EMZETT.SD Cards
In short: Small, removable flash memory cards in the “Secure Digital” format — common in cameras, some laptops and single-board computers.
In more detail: SD cards differ in capacity (SD, SDHC, SDXC) and speed class (e.g. UHS-I/UHS-II) — for high-resolution video recording, sufficiently fast cards are needed, otherwise the camera can’t write fast enough.
In Depth
Capacity classes and file-system pitfalls
SD card capacity classes (SD up to 2 GB, SDHC up to 32 GB, SDXC up to 2 TB) also reflect different file systems the cards are factory-formatted with: SD and SDHC usually use FAT16/FAT32, SDXC, by contrast, exFAT, since FAT32 doesn’t support individual files over 4 GB — a problem for long video recordings. Older devices that can only read an older file system may not recognise an SDXC card at all, even if it physically fits into the card slot — a well-known compatibility pitfall when buying new, high-capacity cards for older cameras.
Speed classes in detail
Speed classes state the guaranteed MINIMUM write speed, not the maximum — an important distinction from the often-advertised peak values. Several parallel, historically grown classification systems exist: the original Class rating (Class 2 = at least 2 MB/s, up to Class 10 = at least 10 MB/s), the UHS Speed Class (U1 = at least 10 MB/s, U3 = at least 30 MB/s) for the faster UHS bus interfaces, and the newer Video Speed Class (V6 through V90), specifically tailored to the requirements of modern video formats, explicitly guaranteeing a consistent minimum speed over the entire recording duration, not just in a short test.
Why video recording is especially demanding
For video recording, this minimum speed is crucial, because a camera continuously delivers image data, with no pause to “catch its breath”: if the card’s write speed isn’t enough, the camera can’t write the incoming data away fast enough, the internal buffer fills up, and the recording either stops or visible image errors appear. For 4K video at a high bitrate, at least a UHS Speed Class U3 or a Video Speed Class V30 is therefore typically recommended, for high-resolution 8K recordings or very high frame rates even V60 or V90 — for simple photos, by contrast, the speed class plays a considerably smaller role, since individual images can be buffered and written out gradually without trouble even with slower cards.
Bus interfaces: UHS-I through UHS-III
Besides the pure speed class, the card’s physical bus interface also determines the maximum possible data rate: UHS-I reaches up to 104 MB/s, UHS-II (recognisable by a second row of contacts on the back of the card) up to 312 MB/s, UHS-III even more. For the higher speed to actually be usable, both the card and the card reader/camera have to support the same UHS interface — a UHS-II card in a device only capable of UHS-I automatically runs at the lower UHS-I speed.
See also: microSD Cards