MicroSD Memory Cards: The MicroSD Card standard

The most popular memory cards today for mobile phones and other small devices is the MicroSD Card. The MicroSD card is effectively just a SD card in a reduced package, it is not actually the size of the card that has made it so popular with manufacturers but rather the reduced size of the socket in the device.


MicroSD Memory Cards: The MicroSD Size
Such a small card is not the best choice for devices which are lightly to be changed regularly due to it's potential ease of loss, devices which need cards changed regularly will use a full size SD socket. microSD cards have support for the SDIO mode (input/output) which is normally required for Bluetooth and GPS, if a device does not require the SDO mode a TransFlash card will be usable in the device as well as a microSD. As there are times when the MicroSD card will need to temporarily connect to a device with a full size SD socket the microSD cards are often bundled with an appropriate adapter when sold.

MicroSD Memory Cards: The MicroSDHC standard:
The microSDHC (Secure Digital high Capacity) used the FAT32 file system which had been in use on most PC computers, this resulted in the ability to support up to a maximum of 32gb capacity. 2009 saw the introduction of the UHS-I (Ultra High Speed) memory bus which facilitates higher transfer speeds on devises which support it. Devices built for microSDHC cards will work with older lower capacity microSDSC cards as well!

MicroSD Memory Cards: The MicroSDXC standard
Advances in the performance and capabilities of devices using memory cards resulted in the introduction of microSDXC (Secure Digital Extended Capacity) cards . microSDXC cards can support up to 2 TB (2048 GB) of data, to achieve this they utilise Microsoft's exFAT file allocation system and therefore microSDXC memory cards are totally incompatible with older microSDSC and microSDXC card system devices! These cards can also have the UHS-I (Ultra High Speed) memory bus which facilitates higher transfer speeds on UHS-1 compatible Devises. 2011 heralded the introduction of the UHS-2 (Ultra High Speed) memory bus system, this increases memory transfer speed capability even further but requires an extra row of electrical contacts, it will only work at higher speeds with devises built with a memory socket with these contacts, because of this a firmware update can never reach these increased speeds if the extra contacts required were not fitted when the device was manufactured.

MicroSD Memory Cards: Memory transfer speeds
An important part of a memory cards performance is how fast it can read and write, read speeds are always faster than write speeds, if only one speed is given it can be assumed it is the read speed which looks most impressive. Originally speeds were given relative to the speed of a standard CD disks 150 kByte/s read speed, e.g. x10 (10 times faster than a standard CD) but nowadays is given as the number of megabytes that can be transferred in a second e.g. 30MB/s.

MicroSD Memory Cards: How to find memory transfer speeds
Speeds of various cards are normally given on the memory card itself, the original packaging, or it's data sheet. Read speeds are normally not use critical as it just relates to how long it will take to transfer a given amount of data from one device to another. Write speeds however if to low can render the device using the card to be unusable until the correct card is sourced, the most obvious device this applies to is video cameras, if the card can not write as fast or faster than the video stream needed to be recorded all is lost!

MicroSD Memory Cards: Memory speed Class
As video camera users require different minimum data write speed depending on the frame rate and resolution the camera is recording at it was decided to create a class system to make the correct choice of memory card a little easier. The system uses a letter "C" with a number in it which gives a write speed in MB/s that the memory card is guaranteed to meet or exceed, on cards supporting "Ultra High Speed" memory cards the "C" is replaced with a "U" and the number is how many 10x1MB data packets can be written in one second, this means that a class C3 card can be written up to to 3MB/s guaranteed, a U3 Class card can be written to at up to 30MB/s guaranteed.


Class of Card

Minimum Guaranteed Write Speed


Video Recording Capability Guide

2 MB/s
  Adequate for standard non HD video recording.  
4 MB/s
  Adequate for 720p HD video recording.  
6 MB/s
  Adequate for 1080p/1080i HD video recording.  
10 MB/s
  Adequate for  1080p/1080i HD video recording and HD stills.  
10 MB/s
  Adequate for Real-time broadcasts + large HD video files + 3D video capture.  
20 MB/s
  Adequate for Ultra HD 4K resolution digital television and digital cinematography.  
30 MB/s
  Adequate for Ultra HD 4K resolution digital television and digital cinematography with improved inter-device transfer speeds.  

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