jeff227 skrev:> OK, I think I understand where my original thoughts were wrong. > > If you hit a mechanical mass, no matter how big it is, with an impulse it > will move - maybe very small - but it WILL move. It will oscillate in a > decaying amplitude forever. This is an IIR filter, an RC filter, etc. > > My confusion was "transition band" vs. "transition frequency" in a FIR > filter. A "perfect" LP FIR has a perfect (zero width) transition band, > infinite length and a sinc function that "oscillates" forever. > > What my analogy of the infinite mass incorrectly represented is NOT the > sinc function - it is Fc, the transistion frequency. As the mass goes to > infinity Fc goes to zero. At infinite mass (and zero Fc) the integration > becomes a flat line - there is no change. > > Fc vs. transition band. THAT was my error. > > Do I understand it now?You might want to add to your confusion by taking into account that your mass/spring/dashpot analogy corresponds to an IIR filter, not an FIR filter... Rune
Theory 101 - Impulse response of "perfect" LPF
Started by ●December 12, 2006
Reply by ●December 13, 20062006-12-13
Reply by ●December 13, 20062006-12-13
glen herrmannsfeldt wrote:> Jerry Avins wrote: > > (snip) > >> But not *every* LPF. Your analogy depends on a "perfect" lowpass, >> which an inductor is not. > > Superconducting inductors are pretty close, though.Interesting! How is the cutoff frequency set? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●December 13, 20062006-12-13
glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote in news:O4qdnZdDm- j1KOLYnZ2dnUVZ_vPinZ2d@comcast.com:> Jerry Avins wrote: > > (snip) > >> But not *every* LPF. Your analogy depends on a "perfect" lowpass, which >> an inductor is not. > > Superconducting inductors are pretty close, though. > > -- glen >When people say "perfect" LP filter, they are usually referring to a filter with a zero-phase "boxcar" in the frequency domain. They aren't refering to a filter constructed with "perfect" inductors or capacitors. -- Scott Reverse name to reply
Reply by ●December 13, 20062006-12-13
Scott Seidman wrote:> glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote in news:O4qdnZdDm- > j1KOLYnZ2dnUVZ_vPinZ2d@comcast.com: > >> Jerry Avins wrote: >> >> (snip) >> >>> But not *every* LPF. Your analogy depends on a "perfect" lowpass, which >>> an inductor is not. >> Superconducting inductors are pretty close, though. >> >> -- glen >> > > > When people say "perfect" LP filter, they are usually referring to a filter > with a zero-phase "boxcar" in the frequency domain. They aren't refering > to a filter constructed with "perfect" inductors or capacitors.Agreed. How do superconducting inductors resemble a brick-wall lowpass? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●December 13, 20062006-12-13
Clay wrote:> PeteS wrote: >> Rune Allnor wrote: >> >> [1] I have a BA in mathematics, and it was stated to me that >> 'one gets a BA because mathematics is not science, it is one of >> the arts'. > > Hello Pete, whether it is a BA or a BS in math is up to the > university or the underwriting authority. Many schools offer > both with the BA trading a few hours of math for a few hours of > a language such as Spanish, French, German, etc. > > IIRC, Newton called Mathematics the Queen of the sciences.The problem is placing mathematics' object of study. Does our concept of mathematical truth approximate a property of the universe or is it a function of the way humans structure their perceptions? The answer may well be unknowable -- or humanly unknowable if you prefer. Martin -- The biggest difference between time and space is that you can't reuse time. --Merrick Furst
Reply by ●December 13, 20062006-12-13
