Storage
The nominal capacity of a memory card is the amount of data that can be written to it. For technical reasons, the real working volume of modern memory cards is usually somewhat less than the nominal one (for example, 14.9 GB with the claimed 16 GB). However, with the same type of card, differences in nominal capacity usually correspond to differences in actual capacity: for example, a 64 GB SDXC card will actually have twice the capacity of a 32 GB card of the same card.
Recall that different types of media have different limits on the maximum volume; see "Type" for details. And a specific volume is most often chosen by the manufacturer not arbitrarily, but from a list of standard options. The most widely used cards nowadays are
16 GB,
32 GB,
64 GB,
128 GB and
256 GB ; capacities of
8 GB or less are considered modest, with the most capacious models holding
512 GB or more.
Note that the price of the card directly depends on this parameter. Thus, many models are available in several variations, differing solely in capacity; and it is worth choosing by volume depending on the types of data that are planned to be stored on the card. Of the popular formats, text documents occupy the least space — usually tens, at most hundreds of kilo
...bytes. The size of a photo from a modern camera or an MP3 file is already calculated in megabytes, music in lossless formats "weighs" already tens of megabytes, and the volume of a video depends on its duration, resolution and the codecs used.UHS class
UHS bus version supported by the memory card.
UHS is a data exchange technology used in SD/microSD cards of the (micro)SD HC and (micro)SD XC standards. It allows you to significantly increase the speed of data exchange compared to previous standards; at the same time, media supporting this technology are quite compatible with equipment that does not have UHS - except that the speeds in such cases will be limited by the capabilities of the card reader, and not the card itself.
As for the versions, their names consist of two parts: UHS generation (I or II) and class (1 or 3) - for example,
UHS-I Class 1. Actually, this is the most modest version of UHS found in modern memory cards; in addition to it,
UHS-I Class 3 and
UHS-II Class 3 are popular. The maximum data transfer speed (usually referring to read mode) depends on the UHS generation - it is 104 MB/s for version I and 312 MB/s for version 2. “Class”, in turn, determines the lowest (guaranteed) write speed : it is at least 10 MB/s for Class 1 and at least 30 MB/s for Class 3. Different versions of UHS are quite compatible with each other, except that the speed in such cases will be limited by the slower standard.
Note that for UHS cards, more advanced characteristics may be indicated in other paragraphs - for example, the video recording class (see below) V90 for a UHS-II Class 3 card. There
...is no contradiction here: the speed of 90 MB/s provided by such a card in recording mode, it fully satisfies the “not lower than 30 MB/s” condition provided for UHS Class 3.Video recording class
The video recording class supported by the memory card.
This parameter is indicated for SD/microSD cards and their variations, as well as CompactFlash (see “Type”). In the first case, the format designation V* is used, in the second - VPG*, but the idea is the same in both cases: the numbers indicated instead of an asterisk are the lowest write speed (in MB/s) that the card is guaranteed to provide. This speed is of key importance primarily in video recording, which is why this parameter is called “Video Recording Class”; the higher it is, the higher the resolution and frame rate at which you can record using this card.
The minimum video recording class found in modern SD/microSD family cards is
V6, which already allows you to work with HD and Full HD video. However, for Full HD it is advisable to have a card of at least
V10 level. And the
V30 class already allows you to comfortably work with Ultra HD 4K. Faster cards are also being released - V60 and even V90, suitable for resolutions of 8K and higher.
In turn, in CompactFlash cards you can find models of the
VPG20,
VPG65 and
VPG130 classes
Note that the video recording class may well be higher than the general speed class and/or UHS class (see above). However, there are no contradic
...tions here. UHS class requirements require write speeds of “not less than...”; Thus, for example, all cards with a video recording class of V30 and higher will comply with the UHS Class 3 standard (“not lower than 30 MB/s”). And the general speed class describes the capabilities of the card when working with equipment without UHS support (and such equipment is practically not used in video recording).Application class
Speed class when working with applications to which the memory card corresponds.
When working with applications that use a memory card (for example, mobile games), information is written and read not in a continuous stream, as when recording video or copying data, but in separate small fragments, and from random areas in the drive’s memory (the so-called random write/read). So a high-class video recording (see above) does not always mean good flow Rate when working with demanding games and other resource-intensive programs. Therefore, the characteristics of memory cards began to separately indicate the speed class for applications.
The key indicator for assigning a particular class is the minimum number of IOPS (input/output operations per second) that the media can provide in random write and read mode. Nowadays, the following classes are relevant:
—
A1 — at least 1500 IOPS for reading and 500 IOPS for writing;
—
A2 — at least 4000 IOPS and 2000 IOPS, respectively.
The continuous recording speed (for example, when recording video) in both cases must be at least 10 MB/s (often it is higher - see “Speed class”, “Video recording class”).
In the future, we can expect the emergence of new, more productive classes for applications. We also note that this parameter significantly affects the price of the card.
Read speed
The maximum speed of reading data from a memory card.
Any modern memory card provides sufficient reading speed for simple everyday tasks — like copying photos from a camera to a laptop. Therefore, it makes sense to pay attention to this parameter mainly in cases where the card is purchased for professional use, or if high speed is critical for you — for example, if you often have to copy photos from the camera in a hurry. In doing so, two points should be taken into account. Firstly, the corresponding speed must be supported by the card reader where the card is used. Secondly, this paragraph indicates the theoretical maximum — in perfect conditions; the actual reading speed may be lower (if we are talking about cards with UHS, this speed can be estimated by the UHS class, see above about it). Nevertheless, it is quite possible to compare different models of cards by this indicator: differences in theoretical speed usually quite correspond to the difference in actual speed.
Write speed
The maximum speed for writing data to a memory card.
High write speed is important for tasks that involve storing large amounts of data in a limited amount of time. However, note that this paragraph indicates the theoretical maximum speed for perfect conditions; in fact, this value can be noticeably lower. Therefore, the characteristics of memory cards also indicate the minimum guaranteed write speeds in different modes — the general speed class, the UHS class, the video recording class and/or the class for applications. All of them are described in more detail above. However, you can also evaluate the capabilities of the carrier by the overall speed: other things being equal, a card with a higher theoretical speed will work faster in fact.
As for specific values, speeds
from 4 MB / s and
from 6 MB / s are practically not found nowadays,
from 10 MB / s and from
16 MB / s can be called average indicators,
from 50 MB / s — above average, and the fastest models have a maximum write speed of
100 Mbps or more.
