I'm trying to think of examples where using an anti-alias filter is a bad idea, or must at least be approached with extreme caution. I already know about control systems and video applications, and I believe that this is a big issue with EKG machines. Does anyone else have any examples that they can share? If someone has a sampled EKG output in some readable file format that they'd be willing to share I would be grateful -- I can whomp something up out of nothing, but then some doctor will read my article and know it's BS. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Examples of Anti-Anti-Alias Requirements
Started by ●September 25, 2006
Reply by ●September 25, 20062006-09-25
Tim Wescott wrote:> I'm trying to think of examples where using an anti-alias filter is a > bad idea, or must at least be approached with extreme caution.Digital Oscilloscopes and misc. sensor data acquisition equipment, the control systems with the ADC in the loop. Anywhere you may be interested in the instantaneous value rather then in the averaged waveform. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●September 25, 20062006-09-25
Vladimir Vassilevsky wrote:> > > Tim Wescott wrote: > >> I'm trying to think of examples where using an anti-alias filter is a >> bad idea, or must at least be approached with extreme caution. > > > Digital OscilloscopesCheck.> the control systems with the ADC in the loop.Check. Actually treating this separately because measurement equipment can have _delay_ but no time distortion, while a control loop can't even have delay.> and misc. sensor data acquisition equipment, Anywhere you may be interested > in the instantaneous value rather then in the averaged waveform. >Yes, but do you have some specific examples that will make good charts for the non expert in the field of the measurement? -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 25, 20062006-09-25
Tim Wescott wrote:> I'm trying to think of examples where using an anti-alias filter is a > bad idea, or must at least be approached with extreme caution. I > already know about control systems and video applications, and I believe > that this is a big issue with EKG machines. > > Does anyone else have any examples that they can share? >I believe that this would be a consideration any time transient or impulsive phenomena are being digitized. However, Nyquist still holds, and if the anti-aliasing filter is interfering with your recording, it's an indication that you aren't sampling fast enough and the filter bandwidth needs to be wider. Some people try to use overly-sharp (eg: Butterworth or Chebyshev) analog AA filters to push the cutoff closer to Nyquist. These filters have a tendency to ring, even when implemented at low orders--not good for sampling transients. Why would EKG recorders be especially problematic? If they are, then maybe EEG and polygraph recorders too? One trick that I used in process control instrumentation was to oversample by a factor of several hundred, do the main anti-aliasing digitally and then decimate to a more appropriate sample rate. This means that the analog anti-aliasing filter can be very cheap and when the sampling can be synchronized to a multiple of the power line frequency, you can get a double whammy.
Reply by ●September 25, 20062006-09-25
Tim Wescott <tim@seemywebsite.com> wrote in news:1q- dndrgmcy9n4XYnZ2dnUVZ_vWdnZ2d@web-ster.com:> while a control loop can't even > have delay. >That seems somewhat overly generalized, no? -- Scott Reverse name to reply
Reply by ●September 25, 20062006-09-25
John Hadstate wrote:> Tim Wescott wrote: > >>I'm trying to think of examples where using an anti-alias filter is a >>bad idea, or must at least be approached with extreme caution. I >>already know about control systems and video applications, and I believe >>that this is a big issue with EKG machines. >> >>Does anyone else have any examples that they can share? >> > > > I believe that this would be a consideration any time transient or > impulsive phenomena are being digitized. However, Nyquist still holds, > and if the anti-aliasing filter is interfering with your recording, > it's an indication that you aren't sampling fast enough and the filter > bandwidth needs to be wider.Nyquist holds, but it can be difficult to go from a real-world (i.e. inherently infinite bandwidth) signal to a bandlimited one without ringing -- I'm discussing this, in the context of how you need to be aware of the issue when establishing your sampling rate.> > Some people try to use overly-sharp (eg: Butterworth or Chebyshev) > analog AA filters to push the cutoff closer to Nyquist. These filters > have a tendency to ring, even when implemented at low orders--not good > for sampling transients.No kidding!