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Digital Drive: Re: This Wasn't as Bad as Expected... by Werner

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Re: This Wasn't as Bad as Expected...

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"If there is no oversampling, there is no filter function the base media signal is convolved with."

Yes there is ...

Theory holds only for a sample train implemented as a train of Dirac
impulses, which is clearly ever so slightly impossible in the known
universe. Practical DACs do no output impulses, but stable levels each with a duration 1/fs. This aberration from theory has a frequency
domain representation which follows a rolloff with the shape of Sin(f)/f. Which happens to be Sinc(f).

This may seem to be annoying, as it gives -2dB at 20kHz for a 44.1k system, and associated phase shift (or rather group delay is what we're after, phase shift *can* be quite harmless). On the other hand, this alone is enough to suppress the high-activity region of the first and second image, say 40-48kHz, in a way sufficient for NONOS DACs to get away without any filtering at all.




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Topic - upsampling article - solidgore 08:08:38 08/22/03 ( 47)