CD-ROM disc
        It is standard CD-ROMses there is in diameters discs the 120th mm.s ( 4. The 72nd inch),it it lasts holes centres to be the 15th mm., thicknesses for 1. 2 mm. , weight is about 14~18 grams.
       The radial cross-section of CD-ROM disc is three layers: (1)Gathered the carbonic acid ester( Polycarbonate) and made in the transparent lining bottom; (2)The aluminium is reflected layer; (3)Paint the antiabrasion layer . One one the form CD-ROM, it makes two-sided the reasons of ones into it is technology that make, But make the two-sided cost of one of all in that place than the ones that do cost of one two slices of forms and higher. So, CD-ROM one is used for printing the trade marks specially, and is used for storing the data another. Laser beam wear transparent to line bottom reach concave holes, read datum of producing, However, disc leave the that one sides of data, it is any damaged by dirt to influence to read performance of publishing data to be on the surface.

The code
       In order to store the data on the physics medium, must change the data into physics suitable for storing on the medium and express the form . The habit is had, various kinds of codes gotten after changing the data are called passway yard. Call passway yard , because these codes should undergo the communication passway. Passway yard is not any new concept , the tape, magnetic disc, network all use passway yard. We can say , all high density figure memories use 0 passways yard expressed with 1. Such as the disk, it uses the frequency modulation improved to make( MFM, Frequency Modulation of Modified) the code, Change the data into passway yard through MFM code.
       CD-ROM changes a piece of 8 data into 14's passway yard, is called 8-14 and modulate the code as CD-DA, Write it for EFM( Eight-to-Fourteen Modulation). Can define the length of concave hole and non- concave hole of the CD according to passway yard.

The structure of the data
       Because technological backgrounds who CD-ROM produce CD-DA, in addition, its helical line say structure , not at invariable linear velocity( CLV) rotate, heavy capacity,etc. a great deal of factors s only by type, It is much more complicated than the data structure of the hard disc and disk to cause the data structure of CD-ROM.
       CD-ROMs divide dishes of district into three pieces district of,namely channelling into area( Lead-in Areas)s, user data districts( and can't lead publishing area of at User Data Areas( Lead-out Areas). These three districts all contain physics and only say. 360 &-#176; one circles are helical in succession that the so-called physics is only said to mean only say. All physics in these three districts is only said the district formed to call the information district( Information Area). At information district, it says information of containing only at some only say and does not suck the information at some. A information of calling of light with information is only said( Information Track). Each a piece of informations it says to be can have been a part who physics is only said only, or an intact physics is only said, Can be only said and form by a lot of physics too.
       The information is only said and can leave digital data, sound information, picture information,etc. . The information with user's digital data is only says the figure of calling and only said, write it for DDT( Date Track of Digital); With sound information lights say and call sound say and write only( Audio Track). One a slice of CD-ROM ones, can only digital datas say and have digital data only said already only, There is sound that is only said .
When and leads these three districts in the appearing area in the channelling into area, user's data district, all there is information that is only said. Only a piece of information is only said in the butting channel into area, called and channel into and only say( Lead-in Track); A piece of information is only said, called and lead and appear and only say to only lead the appearing area too( Lead-out Track)
       On writing down in user's data and only says in the information in user's data district. All information with digital data is only said and be constructed with the sector , Some physics is only said and can used and only say and join the information in the information district .

The mistake is measured and correct
       Laser disk writes down the media the samly with magnetic disc, one kind of datum of tape, Receive the performance of the making material of one, produce the engineering level, driver and restriction of using levels of personnel,etc., The data reading out from plate are very difficult to be totally correct. Testing and count according to relevant research institutions, a slice of read-only optical disc not used , Its is original by mistake a yard of rates is about 3*10 - 4; One with scar is about 5*10 - 3. To this kind of situation, the laser took inventory and stores and adopt mistake yard with strong function and measure and correct the measure, It has three kinds that the concrete countermeasure adopted is summed up:
       (1)The mistake is measured and piled up EDC( Detection Code of Error). Finish adopting CRC one and measure and read and publish mistaken datum. CRC it has to be very strong examining function by mistake by one, it has and develop to correct function by mistake it so only examine it by mistake.
       (2)Mistake correct one or call and correct yards of ECC by mistake( Correction Code of Error). Adopt Reed Solow to cheats yard, abbreviates as RS yard, corrects it by mistake. RS yard is considered to be performance very good correcting yard by mistake.
       (3)Report to the leadship after accomplish it Solow cover with yards of CIRC - Reed( Interleaved Reed-Solomon Code of Cross). This each understand for weaving decipher with RS, go on the inserting value and punish alternately to data.