SDXC memory cards are the next generation of memory cards after SDHC. The SDXC is developed to overcome the limitations of SDHC in capacity, speed, and performance.
In a previous post, I explained the SDHC memory cards. You can read it here if interested. In this post, SDXC memory cards are explained to you.
The Secure Digital eXtended Capacity (SDXC) was announced in January 2009 by SDA. It is defined in SD specification version 3.0. SDXC specification supports cards up to 2TB. And have a much faster read and write speeds compared to SDHC with the standard bus.
SDHC vs SDXC memory cards
As I mentioned before, SDXC is the next generation of memory cards after SDHC. And it will be superseded by SDUC which is currently the latest SD specification (version 7.0).
Although SDUC memory cards are not manufactured as of this writing, they will appear in the market in the near future.
SDXC was developed to overcome the limitations of SDHC memory cards due to the increasing demand for more storage and faster speeds. Any memory card larger than 32GB is an SDXC memory card.
To achieve faster speeds, Ultra-High-Speed (UHS) bus was introduced. It is now implemented in both of these memory cards.
But UHS has a speed range. SDHC memory cards are on the lower side of the speed range and SDXC memory cards have faster speeds even with the same UHS bus.
So there is a big difference between SDHC and SDXC memory cards in capacity, speed, and performance. The speed of SDXC memory cards is much faster than SDHC memory cards and the capacity much higher.
Now let’s go through the important things about SDXC memory cards.
Capacity range of SDXC memory cards
The capacity of SDXC memory cards starts at >32GB up to 2TB. Common capacity of SDXC memory cards are 64GB, 128GB, 200GB, 256GB, 400GB, and 512GB. Currently up to 1TB memory cards could be found in the market.
But 1TB memory cards are expensive. For example, as of this writing, a 1TB microSDXC card has a price between $200 – $300. People buy these higher capacity memory cards only if they have special requirements.
Ultra-High-Speed for fast read and write
SD specification version 3.01, and 4.0 introduced UHS-I and UHS-II. These buses define a speed range for memory cards. The speed range for UHS-I is 50MB/s up to 104MB/s. And the speed range for UHS-II is 156MB/s up to 312MB/s.
Note that the UHS speed range does not mean the maximum and minimum speeds of the memory card. A UHS-I card can have an advertised speed between 50MB/s and 104MB/s. No matter what the speed is, it is the maximum.
For example, if the speed of a UHS-I memory card is 50MB/s, that will be the maximum speed it can achieve, and it can’t go beyond that. But the speed may become slower than that.
That was for the maximum speed. For the minimum speed of a memory card, you must check UHS Speed Class. I will explain it later in this post.
At the time of this writing, memory cards with UHS-II bus are much more expensive than UHS-I, even when everything else is the same.
The bus mark for UHS-I is shown as “I” on the memory cards and for UHS-II it is shown as “II”. In the image below the symbol “I” is shown beside the SDXC logo, which indicates that the card supports UHS-I.
UHS-III was introduced in SD specification 6.0. The speed range for UHS-III is 312MB/s up to 624MB/s. As of this writing, I couldn’t find any memory card in the market with UHS-III interface. The standard mark for UHS-III is “III”.
And finally, SD specification 7.0 introduced the SD Express bus. The speed range for SD Express is 985MB/s up to 3938MB/s.
The standard mark for SD express is shown below. SD Express is intended for SDUC (capacity >2TB up to 128TB) and SDXC memory cards. But SDHC memory cards can also apply it, although it will make them very expensive.
Mandatory exFAT file system
The SD Specification 3.01 makes the exFAT file system mandatory for SDXC memory cards. This new file system was introduced by Microsoft in 2006 and it is specially optimized for flash drives and memory cards.
Although exFAT has its improved features, some operating systems do not support it. This makes the use of such memory cards restricted to a limited set of operating systems.
