SDHC Card Question - Speed ratings? Confused

    OK so the new SDHC cards are either class 2, 4 or 6 based on the minimum read/write speeds of 2, 4 or 6mb per second.

    So I was looking for a Sandisk 8gb Ultra II SDHC card and have read that it performs at 15mb why is it a class 2 (or 4 on some sites). Surely it should be a class 6 at least? or in an ideal world a class 15?

    I think the new extreme version performs at 30mb second but is only a class 6?


    The SD speed rating is the minimum guaranteed speed the card can work at, there's no independent evaluation on the what the manufacturer claims so it may be the top speed they are quoting is only achievable in a very specific set of circumstances which don't apply to general use. Class 6 is the hightest, there are no further defined classes regardless of the speed.

    I agree with the post above to check the device you're buying the card for can actually benefit from the higher speeds as many cannot. My compact camera which can buffer raw files and record 720p video at 30fps works fine with either a class 2 or a class 6 card but no faster whereas the SLRs are noticeably slower emptying their buffers on lower performance memory cards.


    Original Poster

    Cheers for the replies. I will be using it in my Panasonic SD9D high def camcorder.

    I went for a class 4 Sandisk HD Movie one in the end, the camera came with a Panasonic Class4 card & it seems to work fine.

    The ratings are a bit confusing but they are minimum ratings.

    The card may well be capable of a peak transfer rate of 20MB much like a runner might be able to reach a top speed or 20mph but they can not sustain that speed. To qualify for class 6 the card must be able to maintain 6MB continually, that is if the card were to be used non stop with data being added to it until it is full, with no pause in the data flow at no point must the speed drop below 6MB.

    It would be like having a marathon runner, that must run the whole course at no less than 6mph, if at any time they droped below 6mph for even a second they would be disqualified. But if they wanted to they could run a burst of it, perhaps the downhill part at 20mph if they were able to but would still only win a trophy saying that they qualified in the 6mph marathon.
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