Optical Storage - CDs and DVDs
Optical storage media such as CDs and DVDs are polymer disks which contain the "ones" and "zeros" as "pits" and "lands" that vary the strength of the drive's laser beam as it passes through the polymer and gets bounced back to the receiver via the "mirror", which is at the back of the printed top surface.
Writable CDs (CD-Rs) come in 650 and 700 MB versions. An average CD-R costs 25 cents and can hold about 250 six megapixel JPEG or 80 six megapixel uncompressed RAW images, so about 1,000 JPEG or 320 RAW images per dollar.
Single layer DVDs currently have a maximum capacity of 4.38 GB, about 6 times more than a CD. DVD drives can read CDs but not the other way around. A key benefit of CDs is that they are very universal. DVDs come in DVD-R, DVD+R, DVD-RW, and DVD+RW formats. Not all DVD drives recognize all formats but nowadays most DVD burners support at least -R and +R. It is not recommended to use -RW or +RW for long term archiving purposes as they are rewritable (and pricier). The -R and +R read-only formats prevent accidental overwriting.
If you burn for example 300 MB of images onto a blank 700 MB CD-R and "close" the disk, you will not be able to add information in the future. However, if you select the "multi-session" option in your CD-writer software, you can add additional information in subsequent sessions in the future until the CD is full. Note that this is not without risks. Sometimes adding a new session can render previous sessions inaccessible. This is especially true if previous sessions were created using another CD-writer and/or burning software. One of many reasons to have more than one backup copy.
Most burning software packages have a "verification" option. This will considerably lengthen the burning session but is safer because the software will verify that the data on your hard disk corresponds to that on the CD/DVD. Burning errors are not uncommon, especially if you have been using other applications during the burning process.
Note that the "X" read and write speed specifications of DVD-writers are lower than those on CD-writers. However, for a CD-writer, 1X stands for 150 KB of data per second, while for a DVD-writer it is 1,385 KB/s or 9.2 times more. So an 8X DVD-writer will write as much data per second as a 74X CD-writer (8 x 1,385 KB/s = 74 x 150 KB/s).
- (1) Double layer DVDs allow for 8.5 Billion Bytes but are less compatible.
- (2) 4.38 GigaByte or 4.7 Billion Bytes (which is what is usually printed on the packaging).
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com