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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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In Reply to: Question about "upsampling" posted by Slider on February 11, 2009 at 11:38:31:
"There does seem to be a problem with the band limiting of the images shown in the original post, hard to tell what the sample rate is though."
Looking at the pics, the ringing frequency is obviously close to the signal frequency, which suggests the signal frequency is close to half the sample-rate frequency, aka "Nyquist" frequency. It is not "seven octaves below" as claimed.
"And I agree, there also seems to be some confusion on the Nyquist frequency that applies to the digital filter, as compared to the output sample rate Nyquist, which is usually taken care of with just a simple analog post filter."
You have this correct- The digital filter acts on the input sample rate, the analog post filter acts on the digital filter's output sample rate.
"Regardless, this is on a little different subject, and while I know there are some here that will disparage any type of upsampling applied to the signal before the standard oversampling digital filter, if the upsampling ratio is high enough, say to 4x to 5x, aren't you able to reduce the filter slope quite a lot since the upsampler will have a stop band that can stretch from around 20K all the way up to the new sample rate Nyquist, instead of just up to the original Nyquist, and the digital filter could be set to act at the new sample rate, with whatever filter type it is set for?"
Yes, provided the oversampling DAC still oversamples at the higher rate. (Not all do, but the Wolfson apparently does.) The only advantage is it would require an even smaller post filter, but I think a second oversampling stage is totally unnecessary, since it would require intensive number-crunching for a filtering function that's non-critical. It's only critical the first time, since the Nyquist frequency is initially so close to the top of the audible 20-20k band.
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Follow Ups
- RE: Question about "upsampling" - Todd Krieger 14:59:38 02/11/09 (0)
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