The markings found on SD or microSD cards provide essential information for users to determine the card's suitability for specific purposes. These markings include details about memory capacity, minimum read and write speeds, and various speed classes.
Cards prominently display their memory capacity, typically in gigabytes (GB). While less common, some cards may indicate capacity in megabytes (MB). For instance, a card might be labeled as 64 GB.
Four memory format markings categorize cards based on their range:
SD or SDSC (Standard Capacity): Maximum of 2 GB (could be less).
SDHC (High Capacity): Ranges from 2 to 32 GB.
SDXC (Extended Capacity): Ranges from 32 GB to 2 Terabytes.
SDUC (Ultra Capacity): Ranges from 2 to 128 TB (uncommon).
Speed Classes
Three main speed classes are employed to determine a card's suitability for specific applications:
Speed Class: Represented by a number in a circle (e.g., 2, 4, 6, 10). Class 10 is suitable for burst shooting, full HD, some 4K filming, and raw photography.
UHS (Ultra High) Speed Class: Indicated by a number inside a U, with Class 1 at 10 MB/s (equivalent to speed class 10) and Class 3 at 30 MB/s. UHS Class 3 is often required for ultra-high resolution filmmaking.
Video Speed Class: Varies from V6 (6 MB/s) to V90 (90 MB/s). V60 and V90 speeds are essential for filming in 8K.
Bus Speed Class
Another relevant speed class is the Bus Speed Class, determined by peak bus throughput:
Default Speed (DS): 12.5 MB/s
High Speed (HS): 25 MB/s
Ultra-High Speed I (UHS-I): 50-104 MB/s
Ultra-High Speed II (UHS-II): 156-312 MB/s
Ultra-High Speed III (UHS-III): 213-624 MB/s
SD Express: 985-1969 MB/s, 1969-3964 MB/s
UHS-I operates in Half Duplex mode, while UHS-II and III support Full Duplex. SD Express always runs in Full Duplex.
Random Read Performance
The ability to handle random data instructions (measured in IOPs) is crucial for consistent system performance. The Application Class A1 and A2 standards, introduced in 2015, include the following ratings:
A1: Random read performance 1500 IOPs, random write performance 500 IOPs.
A2: Random read performance 4000 IOPs, random write performance 2000 IOPs. A2-rated cards also require hardware support and ensure a video speed rating of at least V10 (10 MB/s minimum read and write speed).
