You might have seen new A1 and A2 speed ratings on some of the new SD and microSD cards. So what are they referring to, and should you take any notice of them?
SD and microSD cards have had several different speed rating categories. You see them in those small symbols and numbers on the cards and their packaging.
First there was the speed class, which went from Class 2 to Class 10. Then there was the UHS Speed Class, where the cards fall into either U1 or U3. More recently, there’s a newer system, the Video Speed Class, that uses categories like V30, V60, and V90.
All of those are categories where the card is rated to meet or exceed a minimum threshold for sustained sequential write speed. Basically, that means they can keep up with a certain amount of data being recorded to them constantly. The obvious case where that comes into play is when recording high-definition (and ultra-high-defintion (UHD)) video. The higher the quality and bitrate that the camera wants to record at, the faster the card needs to be to keep up with the stream of data coming at it. That’s why using a card that’s too slow in a 4K camera like a GoPro causes lockups and recording to stop–the card simply can’t keep up. One thing worth noting is that the actual speed you see in practical use is also very much dependent on the device you’re using it in, such as a camera or smartphone or other mobile device. In practice, the slowest link in the chain will determine the maximum speeds you get. So just because you have a very fast memory card doesn’t necessarily mean that you’ll see faster performance in practice.
So far, recording media like video and photos and audio has been the primary market that SD and microSD cards have been aimed at. So the speed rating systems have reflected that emphasis on recording high-resolution video.
But more and more, memory cards are being used to extend the memory of devices that run apps. Devices like smartphones and mobile gaming consoles. The apps that run on these devices interact with memory space differently. Rather than a stream of sequential data, they want to write a lot of small chunks of data wherever there’s space available. That’s known as random read/write (compared to sequential read/write that’s important for video).
As you can clearly see in the speed test results I’ve posted, a card that is fast for sequential reading and writing isn’t necessarily fast for random reading and writing. So just because a card might be able to record very high bitrate 4K or even 8K video doesn’t necessarily mean that it’s going to be a good fit for the different kind of use that applications might need.
As a result, the SD Association has come out with a new rating system specifically to categorize suitability for SD and microSD cards in application-heavy uses. It’s known as the Application Performance Class Specification, and it’s written in the form of A1 or A2. The idea behind it is that if you’re using the SD or microSD card to run applications on your mobile device, you can have confidence that a given card will be suitable before you buy it and will hopefully reduce the frustration of users buying cards blind that simply aren’t well-suited to running apps.
So far, there’s only two designated classes: A1 and A2. The SD Association will add more as they’re needed. The thresholds to meet these categories are:
To display the A1 symbol, a card must meet or exceed these thresholds:
- Random Read: 1500 IOPS
- Random Write: 500 IOPS
- Sustained Sequential Write: 10 megabytes per second
For the A2 category, it should be able to do at least:
- Random Read: 4000 IOPS
- Random Write: 2000 IOPS
- Sustained Sequential Write: 10 megabytes per second
The “IOPS” here stands for input-output access per second. It’s a different measure than we’re used to with memory cards where the speed has typically been measured in megabits per second or megabytes per second in keeping with how video bitrate is measured. But the IOPS measure is more meaningful for measuring the kinds of operations that apps do.
You’ll also notice that the specification also includes a minimum requirement for sequential write. The 10MB/s threshold corresponds to a Class 10 video speed rating, which is too slow for many of today’s cameras that record 4K video.
Is the Application Performance Class Specification Relevant to Cameras?
Not really. At least, not yet. If mobile cameras end up evolving into more app-heavy devices, maybe it will be more relevant one day.
It’s also worth pointing out that the A rating is a parallel rating system to the V and U systems. What I mean by that is that it doesn’t replace them, and it’s entirely possible–even likely–to have a card that carries both the A1 rating and a V30 rating. They’re referring to two different things.
So if you’re choosing a memory card for something like a 4K camera, you can safely ignore the A rating and focus instead on the speed ratings that start with V or U. If you’re using the card in a smartphone or gaming device, then the A rating is much more useful.
Mikko Rantalainen says
I wrote about A1 vs A2 card more at https://android.stackexchange.com/q/214545/5738
In short, A1 is better in practice even if A2 cards are supposedly better spec’d.
Claudio says
Clear info, thank you.
I understand a DJI flying camera as the Zenmuse X5S, X7 of Inspire 2 and those on Phantom 4 Pro/Advanced, and Mavic series doesn’t need A2 cards, the A1 is perfectly usable, it wouldn’t create speed bottlenecks.
I always use SanDisk microSD cards, A2, V30, U3, but now have seen a sale of Kodak cards having the same parameters, but A1 and I would like to buy someone.
However I don’t know the actual qualty and reliability over time of this so branded cards.
I think Kodak is not the producer.
