Suppose I take samples at once per minute for a data logger. Do I still need an anti-aliasing filter? The sampling freq would be 1/T where T=60 secs...so how is this possible? The values of the capacitors etc would be hugh. Of course I could over-sample1000s of times...is this the norm? K.
Sampling Question
Started by ●August 1, 2008
Reply by ●August 1, 20082008-08-01
On 1 Aug, 22:12, kronec...@yahoo.co.uk wrote:> Suppose I take samples at once per minute for a data logger. Do I > still need an anti-aliasing filter? The sampling freq would be 1/T > where T=60 secs...so how is this possible? The values of �the > capacitors etc would be hugh. Of course I could over-sample1000s of > times...is this the norm?What kind of process is this? What will you use the data for? Rune
Reply by ●August 1, 20082008-08-01
kronecker@yahoo.co.uk wrote:> Suppose I take samples at once per minute for a data logger. Do I > still need an anti-aliasing filter? The sampling freq would be 1/T > where T=60 secs...so how is this possible? The values of the > capacitors etc would be hugh. Of course I could over-sample1000s of > times...is this the norm?An anti-alias filter is rarely used for such measurements. Similarly, neither for tide measurements, daily temperatures, or closing stock prices. For many applications this is fine, but it imposes limits on how that data can be used. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●August 1, 20082008-08-01
On 2008-08-01, kronecker@yahoo.co.uk <kronecker@yahoo.co.uk> wrote:> Suppose I take samples at once per minute for a data logger. Do I > still need an anti-aliasing filter?Imagine you took samples of a GPS position once a minute. If you put it in a car going cross-country you might get perfectly usable data. If you put it in an Indy car on an oval track you might have trouble telling the difference between the car going around the track fast vs forward slow vs backwards (!). So you have to know something about what you're measuring before you know what you stand to lose at your sample rate. Keep in mind that you don't have to sample at your logging rate. You could sample at 100Hz and filter that in software and decimate by 60000. Choose your sample rate to be highly over-sampled for your process, and then you can log at any rate greater than the Nyquist rate. -- Ben Jackson AD7GD <ben@ben.com> http://www.ben.com/
Reply by ●August 1, 20082008-08-01
<kronecker@yahoo.co.uk> wrote in message news:337f705e-90cd-4b04-957c-f442443eed7b@v13g2000pro.googlegroups.com...> Suppose I take samples at once per minute for a data logger. Do I > still need an anti-aliasing filter? The sampling freq would be 1/T > where T=60 secs...so how is this possible? The values of the > capacitors etc would be hugh. Of course I could over-sample1000s of > times...is this the norm? > > > K.Only you can really answer the question "do I still need an anti-aliasing filter?". Your idea of over-sampling and then digital filtering might well be a good one. No capacitors..... You might want to examine what aliasing does and what that means to you. - If you are going to do frequency analysis then you need to know that higher frequencies can be aliased into the analysis range. If you aren't going to do that then does it matter? - If you are going to do reconstruction then you aren't going to be able to do it perfectly and perhaps it will even be done terribly. Reconstruction is closely related to interpolation - so if you're planning to interpolate to a high degree then that might be a problem too. - If all you're going to do is compute statistics then maybe you will be OK as the samples you take *are* valid samples. We often are forced to live with this situation. Sometimes the best thing to do is just try it and see. For example, compare oversampled data that's "properly" decimated with undersampled data from the same time span in your process - whatever that may be. Are the results comparable or not? Fred
Reply by ●August 1, 20082008-08-01
kronecker@yahoo.co.uk wrote:> Suppose I take samples at once per minute for a data logger. Do I > still need an anti-aliasing filter? The sampling freq would be 1/T > where T=60 secs...so how is this possible? The values of the > capacitors etc would be hugh. Of course I could over-sample1000s of > times...is this the norm? > > > K.Nyquist never said you _had_ to have anti-aliasing filters. This may not answer your question, but it may shed some light: http://www.wescottdesign.com/articles/Sampling/sampling.html. If the thing you're sampling just doesn't have any content above 1/120th of a Hz, then you don't need to anti-alias at all. If it has content at 10Hz and you desperately need to know what that content is, then sampling at 1/60th Hz will never, ever get you what you want. Consider that you may do better sampling once/three minutes, and saving away the minimum, maximum, and average signal seen in that period. And read Ben's post - he makes some good points, from a time-domain point of view. Sometimes we get so wound up in the frequency-domain analysis that we forget that we live in the time domain, and a little bit of reasoning in the here-and-now will save us a long trip through mathemagic land. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●August 1, 20082008-08-01
On Aug 2, 8:31 am, Rune Allnor <all...@tele.ntnu.no> wrote:> On 1 Aug, 22:12, kronec...@yahoo.co.uk wrote: > > > Suppose I take samples at once per minute for a data logger. Do I > > still need an anti-aliasing filter? The sampling freq would be 1/T > > where T=60 secs...so how is this possible? The values of the > > capacitors etc would be hugh. Of course I could over-sample1000s of > > times...is this the norm? > > What kind of process is this? What will you use the data for? > > RuneIt's strain measurements. K.
