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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 were multiplied by manufacturers, far exceeding the drive speeds originally released onto the market. In order to represent this exponential growth in drive speeds, manufacturers used the symbol ''nX'', whereby ''n'' is the multiple of the original speed. For example, writing to a CD at 8X will be twice as fast as writing onto a disc at 4X.〔(【引用サイトリンク】 title=What is Read Speed? What is Write Speed? )〕
== CD, DVD and Blu-ray writing speeds ==
Modern compact discs support a writing speed of 52X and higher, with some modern DVDs supporting speeds of 16X and higher. It is important to note that the speed of writing a DVD at 1X (1,385,000 bytes per second)〔http://www.osta.org/technology/dvdqa/dvdqa4.htm〕 is approximately 9 times faster compared to writing a CD at 1X (153,600 bytes per second).〔http://www.osta.org/technology/cdqa5.htm〕 However, the actual speeds depend on the type of data being written to the disc.〔(【引用サイトリンク】title=Understanding CD-R and CD-RW recording speed )〕
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).〔(【引用サイトリンク】 Blu-ray writing speed )〕 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 2X.
Historically, the 1X writing speed is equivalent to the 1X 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 (audio CD standard) specifies for a digital audio CD (CD-DA); 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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