> > Why would EKG recorders be especially problematic? If they are, then > maybe EEG and polygraph recorders too?AFAIK they aren't especially problematic, it's just one that I'm familiar with being an issue -- goodness knows why, since I've never worked with medical electronics. I just recall discussions about the impact of sampling on EKG waveforms. I do know that if you're going to have problems with ringing or rounding that big old spike in the middle of an EKG waveform is going to make it happen -- particularly since there are interesting features of the waveform that happen just before and after the spike, and which would be obscured by ringing.> > One trick that I used in process control instrumentation was to > oversample by a factor of several hundred, do the main anti-aliasing > digitally and then decimate to a more appropriate sample rate. This > means that the analog anti-aliasing filter can be very cheap and when > the sampling can be synchronized to a multiple of the power line > frequency, you can get a double whammy. >I do that too. Doing the anti-aliasing as an average of all the samples leading up to the current decimated sample passes DC unmolested, but puts a notch at all the harmonics of your decimated sampling frequency -- so any noise that _would_ alias into your system's passband gets notched, at the least cost in phase shift in the anti-alias filter. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 25, 20062006-09-25
Tim Wescott skrev:> > Why would EKG recorders be especially problematic? If they are, then > > maybe EEG and polygraph recorders too? > > AFAIK they aren't especially problematic, it's just one that I'm > familiar with being an issue -- goodness knows why,I once attended a conference on DSP where somebody presented some scheme for processing EKG (that's the heart monitor, right?) signals. According to that presentation, ringing and transients are severe issues since physicians inspect the transient characteristic of the heartbeat in order to come up with a diagnostics. Any transient that is not "natural" (i.e. does not conform with text-book descriptions of normal heartbeat behaviour) is, by necessity, interpreted as an indicator of some non-healthy condition. It would be very, very dangerous to meddle with these systems such that the sensor system somehow alters the transient shape of the signal. Rune
Reply by ●September 25, 20062006-09-25
Scott Seidman wrote:> Tim Wescott <tim@seemywebsite.com> wrote in news:1q- > dndrgmcy9n4XYnZ2dnUVZ_vWdnZ2d@web-ster.com: > > >>while a control loop can't even >>have delay. >> > > > That seems somewhat overly generalized, no? >Yes. I can either say "that's very clever of you to have caught that", or I can fess up to leaving an "almost" out of there somewhere. Certainly the performance of many control loops out there are limited by delay, and the performance of almost all "high performance" control loops ends up getting limited by the phase shift in the plant, sensors or controller. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 25, 20062006-09-25
Rune Allnor wrote:> Tim Wescott skrev: > >>>Why would EKG recorders be especially problematic? If they are, then >>>maybe EEG and polygraph recorders too? >> >>AFAIK they aren't especially problematic, it's just one that I'm >>familiar with being an issue -- goodness knows why, > > > I once attended a conference on DSP where somebody presented > some scheme for processing EKG (that's the heart monitor, right?) > signals. > > According to that presentation, ringing and transients are severe > issues since physicians inspect the transient characteristic of the > heartbeat in order to come up with a diagnostics. Any transient > that is not "natural" (i.e. does not conform with text-book > descriptions of normal heartbeat behaviour) is, by necessity, > interpreted as an indicator of some non-healthy condition. > It would be very, very dangerous to meddle with these > systems such that the sensor system somehow alters > the transient shape of the signal. > > Rune >That is my understanding, too. Come to think of it, I'm not sure if it would be as much of a case with EEG and polygraph machines, because they don't have the same sort of sharp transient embedded in bumps the way EKGs do. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●September 25, 20062006-09-25
"Tim Wescott" <tim@seemywebsite.com> wrote in message news:D92dnYQDa9DIjYXYnZ2dnUVZ_sGdnZ2d@web-ster.com...> > Nyquist holds, but it can be difficult to go from a real-world (i.e. > inherently infinite bandwidth) signal to a bandlimited one without > ringing -- I'm discussing this, in the context of how you need to be aware > of the issue when establishing your sampling rate.Tim, There are classes of windows or filters that have no ringing in one domain and have minimum transition width in the other. That's as good as it gets! See: Temes, Barcilon and Marshall "The Optimization of Bandlimited Sytems" Proc IEEE Vol 61 No. 2 Feb 1973 pp 196-234 I believe one can find approximations to the optimum that are close enough.... Fred
