an incredibly clever way of reducing errors. The idea is to minimize the number of 0 to 1 and 1-0 transitions -- thus avoiding small pits. In EFM only those combinations of bits are used in which more than two but less than 10 zeros appear continuously. |
For example, a digital 10 given as a binary 0000 1010 is an EFM 1001 0001 0000 00
The use of EFM coding means that pits come in discrete lengths ranging from 3 bits long (often written 3T) to 11 bits long (11T). As the laser beam scans across these pits, a very distinct RF signal is formed. The shortest wavelength in this signal (highest frequency) is produced by the 3T pits. The longest wavelength in the signal (lowest frequency) is produced by the 11T pits. The zero crossings of the RF signal represent the edges of the pits -- and thus the binary "1s" in the data stream[2]. (Notice that the longer the wavelength, the larger the amplitude of the signal.)
It is common to display the photodetector output on a scope with a conventional trigger. This results in a display where the nine possible frequencies (3T to 11T) all add up on top of each other. This type of display is termed an "eye" pattern and provides valuable information about the various alignment parameters of the CD player. Notice that the relationship between size and wavelength is very distinct in the eye pattern[3].
The RF output is converted to a square wave, and then phase locks a clock with the period T. The CD player then begins to hunt for the characteristic start of frame symbol, which is three transitions separated by 11T. (100000000001000000000010 + 3 merge bits) Then, the player isolates the 33 17T symbols, and then kicks off the 3T merge bits -- leaving the 33 14T active symbols.
If this made you intrested I suggest you read on here, if it doesn't don't worry, all you need to know is how many sheeps your CD-RW drive can do, you can check it here.
If you have more questions related to this after reading the article then join the discussions in the CloneCD Experienced Users Forum, if you haven't read it and want more simple information, visit our CloneCD Forum !
Source: Washington.edu