Reply by ●August 1, 20082008-08-01
kronecker@yahoo.co.uk wrote:> Suppose I take samples at once per minute for a data logger. Do I > still need an anti-aliasing filter? The sampling freq would be 1/T > where T=60 secs...so how is this possible? The values of the > capacitors etc would be hugh. Of course I could over-sample1000s of > times...is this the norm?Remember it is RC that makes a filter, either large C or large R will work. With an appropriate amplifier, you can make good filters without huge capacitors. But you really need to understand the problem. The human auditory system is pretty sensitive to non-harmonic signals, so the filters are pretty important for audio signals. The visual system is somewhat less sensitive to aliasing. Also, video hardware naturally has some filtering due to the width of the sensor elements (or beam size in scanning electron tube devices). Even more, the color TV standards put the color information where high spatial frequency video information should go, but most people aren't especially bothered by it. At low frequencies over sampling and digital filtering are fairly easy to do, so that may be a fine solution. Audio and video are fairly easy, as there are upper limits to the audio frequency and spatial frequency that we can hear/see. -- glen -- glen
Reply by ●August 2, 20082008-08-02
On 2 Aug, 02:13, kronec...@yahoo.co.uk wrote:> On Aug 2, 8:31 am, Rune Allnor <all...@tele.ntnu.no> wrote: > > > On 1 Aug, 22:12, kronec...@yahoo.co.uk wrote: > > > > Suppose I take samples at once per minute for a data logger. Do I > > > still need an anti-aliasing filter? The sampling freq would be 1/T > > > where T=60 secs...so how is this possible? The values of �the > > > capacitors etc would be hugh. Of course I could over-sample1000s of > > > times...is this the norm? > > > What kind of process is this? What will you use the data for? > > > Rune > > It's strain measurements.If these are static strains (which the sampling rate might indicate) I wouldn't bother with the antialias filters. Rune
Reply by ●August 2, 20082008-08-02
Rune Allnor wrote:> On 2 Aug, 02:13, kronec...@yahoo.co.uk wrote: >> On Aug 2, 8:31 am, Rune Allnor <all...@tele.ntnu.no> wrote: >> >>> On 1 Aug, 22:12, kronec...@yahoo.co.uk wrote: >>>> Suppose I take samples at once per minute for a data logger. Do I >>>> still need an anti-aliasing filter? The sampling freq would be 1/T >>>> where T=60 secs...so how is this possible? The values of the >>>> capacitors etc would be hugh. Of course I could over-sample1000s of >>>> times...is this the norm? >>> What kind of process is this? What will you use the data for? >>> Rune >> It's strain measurements. > > If these are static strains (which the sampling rate might indicate) > I wouldn't bother with the antialias filters.It depends on the sensitivity, the nature of the phenomena, and the cut-off frequency of the filter. Water-hight gauges used to measure rivers and tides usually incorporate a stilling basin to exclude the effects of waves and ripples. The averaging time is usually much shorter than the sampling interval. The strain gauges in scales for infants and animals are filtered to give steady readings while the loads move. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
