PetaPixel

What the ‘x’ Means When it Comes to Memory Card Speed

Instead of labeling their memory cards in MB/s, some manufacturers choose to use “Times” ratings (e.g. 8x, 12x, 20x, etc…). While it’s pretty clear that a higher number indicates faster speed, what exactly is the number a multiple of?

The answer is that the memory cards are using the same transfer speed rating as the CD-ROM industry, with 1x being equivalent to the data transfer rate of an audio CD, which is 0.15 MB/sec or 1.2288 Mbps.

If a CD-ROM is read at the same rotational speed as an audio CD, the data transfer rate is 150 KiB/s, commonly referred to as “1×”. At this data rate, the track moves along under the laser spot at about 1.2 m/s. [...] By increasing the speed at which the disc is spun, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8× speed spins the disc at 1600 to 4000 rpm, giving a linear velocity of 9.6 m/s and a transfer rate of 1200 KiB/s. [#]

Even though memory cards don’t use moving parts and have nothing to do with audio CDs, some manufacturers continue to use this measurement standard for labeling their cards.

How Fast Should Your Camera’s Memory Card Be? [NYTimes]


Image credit: New SD cards for the new equipment coming by NoWin


 
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  • http://twitter.com/zak Zak Henry

    I always wonder why Manufacturers insist on using essentially meaningless units. If a customer cares about speed, they will understand the concept of KiB/s.

  • Anonymous

    It is very confusing to compare cards when different companies mark them in different units.  I wonder if this is intentional, especially with the “x” notation.

    When buying for a camera, I suggest Rob Galbraith’s site, he tests a lot of cards in a lot of cameras so you can see what their real speed is in a real world use.  Hopefully he has tested cards with your camera or one similar to it.  Then you can see about what speeds max out the camera such that you can decide where you want your price/performance point to be.

  • http://twitter.com/Soiden Sebastián Soto

    Actually, it’s easier and more effective. You can put the real speed, but many people, even looking for something fast, won’t understand how fast it is. By using something simple as a number and an ‘x’, you make them know it’s fast, even if they don’t what’s the original speed it’s based on. They just don’t care, they see it’s faster than something else and they buy it.

  • Chimpy

    Fascinating. I never knew that!:)

  • http://profiles.google.com/slimspidey Spider- Man

    I thought it ment XTREME!!!!!!

  • Joshua Szeto

    most of the numbers that lexar and sandisk put on their memory cards are just marketing fluff.  45mb/s or 400x speeds are mostly just burst speeds and can’t be maintained for more than a few seconds.  It soon bogs down to slower speeds again.   The most relevant info should be the class speed since those are sustained read/write speeds from empty to full.  Class 4 cards are sustained 4MB/s and class 10 are 10MB/s.  Since the class rating system is standardized, it should be the one to be relied on when choosing memory cards. This is especially relevant for video recording since 1080p is 4MB/s minimum and sometimes might even go as high as 5MB/s for complex scenes.