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Testing Methodology (and considerations)

Why are we only testing 2GB SD and 4GB CompactFlash cards when higher capacity models are now for sale? Well, despite the fact that SD cards are available in sizes greater than 2GB, they need a FAT32 system to operate correctly. This means that many devices are not yet compatible. Throw in the newer SD 2.0 standard and the limited compatibility for SDHC cards and you can see why we’ve stuck to testing 2GB SD cards.

As an example, the Canon EOS-1D Mark II (or for that matter any EOS-1D series camera) is not compatible with SD memory cards larger than 2GB (as of January 2006). Whether this will or can be addressed in a future firmware update is difficult to say but for the meantime, we would suggest you check with your camera manufacturer before you purchase a 4GB SD card or higher. CF cards up to 8GB work without problems in the Canon EOS-1D Mark II, but due to the scarcity of these cards at the time of this review, we’ve focused on 4GB CF cards.

Before we go on, we should also be clear on one thing. Testing flash memory cards is not as straightforward as one might think. There are many factors that can affect performance ranging from the type of data that is recorded or copied (and its directory structure), the flash memory chips used, the performance and compatibility of the card’s onboard controller and of course, how well these operate with the card reader and camera that you use. These varying factors and host devices make it very difficult to control and standardise the tests, and will invariably introduce bottlenecks into the equation.

In this respect, you’ll find that most manufacturers quote read and write speeds obtained from tests under ideal conditions where “real world” bottlenecks have been minimised. One of the most popular providers of tests and testing equipment is TestMetrix. The equipment they design and manufacture is geared toward getting the most out of memory cards and as such can reveal their true read and write potential.

For you, and me it’s probably best to regard this as compliance testing so that memory manufacturers can meet the standards set by the CompactFlash, Secure Digital and MultiMediaCard Associations, thereby allowing them to adorn their cards with figures like 20MB/sec and 133X.

In practice, many of us are unlikely to see these kinds of speeds with our “standard” card readers and “everyday” PCs, or for that matter even our digital cameras. It’s often an overlooked point, but many of our digital cameras have limited write speeds too, so the full potential of these so-called high-speed cards will be restricted. However, in time I’m sure we’ll see cameras, especially prosumer/professional models, with high bandwidth memory card interfaces.

For now though, it’s worth noting that the X speed rating is based on the data rate of a single speed CD-ROM, equivalent to 150KB/s. A 50X speed flash card is therefore rated at 7,500KB/s or as many manufacturers like to quote 7.5MB/s. Strictly speaking this should be approximately 7.3MB/s. This is because to convert kilobytes into megabytes requires a divisional factor of 1,024 (binary) and not 1,000 (decimal). After all, we are talking about computer data here and not metres or grams…

In our tests, we’ve accounted for this but for the sake of argument we’ve slapped a red line on each graph to highlight the 50X (7.5MB/s) barrier.

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