Re: [time-nuts] decimation versus decimation
the killing of one in every ten of a group of people as a punishment for the whole group (originally with reference to a mutinous Roman legion). -Original Message- From: Richard Solomon via time-nuts To: Discussion of precise time and frequency measurement Cc: Richard Solomon Sent: Tue, Feb 25, 2020 7:41 am Subject: Re: [time-nuts] decimation versus decimation The Ancient Romans had another, not so nice, definition. 73, Dick, W1KSZ On Tue, Feb 25, 2020 at 7:56 AM Dana Whitlow via time-nuts < time-nuts@lists.febo.com> wrote: > I'm confused: > > I seem to see multiple (at least double) meanings for the term > 'decimation'. > > 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? And if the answer is "both", then shouldn't we all > be very careful to explicitly specify which meaning applies to > the situation at hand whenever we use the term "decimation"? > > Thanks, > > Dana K8YUM > ___ > 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. > ___ 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. ___ 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.
Re: [time-nuts] decimation versus decimation
Precisely, Dick! On Tuesday, February 25, 2020, Richard Solomon via time-nuts < time-nuts@lists.febo.com> wrote: > The Ancient Romans had another, > not so nice, definition. > > 73, Dick, W1KSZ > > On Tue, Feb 25, 2020 at 7:56 AM Dana Whitlow via time-nuts < > time-nuts@lists.febo.com> wrote: > > > I'm confused: > > > > I seem to see multiple (at least double) meanings for the term > > 'decimation'. > > > > 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? And if the answer is "both", then shouldn't we all > > be very careful to explicitly specify which meaning applies to > > the situation at hand whenever we use the term "decimation"? > > > > Thanks, > > > > DanaK8YUM > > ___ > > 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. > > > ___ > 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. > -- Homo sum humani a me nihil alienum puto. ___ 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.
Re: [time-nuts] decimation versus decimation
Please note that I have little real interest in measuring ADEV. My interest lies almost solely in measuring phase variations of a received signal via some propagation path, not in any statistical sense but looking at actual waveforms plotted as phase versus time. I am working towards measurement runs spanning weeks (maybe even months) over some specified bandwidth (for example, 0.1 to 10 Hz), so that long term phase accumulation due to static frequency error and/or slow frequency drift of my local reference versus the incoming signal need not be fatal. There is concern if the phase accumulation gets large enough to threaten loss of numerical precision, however. I'm currently working on a program which includes a rather slow AFC loop so that the phase accumulation is somewhat bounded. I am currently doing a 2-stage decimation. The first stage is by a ratio of about 4 or 5 to 1, simply so that I can get the array size down to the point where an FFT does not take forever. The FFT does the 2nd stage of decimation according to an upper bandwidth I specify at run time, and a much smaller IFFT gets me back to the time domain where I can do my stuff. I'm currently stuck with a cheap DSO whose minimum sample rate is 2 kHz. I'd prefer to run at a sample rate of ~50 to 100 Hz so I can get some reasonable durations. To this end I'm looking at buying an LGR-5325 from Measurement Computing, which should yield all the flexibility I can ever use. But the price- Ouch! Dana On Tue, Feb 25, 2020 at 11:33 AM Anders Wallin via time-nuts < time-nuts@lists.febo.com> wrote: > Some of the confusion might come from different measurement systems dealing > with different data, either phase (in seconds), or frequency (fractional). > > Sub-sampling works for phase-data, just throw away the in-between samples > and look at phase samples that are tau-distance apart. > > Decimation works for frequency-data, if you collect with 1s gate-time and > want to look at tau=10s, then bin together 10 samples and create a new > time-series that is 10-fold shorter, and compute adev from that. > > FWIW IIRC all the allantools examples/tests that (successfully) compare > results against Stable32, using frequency input data, just use simple > non-overlapping 'boxcar' decimation - nothing more fancy or elaborate. > > AW > > > On Tue, Feb 25, 2020 at 7:10 PM Dana Whitlow via time-nuts < > time-nuts@lists.febo.com> wrote: > > > Let's see if I have this correct, then: > > > > "Decimation" refers to the two-step procedure (filtering followed by > > picking every nth sample), and > > "Sub-sampling" (or "downsampling") properly refers to taking every nth > > sample and discarding the rest. > > > > Is this correct? > > > > And thanks for all the responses. > > > > Dana > > > > > > On Tue, Feb 25, 2020 at 10:09 AM George Watson via time-nuts < > > time-nuts@lists.febo.com> wrote: > > > > > > > > ___ > > > 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. > > > > > ___ > > 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. > > > ___ > 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. > ___ 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.
