X (writing speed)

In computer jargon, the speed at which data is written to (or read from) an optical disc (like a CD or a DVD) is shown as a multiple of X. For example, writing to a DVD at 8X will take less time than writing to it at 4X. [cite web|url=http://www.misco.co.uk/content/module/removablestorage/what-is-read-speed.htm?bp=1| title=What is Read Speed? What is Write Speed?|publisher=MISCO|accessdate=2007-09-08]

CD, DVD and Blu-ray writing speeds

Modern Compact Discs support a writing speed of 52X and higher, and some modern DVDs support writing at 16X or higher. It is important to notice that the speed of 1X in CD writing is not the same as the speed of 1X when writing to a DVD. When writing to a DVD at 1X, the data is transferred at a speed of approximately 1352 kilobytes per second [cite web|url=http://www.osta.org/technology/dvdqa/dvdqa4.htm|title=Understanding DVD recording speed|publisher=Optical Storage Technology Association|accessdate=2007-09-08] ; in contrast, writing to a CD at 1X speed results in a data transfer of 150 kilobytes per second. Thus, in brief, one "X" in DVD writing speeds is about 9 times more than one "X" in CD writing speeds. However, these speeds are not constant, and depend on the type of data written to the disc. [cite web|url=http://www.osta.org/technology/cdqa5.htm|title=Understanding CD-R and CD-RW recording speed|publisher=Optical Storage Technology Association|accessdate=2007-09-08]

For Blu-ray disks, 1x speed is defined as 36 megabits per second (Mbit/s), which is equal to 4.5 megabytes per second (MB/s). [cite web|url = http://www.blu-ray.com/faq/#bluray_speed | title = Blu-ray writing speed | publisher = Blu-ray.com | accessdate = 2008-02-21] However, as the minimum required data transfer rate for Blu-ray movie disks is 54 Mbit/s, the minimum writing speed for Blu-ray writers should be 2X.

Historically, the 1X writing speed is equivalent to the 1X reading speed, which in turn represents the speed at which a media can be entirely read in 74 minutes. Those 74 minutes come from the maximum play time that the Red Book (audio CD standard) specified for a digital audio CD (CD-DA), although nowadays most recordable CDs can hold 80 minutes.The DVD and Blu-ray discs have higher capacity, so reading or writing those discs in the same 74 minutes incurs a higher data transfer rate.

Theoretical versus practical writing speed

Almost all of the modern CD/DVD burning software support selection of the speed at which the portable disc is written to. However, the option a user choses only defines the theoretical maximum of disc burning process. There are other factors that influence the time taken for a disc to be written to:

* Resources available to the program: Reading or writing data on a disc consumes moderate to high level of system resources (including memory and CPU resources), and running other programs at the same time may force the CD/DVD drive to choose a lower speed automatically, to accommodate with the available resources.
* Disc quality: Optical disc recorders detect the available speed options based on the data which is available on the disc itself; however, some low quality discs make a high speed option available to the software, while the burning process can never reach that speed in act.
* The reading and writing process does not happen at a steady speed. Both CD and DVD store data with constant linear velocity, so outer tracks contain more data per radian than inner tracks. During reading and writing, data flow speed varies based on the position of the laser under the disc.

Choosing the best writing speed

Choosing a higher writing speed will result in a faster disc burn, but the quality of the product will be lower. It is generally suggested to burn discs at a lower speed to make them last longer. [cite web|url=http://www.soundonsound.com/sos/nov04/articles/qa1104-3.htm|title=Why does the speed at which you burn a CD make a difference?|publisher=SoundOnSound|accessdate=2007-09-08] [cite web|url=http://www.feurio.com/English/faq/faq_writer_speed.shtml|title=Which write speed should I use?|publisher=feurio|accessdate=2007-09-08]

See also

* Reading (technical)
* Optical disc drive
* Optical disc authoring program
* List of optical disc authoring software

References