Speed Class | SD Association

SD Standard for Video Recording

There are wide discrepancies in memory access speed depending on the SD memory card manufacturer and brand. Varying speeds make it difficult to make out which card can surely record streaming contents.Recording video require a constant minimum write speed to avoid ‘frame drop’ during recording for a smooth playback. The SD Association has defined various Speed Class standards to answer a demand for advanced video quality recording. Speed Class symbols indicated to host and card products help users decide the best combination for reliable recording (no frame drop). The various speed class types assures minimum sequential card access performance including multiple stream access (in SD Express Speed Classes) under defined conditions specified in the standard. It is most important for video recording and other applications.  It is also applicable for any write or read applications that requires an assured minimum constant sequential access performance.

There are four kinds of speed indications:

Speed Class*, UHS Speed Class**, Video Speed Class*** and SD Express Speed Class**** symbols with a number indicate minimum writing speed. This is mainly useful for camcorders, video recorders and other devices with video recording capabilities. Regarding bus mode, it is necessary to use a bus mode fast enough that does not affect memory write speed. C10 is used in High Speed mode or faster, U1 and U3 are used in SDR50/DDR50 or faster , V60 and V90 are used in UHS-II mode or faster, E150/E300/E450/E600 are used in SD Express mode.

Video Speed Class and SD Express Speed Class are defined to answer a demand for high resolution and high quality 4K8K video recording and it also has an important feature to support next generation flash memory such as 3D NAND. In addition, the SD Express speed class supports also multi stream access.

The main reason driving many speed class types is the evolution of NAND memory technology.  While the early speed class types – Speed Class and UHS Speed Class were optimized for older memory technologies, the latest types of Video Speed Class and SD Express speed class are both optimized for the latest NAND flash technologies allowing best utilization of today’s NAND technology.

SD Express Speed Class is based on the same concepts of the Video Speed Class access methods with the following unique features:

– Defined to be used with NVMe protocol

– Supports multi-stream access

– Power and Thermal management optimized for the speed class support.