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Notation and Units<br />
<strong>Chapter</strong> 7<br />
Leftovers<br />
There’s an unfortunate confusion of units of measurement in the DVD world.<br />
For example, a single-layer DVD holds 4.7 billion bytes (G bytes), not 4.7<br />
gigabytes (GB). It only holds 4.37 gigabytes. Likewise, a double-sided,<br />
dual-layer DVD holds only 15.90 gigabytes, which is 17 billion bytes.<br />
The problem is that the SI prefixes kilo, mega, and giga normally represent<br />
multiples of 1000 (10 3 , 10 6 , and 10 9 ), but when used in the computer<br />
world to measure bytes they generally represent multiples of 1024 (2 10 , 2 20 ,<br />
and 2 30 ). Both Windows and Mac OS list volume capacities in “true”<br />
megabytes and gigabytes, not millions and billions of bytes.<br />
Most DVD figures are based on multiples of 1000, in spite of using notation<br />
such as GB and KB that traditionally have been based on 1024. The “G<br />
bytes” notation does seem to consistently refer to 10 9 . The closest I have<br />
been able to get to an unambiguous notation is to use kilobytes for 1024<br />
bytes, megabytes for 1,048,576 bytes, gigabytes for 1,073,741,824 bytes,<br />
and BB for 1,000,000,000 bytes.<br />
This may seem like a meaningless distinction, but it’s not trivial to someone<br />
who prepares 4.7 gigabytes of data (according to the OS) and then<br />
wastes a DVD-R or two learning that the disc really holds only 4.3 gigabytes!<br />
(See “What Are the Sizes and Capacities of DVD?” in <strong>Chapter</strong> 3,<br />
“DVD Technical Details,” for a table of capacities.)<br />
Here’s an analogy that might help. A standard mile is 5,280 feet, whereas<br />
a nautical mile is roughly 6,076 feet. If you measure the distance between<br />
two cities you will get a smaller number in nautical miles, since nautical<br />
miles are longer. For example, the distance from Seattle to San Francisco<br />
is about 4,213,968 feet, which is 798 standard miles but only 693 nautical<br />
miles. DVD capacities have similarly confusing units of measurement: a billion<br />
bytes (1,000,000,000 bytes) or a gigabyte (1,073,741,824 bytes). DVD<br />
capacities are usually given in billions of bytes, such as 4.7 billion bytes for<br />
a recordable disc. Computer files are measured in gigabytes. Unfortunately,<br />
both types of measurements are often labeled as “GB.” So a 4.5GB file<br />
Copyright 2004 by Jim Taylor. Click Here for Terms of Use.
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