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RE: Chord up-samples?

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 (samples) can be filtered by a digital low-pass filter, or alternately, an post-DAC analog signal can be filtered by an analog low-pass filter to remove the image bands. Removing the image bands renders the otherwise discrete looking output smooth and 'analog' looking. Keep in mind that the desired complete analog signal was mixed in there all along, it just needed the image bands to be removed in order to be recognized as such.

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 filter's oversampling ratio, where they become much easier to completely remove with a with an relatively simple analog output filter. You'll alternately see the reconstruction filter refered to as an image-rejection filter, or as an interpolation filter.
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Ken Newton



Edits: 06/20/17 06/20/17 06/20/17

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