Velocity Reviews > speeds of SD vs. SDHC cards

# speeds of SD vs. SDHC cards

yawnmoth
Guest
Posts: n/a

 12-07-2007
According to <http://en.wikipedia.org/wiki/Sd_card#Speeds>, SD cards
come in four different possible speeds - 0.9mb/s, 6mb/s, 10mb/s, and
20mb/s. According to <http://en.wikipedia.org/wiki/Sd_card#SDHC>, SDHC
cards come in three different possible speeds - 2mb/s, 4mb/s, and 6mb/
s. From that, it looks like the fastest SDHC card is slower then the
fastest HD card. Is that correct or is there something I'm missing?
And if it is correct... why? According to the article, "SDHC uses a
different memory addressing method". Is that what's slowing it down?

Gene S. Berkowitz
Guest
Posts: n/a

 12-10-2007
In article <86b5b4cd-a8a2-4f3f-9ccf-0a9107e00311
> According to <http://en.wikipedia.org/wiki/Sd_card#Speeds>, SD cards
> come in four different possible speeds - 0.9mb/s, 6mb/s, 10mb/s, and
> 20mb/s. According to <http://en.wikipedia.org/wiki/Sd_card#SDHC>, SDHC
> cards come in three different possible speeds - 2mb/s, 4mb/s, and 6mb/
> s. From that, it looks like the fastest SDHC card is slower then the
> fastest HD card. Is that correct or is there something I'm missing?
> And if it is correct... why? According to the article, "SDHC uses a
> different memory addressing method". Is that what's slowing it down?

No, they're mixing apples and oranges.
The SD speeds listed are _READ_ speeds. The _WRITE_ speed for and SD
card is typically 1/2 to 1/10 of the read speed. Note also that many of
the speeds published on WP are in MB (megaBYTES), not Mb (megaBITS) per
second.

For SDHC, the "class" rating is for _WRITE_ speed:

Class 2: >= 2 MBYTES/sec
Class 4: >= 4 MBYTES/sec
Class 6: >= 6 MBYTES/sec

--Gene

Dave Martindale
Guest
Posts: n/a

 12-12-2007
Gene S. Berkowitz <(E-Mail Removed)> writes:
>In article <86b5b4cd-a8a2-4f3f-9ccf-0a9107e00311
>> According to <http://en.wikipedia.org/wiki/Sd_card#Speeds>, SD cards
>> come in four different possible speeds - 0.9mb/s, 6mb/s, 10mb/s, and
>> 20mb/s. According to <http://en.wikipedia.org/wiki/Sd_card#SDHC>, SDHC
>> cards come in three different possible speeds - 2mb/s, 4mb/s, and 6mb/
>> s. From that, it looks like the fastest SDHC card is slower then the
>> fastest HD card. Is that correct or is there something I'm missing?
>> And if it is correct... why? According to the article, "SDHC uses a
>> different memory addressing method". Is that what's slowing it down?

>No, they're mixing apples and oranges.
>The SD speeds listed are _READ_ speeds. The _WRITE_ speed for and SD
>card is typically 1/2 to 1/10 of the read speed. Note also that many of
>the speeds published on WP are in MB (megaBYTES), not Mb (megaBITS) per
>second.

That may be true for some SD cards. But the Sandisk "Extreme III" cards
are rated by Sandisk as 20 MBytes/sec. minimum for both read and write.
That's faster than SDHC class 6.

So it looks like most SDHC cards are faster than most but not all SD
cards.

By the way, are there SDHC cards smaller than 4 GB? I was looking
yesterday in a Best Buy and could not find any - anything 2 GB and lower
was SD, not SDHC.

>For SDHC, the "class" rating is for _WRITE_ speed:

>Class 2: >= 2 MBYTES/sec
>Class 4: >= 4 MBYTES/sec
>Class 6: >= 6 MBYTES/sec