What are memory cards?

Memory cards, also know as flash cards or memory cartridges are an electronic flash memory (non-volatile memory that can be erased and reprogrammed) data storage device used to store digital data. Nowadays these memory cards are used mainly in portable devices such as digital camera, smartphones, laptop computers, tablets, etc.

Historically speaking memory cards came in a lot of shapes and sizes since their launch in 1994. The very first memory card was the CompactFlash, but due to its larger size it wasn’t the best suitable for portable devices. Although the SD (Secure Digital) card where developed by the SD Card Association in 1999, only by 2005 SD/MMC cards gained the popularity on the market.

Below you can find a table with all the memory cards that were launched together with their form factor:

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NameAbbreviationForm factorDRM
PC CardPCMCIA85.6 × 54 × 3.3 mmNo
CompactFlash ICF-I43 × 36 × 3.3 mmNo
CompactFlash IICF-II43 × 36 × 5.5 mmNo
CFexpressCFX38.5 × 29.8 × 3.8 mmUnknown
SmartMediaSM / SMC45 × 37 × 0.76 mmNo
Memory StickMS50.0 × 21.5 × 2.8 mmMagicGate
Memory Stick DuoMSD31.0 × 20.0 × 1.6 mmMagicGate
Memory Stick PRO DuoMSPD31.0 × 20.0 × 1.6 mmMagicGate
Memory Stick PRO-HG DuoMSPDX31.0 × 20.0 × 1.6 mmMagicGate
Memory Stick Micro M2M215.0 × 12.5 × 1.2 mmMagicGate
Miniature Card37 × 45 × 3.5 mmNo
Multimedia CardMMC32 × 24 × 1.5 mmNo
Reduced Size Multimedia CardRS-MMC16 × 24 × 1.5 mmNo
MMCmicro CardMMCmicro12 × 14 × 1.1 mmNo
P2 cardP2No
Secure Digital cardSD32 × 24 × 2.1 mmCPRM
Universal Flash StorageUFSUnknown
miniSD cardminiSD21.5 × 20 × 1.4 mmCPRM
microSD cardmicroSD15 × 11 × 0.7 mmCPRM
xD-Picture CardxD20 × 25 × 1.7 mmNo
Intelligent StickiStick24 × 18 × 2.8 mmNo
Serial Flash ModuleSFM45 × 15 mmNo
µ cardµcard32 × 24 × 1 mmUnknown
NT CardNT NT+44 × 24 × 2.5 mmNo
XQD cardXQD38.5 × 29.8 × 3.8 mmUnknown

This article aims to present you one of the most popular memory cards used today and that is the SD card family.

SD card family

As already mentioned before the SD card family was developed by the SD Standard Association. Their aim was to deliver high performances products in a standardized way. As you have seen before there were a lot of shapes and sized of the memory cards. Each manufacturer tried to use its own format.

Thanks to this standardized approach the market has a unified product which is compatible with a large number of products. Whether we speak about mobile phone, MP3 players, tablets, car navigation, E-books and many other consumer electronic devices, all have the possibility to utilize the same memory card.

Form factor

When it comes to shape and size the SD card family had initially three defined form factors:

  • Standard Size SD
  • miniSD
  • microSD

As microSD took over the miniSD the number of form factors was reduced to only two sizes.

Original SD Card (alias non UHS-II Card) has the first-row pins including 3.3V VDD that are used for bus interface such as Default Speed, High Speed and UHS-I modes. Standard Size Card has 9 pins (two VSS) and microSD has 8 pins (one VSS).

A new pin layout on the second-row was introduced to support the UHS-II bus interface and also support UHS-III for the two form factors. 1.8V VDD is included in the second-row pins and then UHS-II/UHS-III Cards require two power supply voltages 3.3V and 1.8V.

Pin layout

Original SD Card (alias non UHS-II Card) has the first-row pins including 3.3V VDD that are used for bus interface such as Default Speed, High Speed and UHS-I modes. Standard Size Card has 9 pins (two VSS) and microSD has 8 pins (one VSS).A new pin layout on the second-row was introduced to support the UHS-II bus interface and also support UHS-III for the two form factors. 1.8V VDD is included in the second-row pins and then UHS-II/UHS-III Cards require two power supply voltages 3.3V and 1.8V.

Non UHS-II microSD Card may extend two antenna pads for contactless applications but antenna pads will interfere the second row of UHS-II/UHS-III Cards. NFC (Near Field Communication) Interface was defined alternatively that use Single Wire Protocol via a pad on the second row.

Memory card capacity

When it comes to memory card capacity there are three standards:


Standard SD – have a maximum capacity up to 2 GB using FAT 12 and FAT 16 file system



SDHC standard – which refers to High Capacity includes memory cards with a storage capacity between 2GB – 32GB using FAT 32 file system


SDXC standard – which refers to Extreme Capacity includes memory cards with storage capacity between 32GB – 2 TB using exFAT file system

More details about  each capacity standard can be found in the below table:

Bus speed

SD bus speed of 12.5MB/s Default Mode was defined by SD1.0 and 25MB/s High Speed Mode was defined by SD1.1 to support digital camera. Its high performance function for continuous shooting required higher write speed memory card where SD Association introduced higher speed bus interfaces for SDHC and SDXC memory cards named UHS-I, UHS-II and UHS-III.

