CD and DVD writing speed

Original CD-ROM drives could read data at 150 kibibytes (150 × 2¹⁰ bytes) per second. As faster drives were released, the write speeds and read speeds for optical discs came to be indicated by manufacturers in terms of a multiple of the speed of the drives originally released to market. Using the notation nX, n is the multiple of the original speed that can be achieved. For example, writing to a CD at 8X will be twice as fast as writing to it at 4X.

CD, DVD and Blu-ray writing speeds

Medium	1X speed			Capacity (GB)	Full Read Time (min)
	Mbit/s	kB/s	KiB/s
CD	1.229	153.6	150.0	0.734	80
DVD	11.080	1,385.0	1,352.5	4.7	57
Blu-ray Disc	36.000	4,500.0	4,394.5	25.0	93

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 note 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 1,385 kB/s (1,385,000 bytes per second); in contrast, a CD at 1X is written at 150 KiB (153,600 bytes) of data (CD-ROM Mode 1) per second (KiB/s). Thus, 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.

For Blu-ray discs, 1x speed is defined as 36 megabits per second (Mbit/s), which is equal to 4.5 megabytes per second (MB/s). However, as the minimum required data transfer rate for Blu-ray movie discs are 54 Mbit/s, the minimum speed for a Blu-ray drive intended for commercial movie playback 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 now 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. However, the option a user chooses 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 practice.
• 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 optical quality may be lower (less reflective). If the reflectivity is too low to be accurately read when the CD is played back, it may skip or contain unwanted audio artifacts such as squeaking and clicking sounds. For optimal results burn at the media's rated speed.

Other media

Removable flash based storage is often rated in ratio to standard CD space. For example, a 100X flash card claims to be able to sustain 100 * 154 kB/s or 15.4 MB/s (100 * 150 KiB/s or 14.6 MiB/s). Read and write speeds will usually have different X ratings.

See also

• Reading (technical)
• Optical disc drive
• Data rate units
• Optical disc authoring program
• List of optical disc authoring software