Question: I have read an audio track several times, written to various files, and then compared the files. Unfortunately I notice that the files are completely different, although the audio data sounds fine. Surely this is not normal - I thought that the data is stored digitally -- the files ought to be identical!
Answer: It is true that the data is stored in digital form. Even so, it is difficult for your CD player, your CD-ROM drive and your CD-burner to control an audio track with exact precision. Even the better CD-ROM drives do not always begin reading an audio track at exactly the same byte. The result is that the bytes in the two image files are slightly out of synch (for example, the TEAC CD516S V1.0D tends to vary between 20 to 100 bytes). A byte comparison of such image files will leave the impression that the files differ greatly from one another - in reality, there are merely slightly out of synch by a small number of bytes. One second of music corresponds to 176400 bytes. So, if the drive starts reading 100 bytes too late, you are actually losing a mere 0.0006 seconds of music.
Since there are on error reports and correction for audio data, it is certainly possible that differing data is transferred from time to time, especially if the medium is not of the highest quality. Whether these disturbances are extreme or not very much depends on your CD-ROM drive. Some CD-ROM drives are very good at reading scratched CDs, while others will transfer corrupted data.
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