But you can reformat SDXC memory cards to any file system like FAT32, ext4, etc. if the host device accepts the new file system. Some host devices are designed to only accept exFAT for SDXC memory cards.
If the host device does not accept the new file system, it will display the error message that the memory card is corrupted. And some devices may format the memory card to exFAT. In such cases, you will lose your data.
UHS Speed Class and Standard Speed Class of SDXC memory cards
In 2009 SD Association introduced UHS Speed Class. It determines the minimum progressive write speed of a memory card. The UHS speed class is based on the worst-case scenario. A memory card should be able to write a constant stream of data at least at that speed.
This is very important for some devices like camcorders. A memory card may have an advertised maximum speed of 100MB/s, but its minimum speed may drop to 10MB/s. It will not be able to maintain a constant write speed faster than 10MB/s.
That means you will not be able to use it with some video cameras or camcorders that require a constant write speed of 30MB/s. And you will not be able to record a 4K video, without dropouts.
SD Association introduced two UHS speed classes, U1 and U3. U1 requires the minimum write speed of 10MB/s for the memory card, and the memory card should be able to maintain at least that speed.
U3 requires the minimum write speed of 30MB/s for the memory card. So the write speed should not become slower than 30MB/s, even when files on the memory card are fragmented. The memory card should be able to sustain at least that speed.
|UHS Speed Class||Minimum Progressive Speed|
Standard Speed Class of SDXC memory cards
If you use a UHS memory card with a host device that does not support the UHS bus, it will use the standard bus. Then the minimum speed will be measured with speed class. These are the speed classes of the standard bus:
|Speed Class||Minimum Speed|
Usually, SDXC memory cards have a minimum write speed of 10MB/s with the standard bus. You will see the mark for speed class 10 on the memory card.
Video Speed Class
SD Association created the Video Speed Class to identify memory cards, that are suitable for very high-resolution video recording. These memory cards should provide the features needed for such recordings.
The video speed classes show the minimum sustained speed. This is required for high-resolution video recording. Examples of high-resolution video recordings are as follows:
- Recording of multiple video streams
- Recording from VR camera
- Recording of 8K or higher resolution video
- 360 video capture
Other speed classes or speed ratings can’t express these capabilities. That was the reason for the creation of Video Speed Classes.
The following table shows the standard video speed classes.
|Video Speed Class||Minimum Sustained Speed||Suitable For|
(it depends on recording device)
|V6||6MB/s||SD Video Recording|
|V10||10MB/s||HD and FHD Video Recording|
|V30||30MB/s||4K Video Recording|
|V60||60MB/s||8K Video Recording|
|V90||90MB/s||Recording of 8K and Higher Resolution Videos|
Application Performance Class
This is the new standard defined by the SD Association in version 5.1 and 6.0 of the SD Specification. It is defined to meet the requirements of different applications.
SD Association defined two application performance classes. Class A1 and Class A2.
The Application Performance Class requires a minimum sustained writing speed of 10MB/s. In addition to that, it adds other requirements.
If the memory card claims A1 class, it must support at least 1500 reading and 500 writing operations per second.
And if it claims A2 class, it must support a minimum of 4000 reading and 2000 writing operations per second. A2 class cards require host driver support, otherwise, they might even be slower than A1 cards.
There are many factors that define the performance of an SDXC memory card. The performance is measured based on the usage context. Now the SDXC memory cards explained for you and you know how to decide when choosing a memory card.
If you want a memory card for general data transfer, UHS and UHS Speed Class are important. And if you want a memory card for high-resolution video recording, video speed class is very important.
For applications (like mobile apps), application performance class should be considered.
I have explained all these factors for you. You can also read the article on SDHC cards and compare SDHC vs SDXC memory cards. And then you will be able to see the difference between SDHC and SDXC memory cards.
As the storage and performance requirements grow, the features of the memory card must grow to be able to meet the requirements.
I hope this post helps you in choosing the right memory card for your specific need.
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