SD cards are removable flash memory cards used for storage in portable devices. The SD standard was introduced in 1999 by the SD Card Association as a successor to MultiMediaCards (MMC). SD stands for Secure Digital, reflecting the encryption and security features built into the standard.
The first SD cards had capacities up to 64MB. Over the past two decades, capacities have grown enormously with current SD cards available up to 2TB. The small size and usable capacity of SD cards have made them the standard portable storage medium for consumer devices like digital cameras, smartphones, handheld gaming devices, and tablets.
There are three main types of SD cards:
- SD – The original SD card format.
- SDHC – SD High Capacity cards introduced faster speeds and up to 32GB capacity.
- SDXC – SD eXtended Capacity cards allow capacities up to 2TB.
All types maintain the same physical dimensions but differ primarily in supported capacities. Newer card types are backwards compatible with older host devices.
SD Card Sizes
SD cards come in a range of storage capacities to suit different needs. Some of the most common sizes available are:
- 4GB
- 8GB
- 16GB
- 32GB
- 64GB
- 128GB
- 256GB
- 512GB
The maximum capacity depends on the SD card standard:
- SDHC supports up to 32GB
- SDXC supports up to 2TB
- SDUC supports up to 128TB
Currently, the largest capacity SD card available on the consumer market is 1TB, introduced by SanDisk in 2019.
Higher capacity cards allow you to store more photos, videos, music, documents, and apps. However, not all devices support the highest capacities, so it’s important to check compatibility.
SD Speed Classes
SD Speed Classes indicate the minimum guaranteed speeds that SD cards are required to meet for real-world performance. These speed ratings, defined by the SD Association, help consumers choose cards that can handle their intended use case. There are several Speed Classes for SD cards:
The Speed Class 2 guarantees a minimum write speed of 2MB/s. This is the basic speed class for SD cards. Class 4 cards have a minimum of 4MB/s, Class 6 is 6MB/s, and Class 10 is 10MB/s. The higher the speed class number, the faster the card can write data. Class 10 cards are recommended for recording HD video.
In addition to sequential Speed Classes, SD cards can also carry UHS Speed Class ratings for fast random access speeds desirable for 4K video recording. UHS Speed Classes designated as U1 and U3 indicate minimum random read speeds of 10MB/s and 30MB/s, respectively. A Class 10 U1 card would support at least a 10MB/s sequential write speed and a 10MB/s random read speed.1
While Speed Classes provide a baseline, real-world speeds vary. Cards often have faster maximum speeds, so it’s important to look at manufacturers’ ratings. But the Speed Class gives a reliable minimum performance to expect from the card.
UHS Speed Classes
UHS Speed Classes indicate the minimum write speeds that SD cards are capable of when recording video. They allow consumers to easily identify cards that are fast enough for certain video recording applications.
There are three UHS speed classes defined in the SD card specification:
- UHS Speed Class 1 (U1) – Minimum write speed of 10 MB/s
- UHS Speed Class 2 (U2) – Minimum write speed of 30 MB/s
- UHS Speed Class 3 (U3) – Minimum write speed of 30 MB/s
A higher UHS speed class indicates faster write speeds, which allows recording higher resolution or bitrate video without dropping frames. U1 is sufficient for most Full HD video recording. U3 offers support for 4K video recording at higher quality settings.
Checking the UHS speed class when purchasing an SD card ensures it can handle the demands of high quality video recording like 4K (https://camerajabber.com/buyersguides/best-memory-cards-for-video/). Selecting a card that meets the minimum write speed requirements is crucial for optimal video recording performance.
Bus Interfaces
SD cards use different bus interfaces that affect factors like speed and maximum capacity. The main bus interfaces are:
- SDHC – Uses the original SD bus, supporting speeds up to 25 MB/s and capacities up to 32GB.
- SDXC – Uses the high-speed SD bus, supporting speeds up to 104 MB/s and capacities between 32GB and 2TB.
- UHS-I – An optional higher bus speed that supports up to 104 MB/s transfer rates.
- UHS-II – The fastest bus interface, supporting up to 312 MB/s transfer speeds.
The bus interface determines the maximum possible card capacity and speed. Newer bus standards like UHS-II allow for greater performance, while retaining backwards compatibility with older SD hosts and devices. When shopping for high capacity SD cards, choose a model that matches your device’s bus interface to take full advantage of available speed.
