[7] Leftovers

[7.1] Unanswered questions

None at the moment.

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 4.37 gigabytes. Likewise, a double-sided, dual-layer DVD holds only 15.90 gigabytes, which is 17 billion bytes.

The problem is that the SI prefixes "kilo," "mega," and "giga" normally 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 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 "kilobytes" for 1024 bytes, "megabytes" for 1,048,576 bytes, "gigabytes" for 1,073,741,824 bytes, and "BB" for 1,000,000,000 bytes.

This may seem like a meaningless distinction, but it's not trivial to someone who prepares 4.7 gigabytes of data (according to the OS) and then wastes a DVD-R or two learning that the disc really holds only 4.3 gigabytes! (See 3.3 for a table of capacities.)

Here's an analogy that might help. A standard mile is 5,280 feet, whereas a nautical mile is roughly 6,076 feet. If you measure the distance between two cities you will get a smaller number in nautical miles, since nautical miles are longer. For example, the distance from Seattle to San Francisco is about 4,213,968 feet, which is 798 standard miles but only 693 nautical miles. DVD capacities have similarly confusing units of measurement: a billion bytes (1,000,000,000 bytes) or a gigabyte (1,073,741,824 bytes). DVD capacities are usually given in billions of bytes, such as 4.7 billion bytes for a recordable disc. Computer files are measured in gigabytes. Unfortunately, both types of measurements are often labeled as "GB." So a 4.5-GB file (4.5 gigabytes) from a computer will not fit on a 4.7-GB disc (4.7 billion bytes), since the file contains 4.8 billion bytes. 

To make things worse, data transfer rates when measured in bits per second are almost always multiples of 1000, but when measured in bytes per second are sometimes multiples of 1024. For example, a 1x DVD drive transfers data at 11.08 million bits per second (Mbps), which is 1.385 million bytes per second, but only 1.321 megabytes per second. The 150 KB/s 1x data rate commonly listed for CD-ROM drives is "true" kilobytes per second, since the data rate is actually 153.6 thousand bytes per second. This FAQ uses "kbps" for thousands of bits/sec, "Mbps" for millions of bits/sec (note the small "k" and big "M").

In December 1998, the IEC produced new prefixes for binary multiples: kibibytes (KiB), mebibytes (MiB), gibibytes (GiB), tebibytes (TiB), and so on. (More details at NIST, also released as IEEE Std 1541-2002) 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 rather silly.

This FAQ is written and maintained by Jim Taylor. The following people contributed to early versions of the DVD FAQ. Their contributions are deeply appreciated. Some information was taken from material distributed at the April 1996 DVD Forum, May 1997 DVD-R/DVD-RAM Conference, and October 1998 DVD Forum Conference, as well as many other conferences and presentations since.

Robert Lundemo Aas
Adam Barratt
David Boulet
Espen Braathen
Wayne Bundrick
Irek Defee
Roger Dressler
Chad Fogg
Dwayne Fujima
Robert "Obi" George
Henrik "Leopold" Herranen
Kilroy Hughes
Mark Johnson
Ralph LaBarge
Martin Leese
Dana Parker
Eric Smith
Steve Tannehill
Geoffrey Tully

Thanks to Videodiscovery for hosting this FAQ for the first two and a half years.

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Copyright 1996-2004 by Jim Taylor. This document may be redistributed only in its entirety with version date, authorship notice, and acknowledgements intact. No part of it may be sold for profit or incorporated in a commercial document without the permission of the copyright holder. Permission will be granted for complete electronic copies to be made available as an archive or mirror service on the condition that the author be notified and that the copy be kept up to date. This document is provided as is without any express or implied warranty.

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