Confused about Speeds in SD Card Specs

Question

I'm looking at buying some SD cards and am having some difficulty with the quoted speeds. For example, take this card:

It has a V30 video speed class, a C10 speed class, UHS 3 speed class and 95MB/s.

All seems fine, but when I look at the following table:

It appears contradictory - UHS3 and V30 are equivalent. However they are not equivalent to C10, and none of these options are anything near 90Mb/s, which is equivalent to V90.

Surely the likes of SanDisk would not be quoting incorrect figures. What have I missed?

Answer 1

The speed mentioned on the card is for reading information, not writing. All the marks (C10, U3, V30) are related to write speed. If you check the specifications of this SD card you will see few things which will change the numbers...

1. The write speed (burst) is slower, around 90MB/s
2. For SanDisk MB=1000000 bytes, which make speed around 85 megabytes/s
3. You need SDXC compatible device to reach this speed
4. All those speeds are: "Based on internal testing"

Answer 2

The video speed class is for sustained write speeds, which are needed for video recording at high bit rates. The card is rated at C10 and UHS 3, the max rating in those classes, because it exceeds the minimum speeds required for those ratings. However, it is rated for only V30 because it is capable of writing only 30 MB/s for extended periods of time.

The advertised 95 MB/s is the max read speed. The burst write speed for the card is 90 MB/s, which can be maintained only for short periods of time.

The SanDisk Extreme Pro SD UHS-I product page states:

Up to 95 MB/s read speed. Write speed up to 90 MB/s. Based on internal testing; performance may be lower depending upon host device. 1MB=1,000,000 bytes. X = 150KB/sec.

UHS Speed Class 3 designates a performance option designed to support 4K UHD video recording with enabled UHS host devices.

UHS Video Speed Class 30 (V30), sustained video capture rate of 30MB/s, designates a performance option designed to support real-time video recording with UHS enabled host devices.

See also Difference between Speed Class, UHS Speed Class, Speed Ratings (performance) and Video Speed Class for SD/SDHC/SDXC cards.

Answer 3

The key word here is minimum.

Any card that writes at a sustained speed of 95 MB/s is compliant with V90, U3, and C10 because it is at least as fast as the minimum standards of all three: 90MB/s, 30MB/s, and 10MB/s, respectively. In the case of U3 and C10, they are the highest rating in those systems and any card faster than the minimum rating for U3 or C10 (or both) is considered a U3 or C10 (or both) compliant card.

The card shown as an example in the question is rated at V30 because SanDisk will only certify it can maintain a sustained write speed of 30 MB/s or more. The 95 MB/s speed printed on the card is probably a peak write speed that can not be sustained over a long enough period to pass the V60 or V90 standards.