SD Card Speed Classes, Grades, Bus Modes, and File Systems Explained
What you never knew you didn't know
While researching a few upcoming SD / microSD product reviews here at PC Perspective, I quickly found myself swimming in a sea of ratings and specifications. This write up was initially meant to explain and clarify these items, but it quickly grew into a reference too large to include in every SD card article, so I have spun it off here as a standalone reference. We hope it is as useful to you as it will be to our upcoming SD card reviews.
SD card speed ratings are a bit of a mess, so I'm going to do my best to clear things up here. I'll start with classes and grades. These are specs that define the *minimum* speed a given SD card should meet when reading or writing (both directions are used for the test). As with all flash devices, the write speed tends to be the more limiting factor. Without getting into gory detail, the tests used assume mostly sequential large writes and random reads occurring at no smaller than the minimum memory unit of the card (typically 512KB). The tests match the typical use case of an SD card, which is typically writing larger files (or sequential video streams), with minimal small writes (file table updates, etc).
In the above chart, we see speed 'Class' 2, 4, 6, and 10. The SD card spec calls out very specific requirements for these specs, but the gist of it is that an unfragmented SD card will be able to write at a minimum MB/s corresponding to its rated class (e.g. Class 6 = 6 MB/s minimum transfer speed). The workload specified is meant to represent a typical media device writing to an SD card, with buffering to account for slower FAT table updates (small writes). With higher bus speed modes (more on that later), we also get higher classes. Older cards that are not rated under this spec are referred to as 'Class 0'.
As we move higher than Class 10, we get to U1 and U3, which are referred to as UHS Speed Grades (contrary to the above table which states 'Class') in the SD card specification. The changeover from Class to Grade has something to do with speed modes, which also relates with the standard capacity of the card being used:
U1 and U3 correspond to 10 and 30 MB/s minimums, but the test conditions are slightly different for these specs (so Class 10 is not *exactly* the same as a U1 rating, even though they both equate to 10 MB/sec). Cards not performing to U1 are classified as 'Speed Grade 0'. One final note here is that a U rating also implies a UHS speed mode (see the next section).
With the classes and grades out of the way, we can get to the physical specs. Unlike the former, which dictate the minimum speed, Bus Modes dictate the maximum possible speed. Note that I said possible speed. We are talking about the maximum throughput over the physical interface to/from the SD card here:
As SD cards have progressed over the years, these upgraded physical specs have been added to improve throughput. The first few jumps in speed were accomplished by increasing the data clock and lowering the signal voltage, but the move to the UHS-II grades required a physical change to the SD cards, adding an additional row of contacts.
This additional row adds two differential pairs for data along with additional supply voltage and ground. The two differential pairs default to one pair for each direction (full duplex - 156 MB/s each direction). If the SD card and reader support it, even higher throughputs are possible by reconfiguring both pairs act in tandem (half duplex). This change can be done on-the-fly and fully doubles the maximum throughput to 312 MB/s. The catch is that the communication can only be in one direction at a time, but that is not really an issue with flash based storage, which is typically not mixing reads with writes simultaneously. Here is one more chart showing the various speeds graphically:
Finally, here is another table showing the approved logos associated with the various speed grades:
Commercial 'x' Speed Ratings
We've covered minimum rated and maximum theoretical speeds so far, but what do you look for when you just want to know how fast you can empty that SD card full of photos onto your PC? There is such a rating, though it is an unofficial figure that is not required per the SD card spec. These optional figures appear in two different ways:
- Commercial 'x' rating
- Optimal MB/s values
The 'x' rating is a throwback from CD-ROM days, where 1x = 150 KB/s. As these are optional specs, they vary by manufacturer. Some brands do not list optimal throughputs at all, while others (Lexar and Transcend) list both styles. In the above example, Panasonic went the extra mile by being the only manufacturer we saw listing read *and* write optimal throughputs directly on their SD card label (most others only list optimal read speeds, as optimal write speeds will typically be a lower value due to the way flash architecture works). The below table is a bit of a wrap up of all of the above, outlining all of the various ratings and where they fall on a MB/s scale. I've also included 'x' rating conversions for those values, but realize they are not nececssarily directly related to the Classes and Grades of a given card:
One big photographer note on these speeds. Most cameras and other portable recording devices do not operate in UHS modes in the interest of power conservation, and many of them limit themselves to SDR25 or SDR50 for that same reason. This means cameras can empty their buffers only as fast as the maximum physical bus speed of the SD interface present in that camera. Most modern SD cards are capable of sustaining those maximums. This means that SD cards with crazy high optimal speed ratings may see no difference in continuous shooting on their expensive camera. Those faster cards will only see the added speed when unloading the card to a PC, or when using that SD card as a secondary storage device. Keep in mind the card reader must be capable of those higher bus speeds.
File Systems and Compatibility
The SD Card Association has mandated the above file systems for the corresponding SD card types. FAT file systems are standard across the board. exFAT is used in devices greater than 32GB, and while it requires a license to use, it remains part of the SDXC spec.
One final note on SD card speeds and readers. The issues you run into with forwards / backwards compatibility of SD readers and cards is usually related to the *logical* issue of addressing larger amounts of available storage. SD cards do their best to be backwards compatible *physically*, in that they will fall back to slower bus speed modes. This was why UHS-II's physical spec adds contacts and leaves the existing ones unaltered. Due to hardware similarities between SDHC and SDXC, it is possible for an SDXC card with an exFAT file system to become corrupted if inserted into an SDHC reader that only understands FAT32. Some SDHC devices may even auto-format the card (to a max 32GB Fat 32 capacity - 2GB FAT16 in even older devices). This is usually a quick format operation which would leave most of the content unaltered, but without the original File Allocation Table, that previously saved media would be inaccessible without the use of data recovery software. This warning also applies to older Operating Systems. Windows Vista and 7 users will need an exFAT Hotfix to be applied and might also need an SDXC specific driver installed. Be sure your hardware and software specifically supports SDXC prior to buying any SD cards greater than 32GB in capacity.
Properly Formatting SD Cards
You might have noted empty space before the first partition of an SD card. This space is actually a part of the SD specification and some cards treat this area differently than the rest of the flash on the card. Some people love to reformat their SD cards to get that tiny bit of extra space back, but this is not recommended for several reasons. Placing the 'improperly' formatted card into a camera or video recording device that has firmware optimized for 'standard' SD formats may corrupt the partition as it is not where it expects it to be. Improper partitioning may also result in the partition being physically misaligned with respect to the flash area, which may throw off any wear leveling routines and wear the flash at an accelerated rate. If you've reformatted all of your cards and are now getting that sinking feeling, don't worry, there's a way to get them back to where they are supposed to be:
The SD Card Association puts out their own format utility (download here). This utility is available for both Mac and Windows and contains the same standard code that sits on SD-compatible cameras. This means you will get the same 'correct' partition alignment and format with this utility as you would get when formatting in a camera, which also happens to match the shipping condition of these cards. If you have repartitioned your SD card, you should delete that partition prior to running the SDFormatter. This will force the creation of a new partition with an offset correct for the specifications of that particular card.
Well that's about all of the information I can cram into this reference. We have some SD and microSD card reviews coming up to help fill in the performance numbers that may have been omitted by the manufacturers. Specification and rating related questions are welcome in the comments below.