7.2 Notation and units
Description
This article is from the DVD Formats FAQ, by jtfrog@usa.net (Jim Taylor) with numerous contributions by others.
7.2 Notation and units
There's an unfortunate confusion of units of measurement in the DVD world.
For example, a single-layer DVD holds 4.7 billion bytes (G bytes), not 4.7
gigabytes (GB). It only holds 4.38 gigabytes. Likewise, a double-sided,
dual-layer DVD holds only 15.90 gigabytes, which is 17 billion bytes.
The problem is that "kilo," "mega," and "giga" generally represent
multiples of 1000 (10^3, 10^6, and 10^9), but when used in the computer
world to measure bytes they generally represent multiples of 1024 (2^10,
2^20, and 2^30). Both Windows and Mac OS list volume capacities in "true"
megabytes and gigabytes, not millions and billions of bytes
Most DVD figures are based on multiples of 1000, in spite of using notation
such as GB and KB/s that traditionally have been based on 1024. The "G
bytes" notation does seem to consistently refer to 10^9. The closest I have
been able to get to an unambiguous notation is to use "kbps" for thousands
of bits/sec, "Mbps" for millions of bits/sec, "kilobytes" for 1024 bytes,
"megabytes" for 1,048,576 bytes, and "gigabytes" for 1,073,741,824 bytes.
In 1999, the IEC produced new prefixes for binary multiples: kibibytes
(KiB), mebibytes (MiB), gibibytes (GiB), and so on. These prefixes may
never catch on, or they may cause even more confusion, but they are a
valiant effort to solve the problem. The big strike against them is that
they sound a bit silly.
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