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7-6] How much data can they hold? 650MB? 680MB?


This article is from the CD-Recordable FAQ, by Andy McFadden (fadden@fadden.com) with numerous contributions by others.

7-6] How much data can they hold? 650MB? 680MB?


There are 21-minute (80mm/3-inch), 74-minute, 80-minute, 90-minute, and
99-minute CD-Rs. These translate into data storage capacities of 184MB,
650MB, 700MB, 790MB, and 870MB respectively (see below for exact figures).
See section (7-14) for more about 80mm CD-Rs, and sections (3-8-1) and
(3-8-2) for notes on 80-, 90-, and 99-minute blanks. There used to be
63-minute CD-Rs, but these have largely vanished.

Typical 74-minute CD-Rs are advertised as holding 650MB, 680MB, or even
700MB of data. The reality is that they're all about the same size, and
while you may get as much as an extra minute or two depending on the exact
construction, you're not usually going to get an extra 30MB out of a disc
labeled as 74-minute media. See section (3-8-3) for information on writing
beyond a disc's stated capacity.

Folks interested in "doing the math" should note that only 2048 bytes of
each 2352-byte sector is used for data on typical (Mode 1) discs. The rest
is used for error correction and miscellaneous fields. This is why you can
fit 747MB of audio WAV files onto a disc that holds 650MB of data.

It should also be noted that hard drive manufacturers don't measure megabytes
in the same way that RAM manufacturers do. The "MB" for RAM means 1024x1024,
but for hard drives it means 1000x1000. A data CD that can hold 650 "RAM"
MB of data holds about 682 "disk" MB of data, which is why many CD-Rs
are mislabeled as having a 680MB capacity. (The notion of "unformatted
capacity" is a nonsensical myth stemming from early hard drives.)

Spelled out simply:

  21 minutes ==  94,500 sectors == 184.6MB CD-ROM == 212.0MB CD-DA
  63 minutes == 283,500 sectors == 553.7MB CD-ROM == 635.9MB CD-DA
  74 minutes == 333,000 sectors == 650.3MB CD-ROM == 746.9MB CD-DA
  80 minutes == 360,000 sectors == 703.1MB CD-ROM == 807.4MB CD-DA
  90 minutes == 405,000 sectors == 791.0MB CD-ROM == 908.4MB CD-DA
  99 minutes == 445,500 sectors == 870.1MB CD-ROM == 999.3MB CD-DA

The International Electrotechnical Commission (IEC) has approved alternate
prefixes for binary powers of two. Instead of kilobytes and megabytes
we would call them kibibytes and mebibytes, with KiB and MiB replacing
KB and MB. This means an 80-minute CD would be rated as holding 703.1MiB
or 737.3MB. These haven't yet fallen into common usage. Check the NIST
site for full details: http://physics.nist.gov/cuu/Units/binary.html.

Many CD recording programs will tell you the exact number of 2K sectors
available on the CD. This is the only reliable way to know exactly
how many sectors are available. 99-minute blanks will actually report
incorrect values.

An informal survey conducted by one user found that the deviation between the
largest and smallest 74-minute CD-R was about 3500 sectors (47 seconds, or
7MB), which while not inconsequential is nowhere near the difference between
650MB and the 680MB or 700MB figures quoted by some manufacturers. All discs
had at least 333,000 sectors, as required by the Red Book specification.

http://www.cdmediaworld.com/ has a fairly detailed listing of how much
data different brands of media will actually hold. Again, bear in mind
that different batches of the same media may have different capacities.

The PCA (Power Calibration Area), PMA (Program Memory Area), TOC (Table
of Contents), lead-in, and lead-out areas don't count against the time
rating on single-session CDs. You really do get all the storage that the
disc is rated for. On standard MODE 1 discs that aren't using packet
writing, there is no "formatting overhead". Bear in mind, however, that
the "cluster" size is 2K, and that the ISO-9660 filesystem may use more or
less space than an MS-DOS FAT or HFS filesystem, so 650MB of files on a
hard disk may occupy a different amount of space on a CD.

On a multisession disc, you lose about 23MB of space when the first session
is closed (to pave the way for the 2nd session), and about 14MB for each
subsequent session. A common mistake when writing multisession CDs is to
overestimate the amount of space that will be available for future sessions,
so be sure to take this into account.

(If you want the details: the first additional session requires 4500 sectors
for the lead-in and 6750 for the lead-out, for a total of 11250 (22.5MB,
or 2.5 minutes). Each additional session requires 4500 for the lead-in
and 2250 for the lead-out, for a total of 6750 (13.5MB, or 1.5 minutes).
You may also need to factor 2-second pre-gaps into the size calculation
for each session. On a single-session disc, the overhead for lead-in
and lead-out are not counted as part of the user data area, so nothing is
"lost" until you go multisession.)

Pressed aluminum CDs are also supposed to hold no more than 74 minutes of
audio, but are often tweaked to hold more (see section (3-8)). To convert
sectors back to seconds, divide the number of sectors by 75. If your blanks
have 333,000 sectors, they have 4440 seconds, which is exactly 74 minutes.

Some packet-writing solutions will take a large bite out of your available
disc space. For example, if you use Roxio DirectCD 2.x with CD-RW media,
it uses fixed-length packets. This allows random file erase, which means
that when you delete a file you actually get the space back, but you're
reduced to about 493MB after formatting the disc. More recent versions can
get closer to 531MB. See section (4-42) for more info.


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