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3-42] How do I make a bit-for-bit copy of a disc?




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

3-42] How do I make a bit-for-bit copy of a disc?

(2005/07/23)

A commonly posed question from the newsgroups: "what software can do
bit-for-bit copies?" The expectation is software that can make an exact
copy of the original.

There isn't any. If this seems counter-intuitive, bear in mind that discs
hold digital data on an analog medium. While "bits" may be what you read
from the drive, at some point those bits have to be stored as marks or
indentations on a piece of polycarbonate.

The "low-level" modes, such as "raw DAO-96", are actually pretty high
level. By the time you've got 2352-byte sectors and 96-bits of subcode
channel data, the drive has converted optical reflections to an analog
signal, converted the analog signal to digital bits, combined individual
bits into 24-byte frames, applied error correction, and assembled the
frames into the data you see. When you're writing a sector, all that
stuff happens on the way out, too, and there's no way for CD recording
software to control it.

What's more, there are copy protection features, such as *physically* damaged
blocks, that a recorder isn't generally capable of writing. Other tricks,
such as out-of-specification track lengths, can't be duplicated by most
CD recorders because the firmware refuses to write them.

Making an exact copy of a disc would require reading and writing the
basic analog signal. In a sense, this is what CD pressing plants do when
they create CDs from a glass master. It's just not possible with the CD
recorders we have today.

Because of these limitations, you have to read a sector of data as a sector
of data, not as a collection of frames scattered over half the circumference
of the disc. The best you can do currently is "raw DAO-96" (section (3-51)),
which reads the subcode data along with the raw sector data.

Bear in mind that CD-ROM drives and CD recorders were designed for people
who want to read and write data, not decipher arcane standards documents
and perform their own error correction. Creating exact one-off copies was
not a major consideration of the original design.

In general, however, you don't *need* a "bit-perfect" duplicate of the
original. If what you're copying is a simple MODE-1 CD-ROM, you can make
an "identical" copy by reading the sectors off the original and writing
them to a duplicate. For most situations this is good enough: you have
copied the bits that matter.

Most copy-protected discs can be copied with more advanced software.
Because the copy protection has to use the same CD-ROM interface that
the copy software does, it's hard to create copy protection schemes
that can't at least be detected.

See also sections (2-4), (2-43), (3-1-1), (3-18), (3-39), and (6-1-49).


 

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