When choosing an optical media type, it pays to know how much actual data storage capacity is available. Optical media is manufactured in several types: 512, 1024, 2048 and most recently, 4096 byte/block 14X media.
The overall capacity may be the same for the different optical media types, but the larger the sector size on the media, the more capacity per surface is obtained. Not all of the space on optical media is devoted to storing data. Each sector must also contain an additional amount of space for the "overhead" contained in each sector.
The breaks between sectors contain an embossed mark, the preformatted header which contains the sector mark, the variable frequency oscillator fields (VFO), the ID fields and error correction (ECC), cyclical redundancy checks (CRC) and sync and resync fields. This is a non-useable area so that the greater the number of sectors in the track, the more non-useable area is allocated; hence, there is less space for data.
For example, two 512 sectors take up more media space than a single 1024 sector because there is overhead associated with two sectors, instead of just one, so that 512-byte 2X media has 600M Bytes per surface while 1024-byte 2X media has 650M Bytes per surface. Therefore, a 1024 cartridge will give a user more storage capacity than the equivalent 512 byte media for optical media of the same total capacity.
About the author: Bob Chohan is the vice president of Services/Continuations Engineering
at Maxoptix, Inc.
This was first published in February 2001