Flash Memory Cards and X-Speed Ratings
With image files quickly
growing in size, the time it takes to store or write those files
to a Flash card is becoming an important buying criterion for prosumers
and professionals. Today, many Flash cards are sold with an "X"
speed rating that is similar to how CD-ROM drives are sold. This
rating system gives users a performance indicator of how fast the
card can transfer images.
When comparing the X-speed ratings of Flash cards from different
manufacturers, users need to beware that manufacturers define this
rating differently and in some cases do not define it at all. Some
definitions refer to the X-speed as "read" speed or "write" speed.
Others use it to mean both and call it "data transfer" speed (referring
to read and write speed).
Definition of X Speed Performance
Flash cards with a higher X-speed rating are designed to perform
faster. The X-speed rating can also be translated into another common
measurement of performance - MegaBytes (MB) per second of write
speed. To determine the actual MB per second, you must know the
value of "X" and do the math using the multiplier number and the
value of X as claimed by the product. For example:
| Value of X = 150
KiloBytes* per second, noted as 150 KB/sec.
| 50X = 50(multiplier of 150KB/sec)
=7,500 KB/sec = 7.5 MB/sec
1 KB = 1,000 Bytes;
1 MB = 1 million Bytes.
Flash Cards "X" Speed Ratings
Most flash manufacturers define X-speed as the write speed, because users generally
care more about how long it takes to write data to a Flash card
when using it in a digital camera or cell phone. Typically, read
speeds are higher than write speeds for Flash cards, so the write
speed is more meaningful when measuring performance.
The table shows X-speed rating versus
the comparable MB/sec as calculated from the write speeds.
| Kingston's Elite Pro/Ultimate Flash Cards
| Elite Pro/Ultimate X Speed
|| Comparable MB/sec.
Source: Kingston Engineering Labs
in benchmark testing;
As an example, if a 2-GB CompactFlash card is rated
at a 50X; this translates to a write speed of 50 * (150 KB/sec.)
meaning 7,500KB/sec or 7.5 MB/sec.
Is a Faster "X" Rating Always Better?
When a Flash card is rated at 50X, does this mean that data will
transfer at 7.5 MB/sec. in any digital camera or other host device?
A digital camera, cell phone, or other host device is itself limited
by the speed of its interface to the Flash card. Often, the host
device has a lower read and write speed than the Flash card, so
the end result is that the Flash card cannot operate at its best
performance level. A good example of this is that a digital camera
may only support 10X write speeds, so using a 45X or faster card will not improve performance. Most cameras or other host
devices do not have clearly documented Flash card interface read
or write speeds, so it may be difficult to know your host device's
As a general rule of thumb, digital cameras with up to 5 megapixels
will work just fine with standard-speed Flash cards (less than 45X).
Newer professional cameras designed for continuous shooting with
specifications of greater than 5 megapixels will benefit the most
from Elite Pro/Ultimate cards rated at 45X or above.
If using a high-quality Flash card reader to transfer data to a
computer, a Flash card with a higher X-speed rating will generally
transfer its data faster, thereby reducing the time to copy data
from the Flash card to the computer's hard drive.