Original CD-ROM drives could read data at 150 kibibytes per second, 1× Constant Linear Velocity, the same speed of compact disc players without buffering. As faster drives were released, the write speeds and read speeds for optical discs were multiplied by manufacturers, far exceeding the drive speeds originally released onto the market. In order to market increasing drive speeds, manufacturers used the symbol n×, whereby n is the multiple of the original speed. For example, writing to a CD at 8× will be twice as fast as writing onto a disc at 4×. There are two main types of disc speed, which are the angular and linear velocities. If the disc spins at a Constant Angular Velocity, the linear velocity is 2.4 times higher at the outer edge.
Modern compact discs support a writing speed of 52× and higher, with some modern DVDs supporting speeds of up to 24×. It is important to note that the speed of writing a DVD at 1× is approximately 9 times as fast as writing a CD at 1×. However, the actual speeds depend on the type of data being written to the disc. For Blu-ray discs, 1× speed is defined as 36 megabits per second, which is equal to 4.5 megabytes per second. However, as the minimum required data transfer rate for Blu-ray movie discs is 54 Mbit/s, the minimum speed for a Blu-ray drive intended for commercial movie playback should be 2×. The fastest Blu-ray speed is 16×. Historically, the 1× writing speed is equivalent to the 1×x reading speed, which in turn represents the speed at which a piece of media can be read in its entirety, 74 minutes. Those 74 minutes come from the maximum playtime that the Red Book specifies for a digital audio CD ; although now, most recordable CDs can hold 80 minutes worth of data. The DVD and Blu-ray discs hold a higher capacity of data, so reading or writing those discs in the same 74-minute time-frame requires a higher data transfer rate.
Theoretical versus practical writing speed
Almost all modernCD/DVD-burning software supports a selection of speeds at which the writeable disc can be 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, and running other programs at the same time may force the CD/DVD drive to choose a lower speed automatically, to accommodate 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.
A higher writing speed results in a faster disc burn, but the optical quality may be lower. If the reflectivity is too low for the disc to be read accurately, some parts may be skipped or it may result in unwanted audio artifacts such as squeaking and clicking sounds. For optimal results, it is suggested that a disc be burnt at its rated speed.
Other media
Removable flash-based storage is often rated in ratio-to-standard CD space. For example, a 100× flash card claims to be able to sustain 100 × 154 kB/s, or 15.4 MB/s. Read and write speeds will usually have different "×" ratings.