UHS-I can provide ultimate bus speed using 1st Row pins. UHS-II and UHS-III have ability to provide much higher bus speed than UHS-I using 1st Row and 2nd Row pins. 2nd Row interface is supported with Low Voltage Differential Signaling (LVDS) technology.
UHS-II and UHS-III has two lanes. Full Duplex (default mode) assigns a lane to downstream direction (host to card) and the other lane to upstream direction (card to host) and then packets can be transferred both directions at the same time. Half Duplex switches lanes to the same direction during data transfer to make data transfer rate twice (156MB/s in Full Duplex can be switched to 312MB/s in Half Duplex). However, UHS-III Card does not support Half Duplex for 312MB/s Full Duplex and 624MB/s Full Duplex to make PHY design simple.

Memory card speed classes

When it comes to purchasing memory cards the most important aspect you need to take into consideration is the speed class of the memory card. But what exactly are these speed classes?

Speed Class*, UHS Speed Class** and Video 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, and V60 and V90 are used in UHS-II mode or faster.


Although many of you would be tented to go with SD XC II memory card as they assure the highest writing speeds, thus you should always read the user manual of your electronic device to check the compatibility with different memory card speed classes.

How to choose the best card for your needs

For video recording

Speed Class supported host can indicate Speed Class symbol somewhere on the product, package or manual. Consumers can find the best card for a host via Speed Class symbol match; choose the same or higher class symbol card than class symbol of the host indicated.

For example, if your host device requires a Speed Class 4 SD memory card, you can use Speed Class 4, 6 or 10 SD memory cards. If your host device requires a UHS Speed Class 1 SD memory card, you can use UHS Speed Class 1 or 3 SD memory cards. Video Speed Class is also the same. Note that expected write speed will not be available by a combination of different class symbols such as Class 10, U1 and V10 even those are indicated to the same 10MB/sec write speed.

In the chart below you can find the illustration of the recommended memory card you should choose based on the resolution of the recorded video and frame rate you use. If for full HD video recording you will get along with a class 10 memory card, the game changes once you go up like 4K or even 8K. While 4K is becoming more and more popular, it seems that the next step will be the use of 8K as standard. Although for the moment it is not popular and there aren’t so many cameras capable to shoot 8K, this could shortly change in the coming years. It is better to be prepared

For photography

Generally speaking a lot of people ask what cards should they choose for their camera. Well the answer really depends. When picking a camera you should consider the following aspects:

  1. The camera you are using. While for low end DSLRs you will be more or less fine with a Class 10 speed card like UHS-I, for higher end models it is recommended to choose a faster card. The reason is quite simple. First of all the size of an image taken at 24 or 36 Mpx is significantly larger than of a 13 Mpx image. Not to mention if you have the possibility to shoot raw. You will need more time to write those larger files to the memory card. Here comes into the scene a faster card like a  UHS-II/UHS-III. But please always check your cameras manual to see whether it is compatible with your camera
  2. Type of photography. If you are shooting still life or something which doesn’t require large amounts of data to be written to the memory card you will be fine with UHS-I cards. If you are for example shooting sports or some wildlife you will want to write the images as fast as possible in order to free up your buffer.
  3. Whether you are using your camera for video recording. As seen in the previous section for video recording we have seen how frame size and frame rate influence the type of card you should choose.

What to choose?

While there will be some people saying that one brand or memory card is better over the other what I can tell is that generally speaking a better branded memory card will perform better over a no name. Nowadays there are a lot of fake or with incorrect information stated memory cards. Therefor it is advised to pay attention where you purchase your memory cards. The second aspect is related to your needs. While you will be probably fine with a Class 10 speed card, for some people a Class 4 or 6 memory card is enough. Therefore below I will make some recommendations based on different speeds:

  • Class 4 memory card when you prefer capacity over writing speed. These cards are cheaper so it won’t let you a deep hole in your pocket
  • Class 10 memory cards when you want speed and capacity, but still don’t need the extreme version memory cards. This is suitable for the vast majority of devices and cameras. I really recommend these as general purpose memory cards.
  • U1 and U3 memory cards when speed is a must. Most modern DSLRs cameras and action cameras really need ultra fast cards to have the job done without any hiccups.

What capacity to choose?

Very often people are tented to buy the highest memory card capacity. There isn’t an issue for that unless you’re data is crucial. Why? Because for example you can go on a holiday with a single 64GB memory card for example. But, what happens if the card fails, gets lost. Well in this case you lose everything. This can be a big issue for example when you go for a photo shoot for a client.

What to do in this case?

Well it is advised to have more memory cards with smaller capacities. This way if one card fails part of your data will be still there. Additionally the probability to have corruption of multiple memory cards at the same time is minimum.

In addition to what we discussed earlier it would be great to store the memory cards in multiple locations just in case your bag is stolen or lost in the airport. Most modern DSLR cameras with 2 card slots allow you to mirror the data on two cards, therefor you can have a back-up.

Final thoughts

We have seen the major aspects you need to take into consideration when purchasing a memory card for your DSLR, action camera, ebook, etc. I really recommend you to read your device’s instruction manual to see compatibility with certain memory cards. Pay attention to the supported memory card speed class and maximum storage capacity.

If you found this article relevant and interesting you can leave a feedback in the comment section. What do you prefer? Having a larger capacity memory card or more cards with smaller capacities?

If you want to buy other camera accessories for your camera I recommend you this article.


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