SD/SDHC/SDXC: Difference between Speed Class, UHS Speed Class, and Speed Ratings
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What is the difference between Speed Class and Speed Ratings for SDTM/SDHCTM cards?
The speed rating measures maximum transfer speed for reading and writing images to and from a memory card, expressed as megabytes per second. However, video doesn't need as big a data pipe because the video format is a smaller "fixed stream" that uses only a portion of the data pipe.
Unlike card write speeds that measure maximum performance, class ratings measure the minimum sustained speed required for recording an even rate of video onto the card. The class rating number corresponds to the transfer rate measured in megabytes per second. Class 2 cards are designed for a minimum sustained transfer rate of 2 megabytes per second (MB/s)1, while Class 10 cards are designed for a minimum sustained transfer rate of 10MB/s2.
What does this difference mean for me?
Rated Speed (e.g. 15MB/s, 30MB/s, etc.) is maximum speed of the card and also what you would expect to approximately see in typical usage of writing or reading files on the card. This measurement is pertinent to still photography, especially for taking pictures with high resolution and/or saving in RAW format where the files created are very large. The faster the card, the faster it can save the file and be ready to take another picture. You can really notice speed differences with high-megapixel DSLR cameras when using multi-shot burst mode.
Still digital images shot on high-megapixel cameras should utilize fast data throughput (a large data pipe), higher speed cards for improved performance. Higher speed cards can also improve how fast you can transfer the files to and from the card and your computer.
Speed Class is a minimum speed based on a worst case scenario test. The Speed Class is important for video mode or camcorders, where the device is actually saving a steady stream of data. The resolution and format of the video determines the amount of steady stream data. This translates to a minimum speed you need to guarantee that the video captured on the cards is recorded at an even, sustained rate with no dropped frames (which would result in lost data and choppy playback).
Compared to high-megapixel photography, video doesn't need as big a data pipe because the video format is a smaller "fixed stream" that uses only a portion of the data pipe. But you do need a minimum guaranteed speed for the SDHC card that satisfies the requirement of the data stream. Your camera's specifications should state the minimum SDHC Class Rating required.
Using a card without the proper class rating on a more advanced camera, such as a high-definition (HD) camcorder or Digital Single Lens Reflex (DSLR) camera with HD video record settings is likely to result in an error message indicating that video can only be recorded at a lower definition setting.
The current SDHC specification defines Class 2, 4, 6, 8 and 10 as follows:
Class - Minimum Speed
2 - 2MB/s
4 - 4MB/s
6 - 6MB/s
8 - 8MB/s
10 - 10MB/s
UHS Speed Class was introduced in 2009 by the SD Association and is designed for SDHC and SDXC memory cards. UHS utilizes a new data bus that will not work in non-UHS host devices. If you use a UHS memory card in a non-UHS host, it will default to the standard data bus and use the "Speed Class" rating instead of the "UHS Speed Class" rating. UHS memory cards have a full higher potential of recording real time broadcasts, capturing large-size HD videos and extremely high quality professional HD.
There are three main types in the SD memory card family. SD, SD High Capacity (SDHC™), and SD Extended Capacity (SDXC™). This article details the different specifications of all three types of SD memory cards and the speed class ratings and compatibilities that are different with each type.
SD capacities range from 128MB to 2GB
Default Format: FAT16
SD cards will work in all host devices that support SD, SDHC, or SDXC
SD High Capacity (SDHC™) card is an SD™ memory card based on the SDA 2.0 specification.
SDHC capacities range from 4GB to 32GB
Default Format: FAT32
Because SDHC works differently than standard SD cards, this new format is NOT backwards compatible with host devices that only take SD (128MB - 2GB) cards. Most readers and host devices built after 2008 should be SDHC compatible.
To ensure compatibility, look for the SDHC logo on cards and host devices (cameras, camcorders, etc.)
card is an SD™ memory card based on the SDA 3.0 specification.
SDXC capacities range from 64GB to 2TB
Default Format: exFAT
Because SDXC uses a different file system called exFAT and it works differently than standard SD cards, this new format is NOT backwards compatible with host devices that only take SD (128MB to 2GB). Most host devices built after 2010 should be SDXC compatible. To ensure compatibility, look for the SDXC logo on cards and host devices (cameras, camcorders, etc.). NOTE: Internal card readers on laptops from 2008 and prior may NOT support SDXC cards. SDXC cards will work in SDHC compatible readers (not SD readers) if the computer OS supports exFAT. For more information on exFat see: Operating Systems that support the exFAT File System
Ultra High Speed, Phase I (UHS-I) bus design for SDHC and SDXC cards was added in SD spec 3.0. This is a design enhancement to increase the performance of SDHC/SDXC cards.
UHS-I specification defines two bus architecture options for up to 50MB/s (UHS-50) and 104MB/s (UHS-104) data transfer rates. These are theoretical maximum limits and actual maximum performance for a specific card is defined on it label or in advertising.
UHS is an enhancement to the original SD interface specifications. Host devices will obtain the UHS maximum speed when both the card and host device support UHS. Otherwise, the host device and card will use the slower SD maximum speed obtainable.
There is no compatibility problem using a UHS card with a non-UHS device.