> One meaning is simply: "take every nth sample and discard the others without > regard to possible aliasing". > The other meaning is: "take every nth sample, but first prefilter as > appropriate to (sensibly) eliminate aliasing". > Which is it? Unless you are doing something tricky, it doesn't make sense to decimate without the filter. I'll phrase your two cases differently. The first is a box that just takes every Nth sample. Some other box did the filtering. The second box includes the filter because that is such a common case. The "box" can be software or hardware. --------- Sometimes the aliasing is a feature. If you start with a 100 MHz signal, run it through a band pass filter, now you can decimate and your signal will get aliased down into baseband. But that's not aliasing bad stuff on top of your signal. It's aliasing your signal on top of emptyness. -- These are my opinions. I hate spam.

Hi If you take a look at how ADEV has been traditionally done for many decades, they *do* indeed “decimate without the filter”. There is no re-filtering process as you go from 1 to 2 to 10,000 second tau. Bob > On Feb 25, 2020, at 10:33 AM, Hal Murray via time-nuts <time-nuts@lists.febo.com> wrote: > > >> One meaning is simply: "take every nth sample and discard the others without >> regard to possible aliasing". > >> The other meaning is: "take every nth sample, but first prefilter as >> appropriate to (sensibly) eliminate aliasing". > >> Which is it? > > Unless you are doing something tricky, it doesn't make sense to decimate > without the filter. > > I'll phrase your two cases differently. The first is a box that just takes > every Nth sample. Some other box did the filtering. The second box includes > the filter because that is such a common case. The "box" can be software or > hardware. > > --------- > > Sometimes the aliasing is a feature. If you start with a 100 MHz signal, run > it through a band pass filter, now you can decimate and your signal will get > aliased down into baseband. But that's not aliasing bad stuff on top of your > signal. It's aliasing your signal on top of emptyness. > > > > -- > These are my opinions. I hate spam. > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there.