03-23-2008 02:01 PM
I'm a bit confused by the various write speeds on SD cards. I have a new digital camera that can record video in HD so I want to get a really fast card. If I understand the speed rating correctly, the new Type 2 SD cards max out at Class 6 which is 6 MB/s. However, I can get an older Type 1 card with a speed of 133x which is 20 MB/s. Why would a newer standard max out at a lower speed? I understand the Type 2 standard allows for larger storage sizes but lower speeds??
Thanks for either confirming my understanding or clarifying for me.
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03-24-2008 10:31 AM
Extreme III SDHC 8GB - Min 20 MB/s read/write
It says class 6 on it still but that's because the SD association only has up to class 6 at the moment. A class 20 does not exist.
Here is also an Ultra II SDHC 32GB - Min 15 MB/s read/write
It's also not called type 2 sd cards it's called SDHC.
03-24-2008 12:41 PM
Thanks for the clarification. It's unfortunate that the "Class" ratings don't truly represent the speed of the cards. That will make it very difficult to compare different cards. Some sites do a better job than others about listing the read/write speeds. I guess I should assume that a Class 6 card is 6MB/s unless otherwise disclosed.
05-22-2008 12:02 PM
Another fact to keep in mind is that the SD class ratings are "guaranteed" minimum speeds and the old "x" ratings are much more wishy-washy. The class rated cards are "supposed" to allow the host device to find unoccupied and unfragmented areas of the card where 6MB/s (for class 6) write speed is assured. Now, the specification is confusing (probably intentionally) because it says the host device can check the fragmented state of the card and calculate the write speed in each part of the card but it doesn't say that such a task is impossible on older non-Class rated cards. The spec also says the speed is "is guaranteed at the best fragmented state where no memory unit is occupied". That's some pretty loose language. But, other rating systems are usually based on "internal testing" and could be the maximum speed capability, an average of the entire card or random sample of addresses. I doubt any "x" rating is the slowest measured speed because it's very time consuming to measure the write speed of every block on a 2GB card.