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08-16-2010, 04:19 PM   #1
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On SDHC 'speed' and 'class'...

I've noticed a bit of confusion in the discussions here with people's experiences with SDHC cards - people often mention the brand, the model, and often the 'Class', however none of that is enough to know the actual performance characteristics of a card (or even exactly which card they're using). This can lead to some significant confusion when people start making assumptions, such as when a new Class 10 card of Brand A is expected to be faster than an older Class 6 card of Brand B, and when it's not for someone, there's an implication or assumption that there's no point in getting any Class 10 card over a Class 6 card because the 'real world' differences were found to be contrary to expectation.

Wikipedia has a decent description of what SDHC class means, saying it "...measures the minimum write speeds based on 'the best fragmented state where no memory unit is occupied.'" This indicates that the Speed Class is a kind of worst-case write performance measure determined when a card is empty (and implies that as a card is used and starts to fill up, it can slow down to below the Class speed).

There is an endnote on Wikipedia which references this SD Association official web page on SD Speed Class. That page no longer has the exact line quoted in Wikipedia, but instead a more vague statement which isn't as clear about what the speed rating exactly means as it relates to fragmentation: "The memory of a card is divided into minimum memory units. The host writes data onto memory units where no data is already stored. As available memory becomes divided into smaller units through normal use, this leads to an increase in non-linear, or fragmented storage. The amount of fragmentation can reduce write speeds, so higher SD card speeds help compensate for fragmentation."

The best I can conclude from my research is that Speed Class reflects some aspect of flash performance, but can't really be used on it's own to tell which particular flash card is faster than another either in it's pristine out-of-box state or after extensive use. (Where I think Speed Class is useful is when a device performs a query on an SDHC card that's plugged in and the card tells what Class it is and then the device can warn a user that the card could be too slow to perform well in that device, i.e. it seems most useful to device makers, and helpful to consumers only as a guide of what flash devices they just shouldn't buy/use in a given device, not necessarily what card will perform better/best).

The vague value of SDHC Speed Classs is really brought into question when you find out that it's not uncommon for manufacturers to change the performance of a particular line of cards without changing the product name - just changing the product model. This is illustrated by this knowledge base article on the SanDisk website, SanDisk memory cards read/write speed change .

Also, while you might be thinking that SanDisk's "speed rating" (mentioned in the KB article above) would be a handy way to tell card performance, since they often print the "speed rating" on the card packaging and sometimes on the card itself. I don't think it is, because all it tells you is the "maximum transfer speed for writing and reading images to and from the card, expressed as megabytes per second." In other words, a best-case scenario that doesn't differentiate between read and write speeds, just one maximum. Reading flash memory is most always faster than writing, yet often the important metric is the write performance - so SanDisk's "speed rating" probably isn't telling you write performance at all!

From what I've been able to determine, one of the factors which may be significantly affecting real-world SDHC write performance between different brands/models is wear leveling algorithms. For SDHC, there is no established or universal standard for wear leveling (unlike for SATA SSD where there is the TRIM command). This means that different SDHC flash controller makers implement different wear leveling algorithms to minimize the progressive performance degradation of write operations on flash storage, and differences/improvements in those algorithms could be affecting performance in a way completely separate from the raw write capability of the flash media.

So ultimately, I think the best information people can give when talking about card performance is to describe the brand, exact model number, method used to test read and write speeds (including specifics on the device used), how full the card was when the speeds were tested, and what the results were. Also, where the card was bought would be a very useful bit of information, since it could help anyone else reading estimate whether or not the card being testing is a counterfeit or not.

Flash card counterfeiting is a very big problem, the copycat cards are now nearly identical looking (but not performing) to the official cards, so where someone buys a flash card can make a big difference. You can read more about this issue here:

Fake Memory Cards - OCAU Wiki

For a very in-depth look at the problem, read this:

On MicroSD Problems

(My research into the counterfeit flash has resulted in me avoiding ebay, amazon/buy.com marketplace sellers, or any other smaller reseller, and buying my flash memory only from large, very established companies now.)

Hopefully this somewhat long post will provide some useful info to people, and help bring clarity to any future discussions of SDHC/flash storage.

08-16-2010, 05:25 PM   #2
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there is also the other issue, will your camera be able to support a class 10 or will the flash saving act no faster than a class 4. no info so far I have read mentions if the camera can go faster with higher class saving speeds.
08-16-2010, 06:03 PM   #3
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QuoteOriginally posted by EvilPentaxUser Quote
there is also the other issue, will your camera be able to support a class 10 or will the flash saving act no faster than a class 4. no info so far I have read mentions if the camera can go faster with higher class saving speeds.
If a camera supports SDHC, the camera will support SDHC Class 10, since all "Class 10" means is that a company has paid a fee to the SD Association to take part in the self-certification program, which then allows them to put the words "Class 10" on their product. The association provides members with a spec, guidelines, and suggested tools for testing, but that's it. No third parties actually verify any claims about a particular product meeting a given Class rating.

