Home How-To Helpline June 2007 HelpLine
Thursday, May 3, 2007

June 2007 HelpLine

Next-Generation Memory


June 2007 HelpLine


• Insecure About SDHC
• A Reminder To Back Up

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Insecure About SDHC

Q) I'm going on the trip of a lifetime in a few months and I was looking to stock up on memory cards so I don't have to bring my laptop with me. With prices coming down and card sizes going up, I thought I'd have an easy decision. When I got to the store, I became confused. There used to be just one flavor of SD; now there's SDHC. Should I try out the new cards?

Jose A.
Via the Internet

A) With some memory cards these days, capacity really doesn't have an effect on which card to choose. I say "these days" because in the early days of digital cameras, some camera operating systems could only work with smaller memory cards. So not only were you concerned about the different types of memory cards, you also had to be a little wary of card capacities.

Digital photographers know a memory card can never have too much room. As manufacturers have increased capacity, they have had to modify some of the original card designs. Look at CompactFlash (CF), for example. Originally, there were Type I cards, but soon Type II cards came out. Electrically, CF Type I is equivalent to CF Type II, but the Type II card is a bit thicker. The thicker cards allowed for larger capacity at first, but these days, the extra thickness isn't needed. Of course, it was fairly easy to tell if the Type II card would work in your camera: If it didn't fit physically, it wouldn't work.

SD or Secure Digital cards now have a new type that you mentioned in your question: SDHC. The HC stands for high capacity. In size and shape, the card is identical to the original SD card, but electrically, they're not the same.

The consortium that tries to control all things SD is the SD Card Association. Formed in 2000 by SanDisk, Panasonic and Toshiba, it now has been joined by a long list of companies. The task of the association is to forge a rock-solid memory card standard to allow for interchange of media across many products.

The SD Card Association developed a new specification (SDA 2.00), which is the basis for the SDHC cards. This new design ups the potential capacity of cards to 32 GB (that capacity is still a ways off).

As for speed, instead of trying to mimic CD-ROM drive speed ratings like 4x, 8x, 16x (which seemed a little strange to me, but I didn't have a better idea other than the raw data rate), SDHC cards now have a Class Speed Rating. This rating specifies a minimum sustained write speed. The three classes are simply known as Class 2, Class 4 and Class 6 and represent a minimum sustained write speed of 2 MBps, 4 MBps and 6 MBps, respectively. Of course, these are minimum speeds and manufacturers will continue to improve performance. While you'll see the new class rating, you'll also see a read/write speed specification that's manufacturer-specific.

Okay, so now you know a little bit more about SDHC, and it has been all good news. Now for the bad news: SDHC cards aren't backward-compatible with SD cards. It's worth repeating: SDHC isn't SD. Yes, physically they're the same size and will fit into each other's slots, but electrically, they're not the same.

Here are the compatibility rules: 1) You can use an SD card in any SDHC slot. In other words, if you buy a camera that supports SDHC, you don't need to throw away all of your old SD cards, as they will work just fine; 2) You can't use an SDHC card in an SD slot.

There are some rare situations (almost not worth talking about, but I want to bring it up for you experimenters out there) where an SD device might be able to access an SDHC card, but it might only have access to part of the storage. This type of access isn't even worth considering-with digital images, you need a consistent, reliable data memory system. So stick with SD cards for SD devices and SDHC or SD cards for SDHC devices.


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