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Many years ago, the SD Association (SDA) came up with a way to standardise the speed ratings for microSD and SD flash memory cards, much like the USB-IF did for USB standards, for manufacturers to follow and for customers to use in determining the right card for their needs. The SDA created three types of speed classifications and recently added a fourth:
Speed Class – represented by a circular “C” symbol
Ultra-High Speed (UHS) Class – represented by a “U” symbol and a continuation of the first Speed Class
Video Speed Class – represented by a “V” symbol
SD Express Speed Class – represented by an “E” logo
Speed Class
UHS Speed Class
Video Speed Class
"Speed Class" Mark
"UHS Speed Class" Mark
"Video Speed Class" Mark
Altogether, these speed classes dictate the absolute minimum sustained write speed, level of video quality recording and host communication mode. If a product is built to meet these defined standards, the memory card manufacturer makes sure to include the appropriate marking on the card so that you, the user, know exactly what you’re getting.
Speed Class and Ultra-High Speed (UHS) Class
Speed Class was one of the original standards and was created to define minimum write speeds for what are now older memory technologies, classified by four performance tiers.
C2 - minimum sustained write speed of 2MB/s
C4 - minimum sustained write speed of 4MB/s
C6 - minimum sustained write speed of 6MB/s
C10 - minimum sustained write speed of 10MB/s
Minimum sequential write speed
Speed Class
Corresponding video format
Speed Class
UHS Speed Class
Video Speed Class (new)
8K video
4K video
Full HD / HD video
Standard Video
Card image
The necessary speed varies by each recording/playback device condition, even in the same format.
90MB/sec
II
60MB/sec
II
30MB/sec
/ I
10MB/sec
/ I
6MB/sec
4MB/sec
2MB/sec
As technology improved and higher write speeds were reached more consistently, the SDA created the Ultra-High Speed (UHS) Class to categorise newer builds into 10MB/s and 30MB/s minimum write performance tiers and add UHS bus interface classification.
U1 – minimum sustained write speed of 10MB/s
U3 – minimum sustained write speed of 30MB/s
Take a look at your microSD or SD card. Does it say C10 and U1? In terms of speed class, don’t those both mean minimum write speed of 10MB/s? Yes, they do! So why do we still include both? Because not everyone has read this article, so spread the word! But seriously, many card manufacturers continue to use both logos on their product markings and in their specification material purely because not everyone is aware they are the same and may be looking for one and not the other. If you do see a card that has one logo and not the other, rest assured, you are still getting a 10MB/s minimum write speed card.
U3 on the other hand, can actually achieve 30MB/s minimum write speed. This is desirable for high-end cameras and those taking photos and videos in 4K resolution. U3 is also shared by the two most popular UHS bus interfaces, whereas U1 is limited to UHS-I:
UHS-I: theoretical maximum read/transfer speeds of up to 104MB/s
UHS-II: theoretical maximum read/transfer speeds of up to 312MB/s
(Of course, we won’t forget to mention UHS-III, the latest classification with theoretical maximum transfer speeds of up to 624MB/s, but its applications are more enterprise and industrial than consumer.)
What does UHS bus interface mean and why is it important? I’m glad you asked! Simply put, the UHS bus interface dictates the method in which the card communicates with the host device, which could be a camera or a reader. If you have a UHS-II camera and use a UHS-I card, the best theoretical transfer performance your card can achieve is 104MB/s, unless it is built with performance-enhancing firmware. And if you have a UHS-II card with U3 markings and nothing else, it means you have a card with a theoretical transfer performance of up to 312MB/s (with UHS-II supporting products) and a minimum write speed of 30MB/s.
But if you have a professional-grade UHS-II camera to shoot 4K, 8K or 3D videos, don’t you need higher than 30MB/s min. write speed?
UHS Speed Class
The next speed class up is the UHS (Ultra-High Speed) Speed Class and it’s denoted with the “U” symbol. There are two ratings within the UHS Speed Class:
U1 (UHS Speed Class 1): minimum write speed of 10MB/s
U3 (UHS Speed Class 3): minimum write speed of 30MB/s
The UHS Speed Class is more commonly used nowadays than the Speed Class and many high-end cameras require at least a U3-rated memory card for many of its functions, such as recording high-resolution videos. The UHS Speed Class mainly refers to the minimum sustained write performance for recording videos and came about due to 4K-capable video recording devices needing faster write speeds. As a rule of thumb, 4K-capable recording cameras will usually require at least a U3-rated SD card.
What makes the U1 and U3 memory cards more advanced than those in the Speed Class are that they use one of two UHS bus interfaces:
UHS-I: theoretical maximum transfer speeds up to 104MB/s
UHS-II: theoretical maximum transfer speeds up to 312MB/s
Both U1 and U3 memory cards can utilise the UHS-I bus interface, but are not compatible with the UHS-II bus interface.
These UHS bus interfaces indicate the theoretical maximum read and write speeds, unlike the sustained write speeds of speed classes. The UHS bus interfaces are denoted by a Roman numeral “I” or “II” symbol on the front of the card. The bus speeds refer to the theoretical data transfer rate of the interface itself while a U3-rated SD card has its own sustained write speed of 30MB/s. For example, a UHS-I U3-rated card guarantees a write speed of 30MB/s but has the potential for a read and write speed of up to 104MB/s if used with a device that supports a UHS-I bus interface.
A UHS-II compatible card has a potential read and write speed of up to 312MB/s. The UHS bus interfaces are backwards compatible so you can use a UHS-II card in a device that supports UHS-I, but you won’t see the speed benefits of UHS-II as the card will default back to the lower specs of UHS-I. Both the card and bus interface must be fully compatible to experience the speed benefits.
Video Speed Class
Like Speed Class and UHS Class, the Video Speed Class has some slight performance metric overlaps but is unique in that it focuses purely on card usage applications, specifically video recording. If you are looking for a microSD card to put in your smartphone, a minimum write spec of 30MB/s is perfect, but if you are planning to record multiple video streams, capture 360-degree footage, create VR content or record in 4K/8K resolution, the Video Speed Class is for you!
V6 - minimum write speed of 6MB/s
V10 - minimum write speed of 10MB/s
V30 - minimum write speed of 30MB/s
V60 - minimum write speed of 60MB/s
V90 - minimum write speed of 90MB/s
UHS-I cards can fall into the Video Speed Class 6, V10 or V30 category but only UHS-II cards can meet the standardised definition of V60 and V90. So, going back to the UHS-II professional-grade camera you are taking 8K videos with, you’re going to be looking for a card that is marked with a UHS-II and a V90 logo to ensure you get the best performance out of your camera, without missed frames or low-quality images. Or maybe you’re looking for a microSD card to use with your mobile gaming device that doesn’t require UHS-II compatibility but needs to be fast enough to avoid lag; look for that U3 and V30 logo instead!
More DRAM means faster PC performance when editing from video, from playback responsiveness to render time. But is 8, 16, 32 or 64GB enough? How much do you need for 1080p, 4K, or 8K?
Robin Trygg is one of Sweden’s most well-known adventurers and he trusted Kingston memory products to support him in documenting two expeditions to Mount Everest.
