SD cards, which stand for Secure Digital cards, are small, removable storage devices commonly used in cameras, phones, tablets, drones, handheld gaming devices, laptops, and more. They provide additional storage space to save photos, videos, music, documents, apps, and other files. The numbers printed on SD cards indicate various specifications like storage capacity, speed, interface, and durability. This article will explain the meaning behind these numbers to help you choose the right SD card for your needs.
Sources
https://www.businessinsider.com/guides/tech/what-is-an-sd-card
https://www.easeus.com/computer-instruction/what-is-sd-card.html
Storage Capacity
One of the main numbers shown on SD cards is the overall storage capacity, generally shown in gigabytes (GB) or terabytes (TB). Some common capacities for SD cards include:
- 16GB
- 32GB
- 64GB
- 128GB
- 256GB
- 512GB
- 1TB
Higher capacity cards allow you to store more photos, videos, music, documents, and other files. The largest currently available SD card capacity is 1TB, introduced in early 2019 by SanDisk [1]. However, capacities are constantly increasing, with a theoretical limit of 2TB for SD cards using the SDXC specification [2].
Speed Class Rating
SD cards display a speed class rating that indicates the minimum guaranteed speed of the card. The rating is typically printed on the front of the card. The SD Association has defined several speed classes:
- Class 2 – Minimum write speed of 2 MB/s
- Class 4 – Minimum write speed of 4 MB/s
- Class 6 – Minimum write speed of 6 MB/s
- Class 10 – Minimum write speed of 10 MB/s (SD Association)
There are also UHS speed classes for ultra high-speed cards:
- U1 – Minimum write speed of 10 MB/s
- U3 – Minimum write speed of 30 MB/s (Kingston)
Higher speed class ratings indicate faster card performance. When choosing an SD card, select one with a speed class rating suitable for your device and usage.
Speed Class Rating
SD cards are given speed ratings known as Speed Classes to indicate their minimum guaranteed speeds. The Speed Classes defined by the SD Association are Class 2, 4, 6 and 10. Class 10 cards have a minimum write speed of 10 MB/s [1].
There are also UHS Speed Classes specifically for ultra high speed cards using the UHS interface. These include U1 and U3. U1 cards have a minimum write speed of 10 MB/s, while U3 cards have a minimum write speed of 30 MB/s [2]. The UHS Speed Classes allow for faster read/write speeds on supported devices, but are backwards compatible with standard SD interfaces.
Bus Interface
The bus interface refers to the data transfer rate supported by the SD card. This determines compatibility with devices and maximum speed. The main bus interfaces for SD cards are:
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SDHC – Uses the High Speed bus, supporting transfer speeds up to 25 MB/s. Compatible with SDHC and earlier devices.
-
SDXC – Uses the High Speed bus and UHSI-I bus, supporting speeds up to 104 MB/s. Backward compatible with SDHC devices.
-
UHS-II – Interface supporting up to 312 MB/s transfer speeds. Only works with UHS-II compatible devices.
-
UHS-III – Newest interface supporting up to 624 MB/s speeds. Backward compatible with UHS-II and UHS-I.
The bus interface is denoted by symbols on the card. SDHC will display a C6 or C10 symbol, while SDXC and newer cards will have a U1 or U3 symbol. This helps determine compatibility and maximum speeds for devices.
Bus Interface
The bus interface determines the maximum bus speed at which a memory card can transfer data. There are several common bus interfaces used in SD cards:
- SD: The original SD bus operates up to 25 MB/s
- SDHC: The High Capacity SD bus operates up to 104 MB/s
- SDXC: The eXtended Capacity SD bus operates up to 312 MB/s
- UHS-I: The Ultra High Speed Phase I bus operates up to 104 MB/s
- UHS-II: The Ultra High Speed Phase II bus operates up to 624 MB/s
- SD Express: Combines the PCIe and NVMe interfaces for speeds up to 4 GB/s
To verify bus interface compatibility, you need to check the specifications of your device that will be reading the SD card. The device needs to support at least the minimum bus speed of the SD card in order to take full advantage of the card’s capabilities.
