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Old Feb 22, 2012, 3:56 PM   #1
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Default SD Card Class & write speed ?s

Two questions about SD memory cards
1.The class of the card is its write speed: Class 4 = 4Mb/sec?
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2. If it takes the HS-20 19.5 sec. to write 6 frames w/ a class 10 card and 19.25 sec. with a class 32 is there any way to know how long it will take w/ class 4 and/or class 6 cards? TIA
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Old Feb 22, 2012, 4:26 PM   #2
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There is no class 32 sd cards.
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Old Feb 22, 2012, 6:12 PM   #3
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The SDHC Card Speed Class represents a guaranteed minimum write speed (under certain very specific conditions.) Thus, a Class 2 card will have a minimum write speed of 2MB/s (That's megabytes per second, not "Mb/sec", which is megabits per second.), a Class 4 card will have a minimum write speed of 4MB/s, etc. (See Secure Digital for more information.)

I suspect your reference to a Class 32 card is based on the mistaken belief that the Speed Class is based on the advertised maximum write speed, which is not the case.

In addition, 6 frames is too small a sample to make any kind of prediction of future performance.
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Old Feb 22, 2012, 6:43 PM   #4
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The bottleneck is often the camera's ability to process the images, and the speed of the camera's interface to the memory card, not the card speed itself.

So, the only way to find out if a faster card will result in faster speeds in a given camera model is to try one (or find someone that's tried a specific camera/card combination to test full buffer flush times, etc.).

As already mentioned, you really can't go by the Class Ratings. I'd look for tests of a specific card brand/model to get an idea of how a given card performs in optimum conditions. One Class 10 Card may be able to handle 10MB/Second Write Speeds, but a different brand/model of Class 10 card may be able to handle 20MB/Second Write speeds.

But, even if you're using a card capable of 30MB/Second write speeds under optimum conditions with a card reader using larger block sizes, that doesn't mean the camera you're using is capable of writing to the card anywhere near that fast. For example, you may find out that the camera's write speed is limited to 4MB/Second instead, no matter how fast the card you use is (because of time needed to process the image from the camera's sensor into a usable format, ability to handle moving data from the internal buffer to the memory card in parallel with processing of other images, speed of the camera's interface to the card and more). Even if the interface from the camera to the card is fast, other processing of the image data before it tries to write to the card may limit the speed you can expect to achieve.

IOW, it's not unusual to see diminishing returns from faster cards in many cameras, where you may see a camera able to process and flush 1 frame per second with a slower card, yet only process and flush 2 frames per second with a card that's 10 times as fast. There are a number of potential bottlenecks.

So, you have to take each camera/card combination on a case by case basis to see how well they perform.
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Old Feb 23, 2012, 8:31 AM   #5
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Default Thanx all y'all

for the informative and prompt replies. My information came from a Review @

http://www.digitalcamerareview.com/d...x+hs20+exr&p=3

"Single shot to shot times take about 2.5 - 2.75 seconds with a class 10 SDHC (30MB/sec) card.
Fire an 8 shot burst at full resolution and JPEG fine image quality with the camera's 8 fps continuous high-speed shooting mode and the write time for those shots approaches 19.5 seconds - make that burst a RAW/JPEG combo and you get only six shots as well as a 30 second write time. Switching to a 45 MB/sec card produced 19.25 and 25 second write times for those same bursts, respectively."
As you all have pointed out the camera appears to be the limiting factor in burst mode and only a hands-on test will tell for other functions.

Sorry my 78 yo brain is more bits & pieces than bytes when it comes to remembering specs.
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Old Feb 23, 2012, 9:31 AM   #6
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There's no guarantee that they would be any faster. But, I do see a hint in that review that UHS-1 mode may be supported. So, you may want to see if one of these cards is any faster in that camera (Sandisk cards rated at 90MB/Second Write and 95MB/Second Read). They may not be faster. You'd have to try one to find out. But, they're about the fastest SDHC cards available now (if the camera is capable of writing to them using the latest transfer modes, and if something else in the processing pipeline isn't the main bottleneck).

8GB
http://www.bhphotovideo.com/c/produc..._Pro_8_GB.html

16GB
http://www.bhphotovideo.com/c/produc...Pro_16_GB.html

32GB
http://www.bhphotovideo.com/c/produc...Pro_32_GB.html
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Old Feb 23, 2012, 9:38 AM   #7
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And, once again, the Speed Class is not an indication of maximum speed, only a guarantee of minimum speed (one that is frequently not met.)

Their attempt to equate the Speed Class of a card to it's advertised maximum speed is irresponsible.
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Old Feb 23, 2012, 10:07 AM   #8
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It's not the UHS-1 class rating that makes those cards faster. It's Sandisk's ratings for them (90MB/Second Write, 95MB/Second Read). ;-)

They wouldn't claim that much speed unless the controller supported the faster UHS transfer modes and the NAND flash was capable of being read from and written to that fast.

Of course, you have to have a UHS-1 Capable camera to take advantage of faster UHS Transfer modes, too. Otherwise, it's not going to use the faster clock speeds needed in order to take advantage of faster cards.

http://en.wikipedia.org/wiki/Secure_...HS_Speed_Class

You can still have other bottlenecks in the pipeline. So, there's no telling how specific camera/card will work without testing it. But, the newer Sandisk Extreme Pro UHS-1 SDHC Cards are probably about the fastest you'll find right now.

Sandisk has also made slower UHS-1 cards (only rated for 45MB/Second). But, their latest generation of cards are rated for much faster speeds (90MB/Second Write, 95MB/Second read).

Of course, that's under optimum conditions, using aligned writes with optimum block sizes. But, as a general rule of thumb, Sandisk's claims are usually closer to the actual potential of the cards compared to some of the other manufacturers around.
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Old Mar 22, 2012, 7:43 PM   #9
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Default SD card speed

When I bought my Sony HX100V, I emailed Sony support to find out the write speed of the camera.

I suggest you do the same and then buy a class that matches or meets the write speed of your camera. In my case, it was class 10.

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Old Apr 6, 2012, 8:11 AM   #10
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Fastest video speed of HX100 is 28 Mbps, these are 3.5 MBps - Class4 would be enough, buuuut Class6 or Class10 are cheaper :-)

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