Information on SD storage media

Capacity:

SD: up to 4 GB capacity
SDHC: 4 GB to 32 GB capacity („High Capacity“)
SDXC: 64 GB to 2 TB capacity („Extended Capacity“)

Class:

Defines the guaranteed minimum constant data rate during writing. Does not indicate the reading speed.
Class 4 (C4): 4 MB/s
Class 10 (C10): 10 MB/s

UHS (Ultra High Speed):

UHS cards have a faster data bus.
In addition to the data bus types Default Speed & HighSpeed, which have been on the market for a long time, there are 3 classes for even faster data transfer:: UHS-I, UHS-II & UHS-III (UHS-III does not play a role in practice at the moment, due to the lack of compatible hardware.)
The UHS specification does not provide information about the guaranteed minimum constant write speed, but just about the maximum possible theoretical bus speed when exchanging data between the memory card and the host device.
In order to achieve the possible transfer speed, the terminal device must also support the respective UHS class

UHS-I: up to 104 MB/s
UHS-II: up to 312 MB/s
UHS-III: up to 624 MB/s

U1 und U3 (UHS Speed Class):

U1/U3 is the class specification for UHS cards. U1/U3, as well as the class specification, provides information about the guaranteed minimum write speed for UHS cards.
U1: 10 MB/s
U3: 30 MB/s

V10, V30, V60 und V90 (Video Speed Class):

The new “Video Speed” classes is provided with a large “V” and a speed indicator which indicates the minimum sequential write speeds (in MB/s):

V10: 10MB/s
V30: 30MB/s
V60: 60MB/s (–> specification only valid from UHS-II interface)
V90: 90MB/s (–> specification only valid from UHS-II interface)

A1 & A2 (APP performance classes):

The new APP performance classes (A1 & A2) are relevant for the outsourcing of APPs to the memory card in smartphones / tablets (from Android 6.0)

Memory cards that meet the A1 or A2 standard can open APPs much faster. The speed is not determined (as usual) in MB/s but in IOPS (Input/Output Operations per Second). The “random access time” to many small (4kb) data packets and not the “sequential access time” to a larger file (e.g. when measuring the “usual” speed rates) is decisive here. A memory card that meets the A1 standard must therefore be able to process at least 1500 (read) and 500 data packets (write) with a size of 4kb per second each.

Compatibility:

SD-UHS cards are compatible with SDXC and SDHC host devices. If they do not have a UHS marking, the transmission rate is reduced accordingly.
SDXC-UHS cards are not compatible with SD and SDHC host devices (SDXC-UHS cards are therefore only compatible with SDXC host devices).
U3 cards are downward compatible to non-U3 hosts.

“Old” class rating on UHS cards:

There is also a regular class specification on UHS cards. It specifies the minimum write speed that is guaranteed when the card (with a host) is operated without UHS bus.

Example:

1) If the host does not support UHS, the card works as a regular Class 10 card with at least 10 MB/s write speed.
2) SDHC card
3) This card has a read speed of up to 95 MB/sec and a write speed of up to 90MB/sec. This is data measured by the manufacturer.
4) UHS-I interface
5) In UHS mode, the card has class U3, i. e. at least 30 MB/s write speed

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