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RE: -75 dB

As far as I could tell, the levels were matched to a tiny fraction of a dB, i.e. to all the significant digits displayed by Soundforge. This was surprisingly good for analog comparisons. A slight drift in temperature could cause a gain differences needed to get a better null. One thing that I noted was that the two files were perfectly synchronized. This indicates that the DAC and the ADCs involved were run off of a common clock for both runs. Otherwise, there would have been clock drift across the file with the samples going in/out of phase.

I've done a lot of null tests, usually digital to digital involving comparison of sample rate conversions of one sort or another. These rarely give 75 dB nulls, even when there is no theoretical reason for a null to be more than a few dB from the resolution given by the bit depth. (An example is resampling 44.1 to 88.2 and back to 44.1. Getting good nulls was impossible due to sub-sample delays in the filters, fractional gain errors, phase shifts and filter pass-band ripple. Eventually, I figured out how to do this kind of conversion without excessive error, except in the pathological cases where there was energy too close to Fs/2.)

Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar

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