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NuJew
SDHC Class & Speed
#1
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เป็นสมาชิกเมื่อ:
11/4/2007 23:16
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The
SD Card Association (SDA) has established a new specification rating
the minimum data transfer both SD/SDHC Memory Cards and SD/SDHC host
products.


Background

Previously, when writing onto an SD Memory Card, the host product
could not determine the storage conditions inside the card. Due to the
nature of flash memory, speed and performance depends on card
fragmentation. This means that memory cards are not always able to
provide the expected performance required by the host product.


Concept

With the new Speed Class specifications, the host product can check
the fragmented state in the card and calculate the write speed in each
part of the card. The host can then determine where to write the data
according to its speed requirement.




* What is fragmented state?

The memory area of a card is divided into minimum memory units. The
host product writes data onto those memory units where no data is
occupied. The fragmented state reflects how these memory units are
occupied. The write speed depends on this state.


The SDA has specified operating conditions for
SDHC Memory Cards that guarantee the minimum speed according to the
card's speed class. Class logos are used to identify cards that comply
with these specifications.

The Class logos must appear near the card's logo on all SDHC Memory Cards (above 2GB).

SD Speed Class logo is optional for SD Memory Cards (2GB and less).






SD Speed Class

The following speed classes have been established:





SD Speed Class in Actual Use





Class 2 Card:
A speed of 2 MB/s or higher is guaranteed at the best fragmented state
where no memory unit is occupied.		 Class 4 Card:
A speed of 4 MB/s or higher is guaranteed at the best fragmented state
where no memory unit is occupied.		 Class 6 Card:
A speed of 6 MB/s or higher is guaranteed at the best fragmented state
where no memory unit is occupied.

*This capability varies depending on the product manufacturer and the model.








If the host product requires or recommends Class 4 data transfer speed or higher-


The host product can achieve the best performance with these cards.


http://www.sdcard.com/b2b/TextPage.asp?Page=3

โพสเมื่อ : 5/2/2008 15:55
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kiat
Re: SDHC Class & Speed
#2
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เป็นสมาชิกเมื่อ:
17/4/2007 16:25
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โพสเมื่อ : 6/2/2008 0:49
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