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Since the development of the audio CD, and its introduction into the world of
the home PC user, there has always been some confusion on how much data a CDR
used by home writers will hold. This confusion is not helped by the manufacturer
of the disk either.
Overburning
In addition to the amount of room available, which manufacturers quote,
there is usually a little more room, and with the use of good CD writers,
overburning can be used to get a little more on the disk.
Size Quoted
You can expect to get close to the quoted size, without having to overburn.
Data Disks
On a 650 Mb (74 Min) you will get 650 Mb of data.
On a 700 Mb (80 Min) you will get 703 Mb of data.
Video CD's (VCD/SVCD)
On a 650 Mb (74 Min) you will get 735 Mb of data.
On a 700 Mb (80 Min) you will get 795 Mb of data.
The information of a CD is very well organized, otherwise it would be
difficult too find things quickly. Like a telephone directory it has a look up
page, this will tell the reader where to find the data on the disk. Similar to
the way a PC's hard drive is organised, the disk is created with organised sectors.
The original specification of the first music CD's were organised and given
75 sectors for each second of audio. This proved to be more than adequate for
home use.
In addition to this the music on the disk was also given a
specification.
Uncompressed audio.
Stereo
16 Bit (Each channel)
44.1Khz
Calculations
Calculations can be made, now that we know the exact specifications, so for 1
second of audio...
16 Times 2 Times 44100 Times 1 = 1411200 bits. 1411200 Divided by 8 =
176400 Bytes
We know there are 75 sectors for 1 second
176400 Divided by 75 = 2352 bytes.
So 1 second on a CD can hold 2352 bytes of data.
Once this standard was set, it was not long before some changes were made so
that these disks could hold other types of data in addition to music, and soon
CD-ROM followed.
When reading music CD's it was not too big a problem. If the disk
contained errors, the CD drive would read from adjacent audio samples, However
this was no use at all for storing data, just a small error would be enough to
render the data useless. So a modification was needed to the original
specification.
So out of the original 2352 byes only 2048 byes can be used and the renaming
set aside for error correction. This type of ROM is either Mode 1 or Mode 2
(Form type 1)
Video disks (VCD/SVCD)
Again the robust error detection is not required for these type of disks, and so
just a small amount is reserved. Video disks are made in Mode 2 (Form 2) when
disks are made in this mode 2324 bytes out of the original 2352 music spec is
available.
So for Video disks we have a new calculation to give us the amount of
usable room.
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75 sectors = 1 second |
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74 Min CD |
333,000 sectors |
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80 Min CD |
360,000 sectors |
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MB |
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Data capacity for an audio CD |
74 min |
333,000 sectors * 2352 bytes |
783216000 bytes |
746.9 |
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80 min |
360,000 sectors * 2352 bytes |
846720000 bytes |
807.5 |
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Data capacity for a CD-ROM |
74 min |
333,000 sectors * 2048 bytes |
681984000 bytes |
650.4 |
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80 min |
360,000 sectors * 2048 bytes |
737280000 bytes |
703.1 |
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Data capacity for a VCD/SVCD |
74 min |
333,000 sectors * 2324 bytes |
773892000 bytes |
738.0 |
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80 min |
360,000 sectors * 2324 bytes |
836640000 bytes |
797.9 |
VCD's and SVCD's use a different mode of burning which does give more usable
data than for other types of disks. In addition to the video stream there is
also normal data written, so the amount of room is slightly less than what is
calculated. Around 735 Mb for 74 min disks and 795 for 80 min disks. So this is
why you may see capacities quoted at 800Mb for 80 min disks and 740 Mb for 74
min disks.
Overburning, and larger capacity disks.
It is quite possible to get up to 2 more min on standard disks with the use of
overburning. Most modern CD writers are capable of doing this, and blank CD's
usually have a little more room than quoted. With the large amount of different
brands of disks, the amount of overburning will very, so its not possible to say
exactly how much more room there is.
Older CD Writers do not support overburning at all, and in some cases damage could
accrue when it was tried. Both the manufacturers of the writers and blank CD
suppliers all said the same thing....
"We don't support overburning". But as time
passed, they had no choice but to support it, as people would buy writers that
did support it. The same can be said for the programs used to make the disks..
In addition to overburning, manufacturers produced 90 min disks, some of the
writers wont use them, luckily most new writers will. So the next time you try
to burn the exact amounts you have been told you can burn, and things don't work
out... Just remember, this is how it suppose to work.
But there are no
guarantees :)-
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