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Thursday, January 27, 2011

Genealogist's View -- More about memory cards -- Part two

http://en.wikipedia.org/wiki/File:Memory-card-comparison.jpg -- Public Domain

Digital memory is measured in bits and bytes. I am not going to get into a discussion of binary numbers and showing how to calculate in decimal and binary, but to understand computer memory and digital memory of all kinds, it is helpful to recognize the terminology and the relative sizes of the steps between different levels of storage media. Before showing the illustration, there are one or two points that need mentioning. First, 8 bits = 1 byte. It doesn't do any good to ask this question any more than it does to ask why there are 4 quarts in a gallon. There is a good historical reason which has long since been almost forgotten or never learned. The second is that commonly the real number of bytes at any level of storage device is rounded to a simple number. The progression of the numbers is usually indicated by a prefix on the word "byte" such as Kilobyte.

There is a simple progression here illustrated:

1 byte = 8 bits
1 Kilobyte KB = 1,024 bytes
1 Megabyte MB = 1,048,576 bytes
1 Gigabyte GB = 1,073,741,824 bytes
1 Terabyte TB = 1,099,511,627,776 bytes
1 Petabyte PT = 1,125,899,906,842,624 bytes

We'll knock it off there, suffice it to say the next three steps are Exabytes, Zettabytes and Yottabytes. In real life these steps are usually expressed much more simply by noting that each level is 1000 of the previous level, i.e. a kilobyte is a 1000 bytes and a megabyte is a 1000 kilobytes, and so forth. Although not strictly true, it makes the whole idea a little less complicated and is near enough for everyone except those who build computers.

OK, let's get on with the discussion of memory cards. They come in sizes expressed in the terminology above (there are no terabyte or petabyte memory cards, yet) Common memory cards today (2011) are usually around 2 to 4 Gigabytes (2GB to 4GB) Since this size card usually sells for less than $25 (some for much less) the discussion should probably end there. But it is interesting to note that the largest cards available today, either a 64GB SDXC card or a 64GB CompactFlash card, cost more than $250.

If you are buying memory cards, buy two or three 8 or 16 GB cards rather than one 64GB card. Some professional photographers recommend a selection of even smaller memory cards, just so that loss of one card does not lose that many pictures. Bear in mind that an 8 GB card can store hundreds of pictures at lower resolution. Even at the highest resolution the camera supports, you can still store well over 100 pictures on an 8GB card. Some 8GB cards are priced at under $10.

Even though the memory cards are robust and can take a lot of abuse, why should you try to make one fail. The cards should be kept very clean and away from dust, moisture and oils, i.e. they should not be carried uncovered in a purse or pocket. Most cards come with a little plastic container. Use it. Don't throw it away.

Be careful when inserting the cards into the slot in your camera or other device. Make sure the contacts, the little metal strips, are facing the right direction and don't force the card into the slot. If it seems hard to insert, it is probably not oriented properly.

Although, as I have finally gotten straight, the cards can potentially last for many years, both the cards and the cameras are rapidly evolving and it is possible that the type of card used by your camera will disappear and no longer be supported by the original manufacturer. If you cannot readily buy a replacement card, it is time to think about upgrading your camera to a new model using a more available card. Unfortunately, most of us think of cameras as a long term investment. With digital cameras, that mind set is in need of change. Cameras have become like any other electronic device, subject to obsolescence.

Next time I will talk about card readers and some other uses for memory cards.

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