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Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it.

RE: No brickwall filter then?

The filter in a DAC is used to remove images created when the samples are converted into a higher sampling rate domain or to analog (think of analog as an "infinite" sampling rate). The images exist because of the original samples—they are not there because of the higher sampling rate. A low pass filter is needed to remove them (or at least to reduce them).

The slope of the filter depends on the difference between the top of the pass band and the bottom of the stop band. The top of the pass band depends upon the desired frequency response. The bottom of the stop band must be at or below the Nyquist frequency for the sampling rate of the input signal. The output sampling rate is not relevant; there might not even be an output sampling rate if the filter is an analog one. The stop band for CD playback starts at 22,050 Hz and extends to infinity.

Tony Lauck

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


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