For example, if you have a UHS-II SD card, you need a device with a UHS-II bus interface to get the full 624 MB/s speeds. Using that card in a UHS-I or SDHC device would limit the maximum speed to 104 MB/s. Checking your device specs or manual can help determine which bus interfaces are supported.
Durability Ratings
SD cards have durability ratings to indicate how resistant they are to water, shock, extreme temperatures, X-rays, and other environmental factors. These ratings help determine which cards are best suited for certain use cases.
For example, some SD cards are rated to withstand up to 72 hours in 1 meter of water. Cards with this IPX7 waterproof rating are ideal for use in outdoor action cameras. Other cards have a shockproof rating that indicates they can withstand a drop from a certain height without data corruption. And some industrial SD cards can operate in extreme temperatures ranging from -25°C to 85°C, making them suitable for specialized applications.
Higher durability ratings generally indicate a more durable SD card. However, there are tradeoffs as more durable cards are often more expensive. When choosing an SD card, it’s important to consider the environmental conditions it will face and select a card with appropriate durability ratings.
According to The New York Times, some of the most durable microSD cards are the Samsung PRO Plus and SanDisk Extreme lines. Both offer shockproof, waterproof, and temperature proof options to handle various conditions.
Example Card
To demonstrate the different numbers and codes found on SD cards, here is an example of a common SD card:
This particular card shows the following details:
- Storage capacity – 32 GB (gigabytes)
- Speed class – Class 10, U3, V30 [1]
- Bus interface – SDHC [2]
- Durability rating – A1
The storage capacity indicates that this SD card can hold up to 32 billion bytes (32 GB) of data. The speed ratings of Class 10, U3, and V30 indicate it has a minimum sequential write speed of 10 MB/s, 30 MB/s, and 30 MB/s respectively. SDHC refers to the SD High Capacity interface supporting cards up to 32 GB. Finally, the A1 durability rating means this card is optimized for app performance.
By looking at these key numbers and ratings, you can get a quick understanding of the card’s capabilities and performance level.
[1] https://learn.sparkfun.com/tutorials/sd-cards-and-writing-images/all
[2] https://support.industry.siemens.com/cs/document/109780231/simatic-iot2050-sd-card-example-image?dti=0&lc=en-WW
Choosing the Right Card
When selecting an SD card, consider the device you plan to use it with and your intended purpose. Certain devices and use cases benefit from higher capacity cards, while others require faster read/write speeds.
For phones, mid-range capacity cards around 32-128GB are common. You’ll want a card with decent speed for capturing photos/video, so look for read speeds of at least 80-100MB/s. The UHS speed class and A1/A2 ratings indicate performance for apps.
Digital cameras need higher capacities and speeds. Aim for 128GB+ and UHS Speed Class 3 (U3) minimum, which supports 4K video recording and burst photography. The V-class ratings indicate maximum video bitrates.
For camcorders and action cams, cards with V30/V60/V90 ratings are ideal for capturing high bitrate 4K/8K video. UHS-I or UHS-II bus interfaces provide faster transfer speeds. Capacities of 128GB or higher are recommended.
When in doubt, check your device’s manual for its SD card recommendations. Also consider future-proofing – buying a higher capacity or faster card than needed today allows room to grow.
Conclusion
In summary, the most important numbers to look for when buying an SD card are the storage capacity, which tells you how much data the card can hold, and the speed class rating, which indicates the minimum guaranteed speeds for reading and writing data. The bus interface (UHS-I or UHS-II) is also key for determining maximum possible speeds. Other specs like durability ratings provide additional details on the card’s expected lifespan and resistance to water, shocks, X-rays and more. While the variety of numbers on SD card labels can be confusing at first, by learning the basics around storage, speed, interface and durability, you’ll be better equipped to choose the right card for your needs and budget. The key is focusing on the capabilities that matter most for your particular device and use case. With a grasp of the core SD card markings covered here, you can shop confidently and find a card that delivers reliable performance.