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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.
45X 6.75
50X 7.5
100X 15
120X 18
133X 20

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? Not necessarily.

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 exact capabilities.

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.