i have a disc that i want to copy exactly. how do i make an exact copy of a disc. i tried the windows default thing for making cd's but it didnt make it exact.
post #1 of 4
11/5/04 at 8:02pm
|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 it helps to have a (convenient albeit somewhat inaccurate) mental image, picture a long string of bits arranged in a spiral. There are bits at the start of the spiral that you can't copy (the lead-in area), there are bits outside the spiral that you can usually copy if you request them ("raw" MODE-1 CD-ROM ECC and sector goop), and there are bits *under* the spiral that are blurry and hard to see (the subcode data).
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.
In no event can you guarantee an exact duplicate of the level 1 ECC (CIRC) encoding. In practice this doesn't matter, since no CD-ROM drive provides an interface for reading it directly.
Making a "bit-for-bit" copy of a disc would require reading the data at the lowest possible level, something that no production CD-ROM drive is capable of doing. Even if it were possible, there aren't any CD recorders that can write that sort of data.
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 the 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.
On the other hand, if it's a copy-protected CD-ROM with index markers in strange places, you have to use software and hardware that can see the "blurry bits" reliably and copy them.