Random Ramblings of Everyday Life

Quick Guide to SD Cards

Posted in Guides by ziptrickhead on July 9, 2010

SD (Secure Digital) cards are very popular and widely used today in digital cameras, digital camcorders, computers, PDAs, media players, and even gaming devices. Standard SD cards have a maximum of 4GB capacity although generally SD cards usually have the capacity of 2GB and less. SDHC (high capacity) cards overlap and start at 4GB, increasing all the way up to 32GB capacity. A few years ago, SDXC (extended capacity) were announced; allowing for up to 2TB capacity by new specifications, but they are not widely available currently and most current devices are not compatible.

It is important to note that although 2GB and 4GB cards are currently under the SD standard, there may be issues with older devices since the standards are constantly revised. SD cards will work in any SDHC capable device though.

Outside of just the capacity rating, speed ratings should also be considered. Usually SD cards have independent read and write speeds; generally the write speed is what you have to be weary of. As technology advances, and things such as digital cameras and camcorders improve, the data transfer rate has to increase. Say you were to use a class 2 SD card in a HD camcorder. You might find that although it’ll record, the video will most likely be choppy since the video data can not be written onto the memory card fast enough.

There are two official units of measure of SD speed. The most current is the “Speed Class Rating” while the older standard is the “x rating”.

The Speed Class Rating measures the minimum write speeds based on “the best fragmented state where no memory unit is occupied”. Basically, memory is divided into memory units (think of them as shelves). The device writes data onto memory units where no data is already stored. Eventually available memory is divided up into smaller and smaller units, fragmenting the memory space. This leads to decrease in write speeds. Think of it like a computer hard drive. The same occurs in hard drives, although most operating systems, such as Windows, offers tools to help with the fragmentation of memory like the Disk Defragmenter. A defragmenter sorts and combines all the unused memory units into larger chunks to allow for faster write speed. So going back to the shelf analogy, the more shelves you put up, the slower it takes to organize things onto those shelves, compared to just having one large shelf.

The Speed Class Rating makes it much easier to remember the minimum write speed. Each value is equivalent to 8Mb/s. Basically the rule of thumb is that whatever number class it is, that is the minimum write speed in megabytes per second (MB/s). So a class 2 SD card has a minimum write speed of 2MB/s and a class 6 card has a minimum write speed of 6MB/s. Remember, that MB is megabyte while Mb is megabit, a difference of 8 times. So a class 2 would be 16 Mb/s and a class 6 would be 48Mb/s. You can tell what class of SD card you have by the label. The label for the Speed Class Rating is the number of the class within a “C”. If there is no class label then it can be assumed that the card is class 2.

Take note that the class ratings quoted by manufacturers are not verified even though they are defined by a governing body (SDA or SD Card Association). There is no enforcement of these speed ratings so if you buy cheap or imitation SD cards, don’t be disappointed when they don’t offer the performance they promise.

The older unit of measure is the “x rating”. Think back to CD drives where they had the x rating as well (like x20). Each x unit is equivalent to 1.2Mb/s. So an SD card with a 40x rating would be the equivalent of a class 6 card (40 x 1.2 Mb/s = 48 Mb/s or 6MB/s). Unlike with the Speed Class rating, the x rating doesn’t specify if the value given is the read speed or the write speed. Generally read speed is higher than write speed on most cards, so manufacturers can just use the read speed on the label, without actually labeling if that speed is read or write.

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One Response

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  1. Hard Disk Camcorders said, on July 14, 2010 at 2:54 am

    What is SDHC SD High Capacity (SDHC) card is the new SD memory card based on the SDA 2.00 specification, introduced by the SD Card Association.


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