Physical Dimensions
SD cards come in three physical sizes: full-size SD cards, miniSD cards, and microSD cards. The most common physical dimension for full-size SD cards is 32 mm x 24 mm x 2.1 mm, according to Homyspire [https://www.homyspire.co.nz/products/adata-premier-microsdxc-uhs-i-a1-v10-card-128gb-adapter-1]. MiniSD cards measure 21.5 mm x 20 mm x 1.4 mm while microSD cards are even smaller at 15 mm x 11 mm x 1 mm, as stated by Homyspire [https://www.homyspire.co.nz/products/adata-premier-microsdxc-uhs-i-a1-v10-card-with-adapter-256gb]. The microSD card is the smallest variant and is commonly used in smartphones, tablets, and other small devices that require compact storage.
Durability Ratings
SD cards are rated for durability against water, dust, shock and extreme temperatures according to industry standards. The IP Code classifies the degrees of protection against intrusion from solid objects like dust, and ingress of water.
Most consumer SD cards have an IPX7 rating, meaning they are waterproof in fresh water up to 1 meter for 30 minutes. However high endurance cards like SanDisk Extreme Pro are rated IP55 for protection from limited dust ingress and low pressure water jets.
For shock protection, SD cards are tested to withstand 1500G of force. They can operate in temperatures ranging from -25°C to 85°C. Premium cards are tested more rigorously, with SanDisk claiming operation in -40°C to 85°C temperatures.
The NAND flash memory cells in SD cards can withstand thousands of read/write cycles. However, real-world lifespan depends on usage and environmental factors.
Security Standards
SD cards support various security standards to protect data. One of the main standards is CPRM, which stands for Content Protection for Recordable Media. CPRM is a type of Digital Rights Management (DRM) that encrypts media files on SD cards to prevent unauthorized copying. CPRM is supported on both SD and microSD cards (ATP Inc.).
Many SD cards also now support full disk encryption using AES-256 bit encryption. This encrypts all data written to the card (ATP Inc.). The data remains encrypted when stored on the card and is only decrypted when read by an authorized host device. This protects the data at rest if the SD card is lost or stolen. Some SD cards are self-encrypting while others need third-party software to enable encryption.
Compatibility
SD cards have various levels of backward and forward compatibility depending on the SD card type and host device:
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SD cards are backwards compatible – SDHC devices can use SD cards, and SDXC devices can use SD and SDHC cards. However, the opposite is not true – SD devices cannot use SDHC/SDXC cards (SD Card Capacity Choices).
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SDXC cards can only be used in SDXC devices, but SDXC devices can use SD, SDHC, and SDXC cards (SD Card Compatible Devices).
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Most current consumer devices like phones, tablets, cameras, and laptops support SD, SDHC and SDXC cards (WD Specifications).
Overall, it’s important to check your device’s compatibility before purchasing an SD card to ensure forward and backward compatibility.
Choosing the Right Card
When selecting an SD card, there are several factors to consider:
Capacity – SD cards come in sizes ranging from 4GB to 1TB. Choose a capacity that fits your storage needs. Larger capacity cards are better for lots of photos/videos, while smaller cards work for basic usage.
Speed – SD cards have speed classes indicating minimum performance. Higher speeds allow for faster write/read speeds when capturing bursts of images or 4K video. UHS-I and UHS-II cards are the fastest options.
Compatibility – Check your device’s manual to see which SD standards it supports. Newer devices usually take the fastest standards. Older hardware may be limited to slower cards.
Brand – Stick with major brands like SanDisk, Samsung, Sony when possible. Avoid cheap generic cards that may have reliability issues.
Use Case – Consider your intended usage. For photos/video, prioritize speed and capacity. For gaming, focus on speeds over 64MB/s. For everyday use, any decent card will suffice.
Some common recommendations:
– Photography: SanDisk Extreme Pro SDXC UHS-II or Samsung PRO Endurance
– Video recording: SanDisk Extreme SDXC UHS-I or Sony Tough SDXC UHS-II
– Gaming: SanDisk Ultra SDXC UHS-I or Samsung EVO Select
– General use: Any UHS-I card from major brand, aim for 64GB+ capacity