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We've been writing about the clues that help us figure out why CDs, hard drives and different medium do not sound the same. And the answer, as many of you have guessed, is a mixed bag. It's instructive to remember that when we discuss 'bits is bits' we refer only to the actual 1's and 0's coming from the device and not the timing or any particulars of them, other than they are the correct data in the correct order. In other words, those 'identical' bits, as you probably guessed in our whodunnit mystery, aren't really identical. Nor, I might add, are identical twins. They may look the same, but as any good detective will tell you, looks can be deceiving. One twin may be capable of murder while the other can be a saint. Identical in appearance does not mean identical in performance. If you take two CDs and scuff the surface of one and leave the other new, chances are good the two will not sound the same. One of the reasons this is true concerns two aspects of the mechanism playing them: the amount of error correction needed as well as the power supply noise that is increased. When your CD player attempts to read the data from the marred disc it must read it many times over and often applies greater amounts of error correction to obtain the data in correct form, thus overworking the laser mechanism and its power supply. Overworking the laser mechanism and power supply has many downstream effects including greater levels of jitter and noise. And in the same manner, the reading mechanism and its byproducts have an impact on sound quality. And here's what most people seem to miss. Just because the data looks identical doesn't mean it is identical. During our little mystery I have tried to be careful not to suggest the data are identical in the true sense of the word, which they are not. Think of it like this: two picket fences with identical alternating black and white pickets can be said to contain the same number and order of vertical boards (just as our data is the same). If the spacing between the vertical boards varies (jitter) then they are not identical, although they contain the same exact elements. Or even simpler, two fences of identical build, one dirty the other clean, are not identical although they are of the same construction. More tomorrow.
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Paul McGowan

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