So sure, a "Class 10" card can be slower than a "Class 4" card, because one company can make a "Class 4" card very fast, and another company can make a "Class 10 card slow (minimum spec or even below spec). A company can make a card fast or slow, there's no upper limit or lower limit, and no restrictions on labelling other than they must pay a fee to use the SD Association "Class" labelling system. That's it.

Electrically and protocol-wise, SDHC "Class 6" and "Class 10" cards operate the same. Sorry if I didn't make any of this clear above.
08-16-2010, 07:07 PM   #4
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I use the free program CrystalDiskMark to test the speed of SDHC cards and another free program H2testw to check if a card is fake.

I have used and tested many different SDHC cards and I have come to the conclusion that one cannot go by what is printed on the card. One company Class 4 16GB card was faster than another company Class 10 16GB card, both companies were established SDHC cards manufacturers. Right now, I buy from only one online store and have no problems so far. The only time I was scammed was when I got a Kingston card from an Ebay seller (hoping to find a cheaper source). Luckily I didn't lose much $ in the deal 'cos Ebay had just introduced buyer's protection.

I am using Class 10 16GB and 32GB SDHC cards in my Pentax KX and Panasonic DMC-ZS3 with no problems.

08-16-2010, 11:22 PM   #5
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What a mess. My motivation to buy a SD Extreme Class 10 was to get faster emptying of my K20's buffer. I'm occasionally filling it up with single shutter actuations during promo shoots for a local theatre plays. I'm always amazed at how long my SD Ultra's take to empty the buffer. Once actors get going, one can't just tell them to stop without disrupting them.

I've ordered a 8 gb Extreme card with 30mb/sec read write speed. I guess my plan now is to test this card with the program discussed above. If its not faster than the Ultra i have, i see no reason to keep it for the money i spent. Also, it appears that just going to a larger size Card actually helps the write speed. Its really annoying that one gets to spend more money for cards without any assurance that they will be faster
08-19-2010, 05:30 AM   #6
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QuoteOriginally posted by nater Quote
Wikipedia has a decent description of what SDHC class means, saying it "...measures the minimum write speeds based on 'the best fragmented state where no memory unit is occupied.'" This indicates that the Speed Class is a kind of worst-case write performance measure determined when a card is empty (and implies that as a card is used and starts to fill up, it can slow down to below the Class speed).
Also note that because individual bits of memory have a limited write lifetime, the card will try to spread out usage automatically and transparently. This is good, but the result is that formatting the card does not reset this problem. It may appear blank, but it's no longer pristine.
05-25-2011, 02:44 PM   #7
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Here is a recent and good review of a range of SD cards (and a good illustration of how you can't just go by speed class):

10 SDXC/SDHC Memory Cards, Rounded Up And Benchmarked

06-07-2013, 10:25 PM   #8
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Thanks Nater. for all the info . I see by the date its been 2 years since your post and a lot of changes in cards have came about . If a person doesn't care how long it takes to transfer information to the card from the camera but is more concerned about wanting all the info from the camera to be written to the card so the best possible photos are reproduced . That being said In your opinion what card reproduces the best picture .
06-08-2013, 05:25 AM   #9
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It's amazing how people are worried that their camera will not support a fast SDHC card. I paid $18.95 ($9.47 each) for two 94mb/s Sony SDHC cards at B&H with free shipping and they upload my HD videos at 90mb/s using my USB 3.0 card reader to my Windows 8 PC. I really don't care if my cameras support the faster speeds or not since I prefer a fast upload speed. I will not purchased less than a Class 10 SDHC card since my last one cost me $6.80 with free shipping for a 16GB version .
06-08-2013, 06:29 AM   #10
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QuoteOriginally posted by jogiba Quote
It's amazing how people are worried that their camera will not support a fast SDHC card. I paid $18.95 ($9.47 each) for two 94mb/s Sony SDHC cards at B&H with free shipping and they upload my HD videos at 90mb/s using my USB 3.0 card reader to my Windows 8 PC. I really don't care if my cameras support the faster speeds or not since I prefer a fast upload speed. I will not purchased less than a Class 10 SDHC card since my last one cost me $6.80 with free shipping for a 16GB version .
I bought those Sony cards too. Just last night, I re-checked those with USB III card reader hooked up to USB III port of my computer. Using h2testw to check and the cards got write speed of 21.8 MB/s and read 72.9 MB/s.
07-06-2013, 10:59 AM   #11
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The specific card I bought - Sony 32GB SDHC/SDXC Class 10 UHS-1 R40 Memory Card (SF32UY/TQMN) -
Did not work in new Pentax K-30 with two different firmwares 1.00 and 1.04, it gets a fatal memory card error.
Nor did formatting by computer or by K-x help. Even went so far as to FULL format which I didn't think would help, and yes it was all a waste of time. My K-x now has my new SONY 32 GB/40mbs card.
Soooo, I'm reluctant to get a 16/32GB sdhc card for K-30/01. My 8/4/2GB cards all work just fine in K-30. Before I purchased it I read others comment about some of these 16/32GB cards not working in K-30 and k-01. Various other comments people made said some other brands don't work as well, and kind of hit and miss type of scenarios going on here. It must be a problem with the K-30 and K-01 hardware.
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