SD Memory Cards: The SD/SDHC Memory Card standard
When introduced in 1999 SDSC cards were far superior to the existing MMC cards and for numerous MMC (MultiMediaCard) devises were fully combatable, the correct designation for these original cards is SDSC (Secure Digital Standard Capacity).

 

SD Memory Cards: SD/SDHC Device Compatability
SDSC devises will not work with the the newer SDHC cards! (unless the device has had a manufactures firmware update specifically designed to facilitate use of the newer cards). They have limited memory bus support with a 4gb maximum capacity, this was ok in 1999 where the cards were used primarily for MP3 players and low pixel count cameras. SDSC devises will not work with the the newer SDHC cards! (unless the device has had a manufactures firmware update specifically designed to facilitate use of the newer cards).

SD Memory Cards: SDHC standard
The SDHC (Secure Digital high Capacity) was introduced in 2006, while being physically the same size as the older SDHC cards and having the same number of electrical connections the Card Specific Data (CSD) register was redefined within the card and now 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 SDHC cards will work with older lower capacity SDSC cards as well!

SD Memory Cards: SDXC standard
Advances in the performance and capabilities of devices using memory cards resulted in the introduction of SDXC (Secure Digital Extended Capacity) cards in 2009. SDXC cards can support up to 2 TB (2048 GB) of data, to achieve this they utilise Microsoft's exFAT file allocation system and therefore SDXC memory cards are totally incompatible with older SDSC and SDXC 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 increased 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 will not reach these increased speeds if the extra contacts were not fitted when the device was manufactured.

SD Memory Cards: The SDIO standard
SDIO (Secure Digital Input Output) cards were created for use with devises which needed an input and output functions, they have the same connections as basic SD cards but will not work correctly with devises not built to the SDIO standard, they will however cause no damage if fitted in error to a SD device not supporting the SDIO standard.

SD 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.

SD 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!

SD 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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