David says
I haven’t tried Kodak cards. My impression was that many of their products were licensed out to other manufacturers these days, but I really don’t know anything about their memory cards. Will see if I can track some down to try.
The DJIs should mainly need fast sequential write speed cards to record the video, so the A1 or A2 ratings really shouldn’t apply. The U3, V30, V60, etc would be the relevant rating system.
ProDigit says
For a raspberry pi, I would presume an A2 card is better suited?
It came with a standard MicroSD card, But updating the system takes hours.
Since an A1 card costs only a few dollars more, and an A2 card a few dollars more than an A1 card, costing roughly 25% more than a standard MicroSD card of 16-64GB, I think it would probably be good to go with an A2?
I was just wondering how they would do on USB 2.0 card readers?
David says
I haven’t specifically tested that combination, but it makes sense. While we’re still in this rollout period for the app speed rating system, there’s no guarantee that a card with the A2 rating is faster than one with an A1 rating (or one without an A rating, for that matter), but one with an A2 rating should have good performance for that kind of use and be a safe bet. Most of these newer, faster cards will have sequential read speeds faster than many USB 2.0 card readers can do, so they’ll just roll back to card reader’s real-world max speed.
ProDigit says
Turns out that the A2 card isn’t compatible with the raspberry pi 3B+ (or older. I don’t know about the pi 4, as I don’t own one).
At least, the A2 cards require a specialized reader, and it wasn’t recognized by itself in a standard reader or in the pi cards lot).
I haven’t tried what would happen, if I would install an OS on the A2, of it still would be incompatible…
But A1 cards are heaps faster than standard class 10 MicroSD cards, plus, they appear to be more stable too. It also looks like you can both read and write to them, or write, while browsing the file list in a file browser.
Installing the update took 15min on an A1 card, vs 7 hours on a standard card (has to do with the pi using read writes on the Swap file, while using regular file operations.. Class 10 MicroSD cards are made to only do 1 file transaction at a time.
ProDigit says
Edit:A2 cards function in a raspberry pi internal card reader, and are about the same speed as an A1 card.
Their IOPS are lots faster than USB 2.0 drives, and work well.
However the pi doesn’t support these cards through an A2 card reader, and regular USB 2.0 microSD card Readers also don’t read them out of the box on Linux.
Sun says
I agree with Hatarno when I was in the shop, micro sd card A1 and non A1 has the same exact specs (class 10 and speed at 100 mb/s)
I bought the A1 cause it’s cheaper and intending to use for my camera. But as I read your post the system might be different as you say phone app cards are going to record in blocks writes rather than sequential writes. So my question is, will the A1 as card now affects for my camera? I would need to go to the shop and get it swop then :/
David says
The A1 is just a rating system, not a different kind of card, as such. For cameras, and more specifically for recording high-bitrate video, you can ignore that and instead focus on the video speed rating systems such as V30, U3, or U1. Not all cameras need cards that fast–it really depends on the specific camera model.
David says
You can ignore the A1 rating when it comes to video recording. Look instead to the C or U rating. It’s further complicated that A1 is a new rating system and not every manufacturer has or will go to the effort and expense of certifying their cards and changing all the packaging and marketing material just to add it. So it’s not necessarily safe to assume that a card with A1 on it is faster or better than one without an A1 rating on it. And in some cases you can find exactly the same card with new packaging with A1 and old packaging that doesn’t have it. So it’s best to simply ignore it if you’re buying for video recording.
sukiadi hartono says
Hi David so now I understand the A1 standard is more benefit for the smartphone user, and for 4k video recording you can just ignore it. But will the A1 card perform the same as for the non A1 card for video recording? Because the A1 card will be cheaper compare the non A1 card with the same spec.
Thanks
Johan says
I presume the A2 standard will not be compatible with many existing phones as the jump in performance is very large?
David says
That would be my assumption too, but I don’t know enough about phone processors and interfaces to say for sure.
Ran Talbott says
No, the A2 should be compatible with existing phones. The speed rating means it CAN go faster, not that it HAS to.
Often, the computer in the phone has to wait because it can move data faster than the card. With an A2 card it will spend less time waiting.
Probably: if the phone isn’t reading/writing lots of data, the difference in wait times may not matter much. So an A2 card might make the app run twice as fast, but it might only make a difference too small to notice. Depends on what the app is doing.
Chizko says
Can this A1 and A2 rating benefit devices like Nintendo Switch?.
David says
It doesn’t look as though Nintendo specifically recommends a card with an A1 or A2 rating, but that might simply be case of the manual being written before the new A rating started appearing on memory cards. But it’s precisely the kind of device that the rating is designed to help with.
ProDigit says
The switch most probably uses a USB 2.0 interface on their card reader.
For small files, an A1 card is better on this interface.