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What Does the X-Speed Mean?

SD cards and CF cards.  They all have performance limits.  Thankfully there seems to be a trend toward actual MBps (megabyte per second) ratings, but a lot of older and some newer memory cards use the X (ex or by) rating to indicate speed.

But 266x What?

266 * 150KBps = 39900KBps / 10 = 39.9MBps (rounded to 40 by most manufacturers) means you have a 40 megabyte per second memory card.

WTF? Why 150KBps??

CDs were born for music.  Your music CD player reads data off the disc at the rate of 150KBps (at least, it did before you put it out in the garage to gather dust.)  The very first CD drives in computers also read discs at this same rate.  When advancements started producing higher performance, that performance was indicated by manufacturers as some multiple of the base speed of 150KBps.  A 2x CD drive (yeah, I actually had one) was a blazing-fast 300KBps device.  The ex or by moniker was born.  A 40x CD drive therefore had a peak speed of 6000KBps, or 6MBps.  (Don’t laugh – we thought they were awesome.)

Most memory cards indicate the speed at which they can read data.  With few exceptions, cards can be read-from faster than they can be written-to.  The read speed and write speed are asynchronous (not the same.)  Sometimes this difference is significant, so do your homework if the write performance is important.

133x = 20MB/s

400x = 60MB/s

600x = 90MB/s

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