Re: [time-nuts] decimation versus decimation
Some of the confusion might come from different measurement systems dealing with different data, either phase (in seconds), or frequency (fractional). Sub-sampling works for phase-data, just throw away the in-between samples and look at phase samples that are tau-distance apart. Decimation works for frequency-data, if you collect with 1s gate-time and want to look at tau=10s, then bin together 10 samples and create a new time-series that is 10-fold shorter, and compute adev from that. FWIW IIRC all the allantools examples/tests that (successfully) compare results against Stable32, using frequency input data, just use simple non-overlapping 'boxcar' decimation - nothing more fancy or elaborate. AW On Tue, Feb 25, 2020 at 7:10 PM Dana Whitlow via time-nuts < time-nuts@lists.febo.com> wrote: > Let's see if I have this correct, then: > > "Decimation" refers to the two-step procedure (filtering followed by > picking every nth sample), and > "Sub-sampling" (or "downsampling") properly refers to taking every nth > sample and discarding the rest. > > Is this correct? > > And thanks for all the responses. > > Dana > > > On Tue, Feb 25, 2020 at 10:09 AM George Watson via time-nuts < > time-nuts@lists.febo.com> wrote: > > > > > ___ > > 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. > > > ___ > 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. > ___ 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.
Re: [time-nuts] decimation versus decimation
Let's see if I have this correct, then: "Decimation" refers to the two-step procedure (filtering followed by picking every nth sample), and "Sub-sampling" (or "downsampling") properly refers to taking every nth sample and discarding the rest. Is this correct? And thanks for all the responses. Dana On Tue, Feb 25, 2020 at 10:09 AM George Watson via time-nuts < time-nuts@lists.febo.com> wrote: > > ___ > 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. > ___ 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.
Re: [time-nuts] decimation versus decimation
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Re: [time-nuts] decimation versus decimation
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 > 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. ___ 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.
Re: [time-nuts] decimation versus decimation
The Ancient Romans had another, not so nice, definition. 73, Dick, W1KSZ On Tue, Feb 25, 2020 at 7:56 AM Dana Whitlow via time-nuts < time-nuts@lists.febo.com> wrote: > I'm confused: > > I seem to see multiple (at least double) meanings for the term > 'decimation'. > > 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? And if the answer is "both", then shouldn't we all > be very careful to explicitly specify which meaning applies to > the situation at hand whenever we use the term "decimation"? > > Thanks, > > DanaK8YUM > ___ > 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. > ___ 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.
Re: [time-nuts] decimation versus decimation
> 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.
Re: [time-nuts] decimation versus decimation
in original -- Latin -- it means; take every tenth 73 KJ6UHN Alex On 2/25/2020 7:15 AM, Bob kb8tq via time-nuts wrote: Hi In the common vernacular “decimation” does indeed mean “just throw the rest away”. You can find papers where that *is* what’s being done with samples and that term is used. Downsample also means “just throw them away”…… Bob On Feb 25, 2020, at 9:55 AM, Dana Whitlow via time-nuts wrote: I'm confused: I seem to see multiple (at least double) meanings for the term 'decimation'. 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? And if the answer is "both", then shouldn't we all be very careful to explicitly specify which meaning applies to the situation at hand whenever we use the term "decimation"? Thanks, DanaK8YUM ___ 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. ___ 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. ___ 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.
Re: [time-nuts] decimation versus decimation
Hi In the common vernacular “decimation” does indeed mean “just throw the rest away”. You can find papers where that *is* what’s being done with samples and that term is used. Downsample also means “just throw them away”…… Bob > On Feb 25, 2020, at 9:55 AM, Dana Whitlow via time-nuts > wrote: > > I'm confused: > > I seem to see multiple (at least double) meanings for the term > 'decimation'. > > 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? And if the answer is "both", then shouldn't we all > be very careful to explicitly specify which meaning applies to > the situation at hand whenever we use the term "decimation"? > > Thanks, > > DanaK8YUM > ___ > 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. ___ 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.
Re: [time-nuts] decimation versus decimation
Hi Dana, On 2020-02-25 15:55, Dana Whitlow via time-nuts wrote: > I'm confused: > > I seem to see multiple (at least double) meanings for the term > 'decimation'. > > One meaning is simply: "take every nth sample and discard > the others without regard to possible aliasing". This is sub-sampling. It performs a very crude decimation, full with aliasing. > The other meaning is: "take every nth sample, but first prefilter > as appropriate to (sensibly) eliminate aliasing". This is what every day decimation is these days. > > Which is it? And if the answer is "both", then shouldn't we all > be very careful to explicitly specify which meaning applies to > the situation at hand whenever we use the term "decimation"? Luckily there is different terms for the two different processes to use. Cheers, Magnus ___ 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.
