Monday, January 7, 2008
Flash Memory Basics
What's the anatomy of a memory card?
The other type of flash Samsung makes is called multilevel cell (MLC). MLC flash differs from SLC flash in the amount of charge each cell can hold (Figure 4b). An SLC flash holds one bit of information per cell. It's either on or off. MLC holds two bits of information per cell, meaning it holds four levels, or twice as much data as SLC.
To help illustrate this, consider a glass of water. For SLC, it's either full or empty. Even if it isn't completely empty or full, the difference between the levels is already large enough that you can easily tell which is which. MLC is a little more complex. Instead of the glass having only two states, full or empty, there's now also a one-third-full and two-thirds-full state.
With the same process geometry as the SLC, MLC die can be produced at 16 Gb or 2 GB each, essentially doubling the total memory capacity of a wafer at the same cost.
So, if you can achieve twice the capacity at a lower cost using MLC, why in the world would anyone in their right mind continue to use SLC? Well, there's a catch, and the answer should be particularly relevant to photographers, especially those earning a living shooting.
Because of its design complexity, MLC flash takes four times longer to write to than SLC, and MLC's life span is only one-tenth that of SLC. This means that it's primarily used in slower non-speed-rated, low-cost cards, USB Flash Drives or non-mission-critical devices, such as MP3 players.
To achieve the high-speed 300x UDMA speeds and offer lifetime warranties, card manufacturers must continue to use SLC flash. As long as consumers continue to value these professional-grade products, manufacturers will continue to make them.
So if you're serious about photography as a hobby or business, you have choices when it comes to memory. Not all cards are created equal. If you see a 2 GB card advertised for $15 or a Pro 2 GB card for $45, consider that there's a difference in the components used to make the cards, the read/write speeds, the warranty and the level of support you can expect when you need it most.
As the Romans used to say, “caveat emptor—buyer beware.” You get what you pay for, but then again, they were also carving their images in stone.
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