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RE: Chord up-samples?

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This Post Has Been Edited by the Author

In Reply to: RE: Chord up-samples? posted by yage on June 20, 2017 at 13:15:43

Understanding what an interpolating reconstruction digital filter does provides insight in to the nature of the analog signal which is output from a D/A converter unit. An unfiltered D/A output contains the complete desired analog signal band, unfortunately, it also contains a series of repeating copies of that signal band, called image bands or simply images.
The audio reconstruction filter's job, whether it's digital or analog, is to remove these image bands by low-pass filtering the desired signal band. It is the image bands which give an unfiltered post DAC signal it's familiar discrete (typically, though not necessarily, stair-stepped) appearance. An pre-DAC digital signal can be filtered by a digital low-pass filter, or alternately, by a post-DAC analog signal can be filtered by an analog low-pass filter to remove the image bands. Removing the image bands renders a the discrete looking output smooth and 'analog' looking, keeping in mind that the desired analog signal was mixed in there all along.
A digital reconstruction filter doesn't really produce samples as though the original native sample rate were higher. There's no increase of encoded signal bandwidth. It simply shifts the repeating image bands up in frequency by a multiple equal to the oversampling ratio as a result of low-pass filtering the signal, where they become easier to completely remove with a with an relatively simple analog filter. You'll alternately see the interpolation filter refered to as an image-rejection filter, or simply as an (signal)reconstruction filter.
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Ken Newton

Edits: 06/20/17

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