Jerry Avins <jya@ieee.org> wrote in news:od-dnUdsho_2gh3YnZ2dnUVZ_rTinZ2d@rcn.net:> Scott Seidman wrote: >> glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote in news:O4qdnZdDm- >> j1KOLYnZ2dnUVZ_vPinZ2d@comcast.com: >> >>> Jerry Avins wrote: >>> >>> (snip) >>> >>>> But not *every* LPF. Your analogy depends on a "perfect" lowpass, >>>> which an inductor is not. >>> Superconducting inductors are pretty close, though. >>> >>> -- glen >>> >> >> >> When people say "perfect" LP filter, they are usually referring to a >> filter with a zero-phase "boxcar" in the frequency domain. They >> aren't refering to a filter constructed with "perfect" inductors or >> capacitors. > > Agreed. How do superconducting inductors resemble a brick-wall > lowpass? > > JerryMy guess is that the poster was referring to the fact that a superconducting inductor has no serial resistance, inferring a "perfect" inductor, and made the incorrect jump that a perfect inductor could be used to build a perfect filter-- i.e., his definition of "perfect low pass filter" was not what others in this thread are using. It wouldn't seem like he meant to say that you could use a superconducting inductor to make a brick wall filter. -- Scott Reverse name to reply
Reply by ●December 13, 20062006-12-13
Scott Seidman wrote:> Jerry Avins <jya@ieee.org> wrote in > news:od-dnUdsho_2gh3YnZ2dnUVZ_rTinZ2d@rcn.net: > >> Scott Seidman wrote: >>> glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote in news:O4qdnZdDm- >>> j1KOLYnZ2dnUVZ_vPinZ2d@comcast.com: >>> >>>> Jerry Avins wrote: >>>> >>>> (snip) >>>> >>>>> But not *every* LPF. Your analogy depends on a "perfect" lowpass, >>>>> which an inductor is not. >>>> Superconducting inductors are pretty close, though. >>>> >>>> -- glen >>>> >>> >>> When people say "perfect" LP filter, they are usually referring to a >>> filter with a zero-phase "boxcar" in the frequency domain. They >>> aren't refering to a filter constructed with "perfect" inductors or >>> capacitors. >> Agreed. How do superconducting inductors resemble a brick-wall >> lowpass? >> >> Jerry > > > My guess is that the poster was referring to the fact that a > superconducting inductor has no serial resistance, inferring a "perfect" > inductor, and made the incorrect jump that a perfect inductor could be > used to build a perfect filter-- i.e., his definition of "perfect low > pass filter" was not what others in this thread are using. It wouldn't > seem like he meant to say that you could use a superconducting inductor > to make a brick wall filter.You're probably right. I've been jumping at a lot of wrong conclusions lately, especially when the wording gives me an initial shove (impulse?) in the wrong direction. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●December 13, 20062006-12-13
Jerry Avins <jya@ieee.org> wrote in news:7pydnZAuDqxC0R3YnZ2dnUVZ_uXinZ2d@rcn.net:> You're probably right. I've been jumping at a lot of wrong conclusions > lately, especially when the wording gives me an initial shove (impulse?) > in the wrong direction. > > JerryIts good training. Much more pleasant doing it here than having a light bulb going off in your head telling you that you and a client have been using different definitions for two months! FWIW, I teach in a biomedical engineering program, and I try my best to make sure that the undergrads we send out into the world know how to write and communicate. I'm in the middle of grading term papers now, and that has me in the habit of casting a big net for definitions, which the students often let drift. -- Scott Reverse name to reply
Reply by ●December 13, 20062006-12-13
"jeff227" <rocksonics@earthlink.net> wrote in message news:8cidnVYW8O0zR-PYnZ2dnUVZ_hm3nZ2d@giganews.com...> OK, I'm going to really stick my naive neck out on this one... > > > Why isn't the impulse response of a perfect LPF a flat line (i.e., zero)? > > For example, an infinitely large mass (the filter) hit by an infinitely > small particle (the impulse) would not move at all because of its inertia > (acceleration goes to zero as masses go to +/-infinity). > > So why is the impulse response of an infinitely long (perfect) LP FIR a > "sinc" function rather than zero?Nice troll. Welcome back, Airy.R.Bean! (Where have you "bean" all this time?) I've read through the thread and you've certainly got them all going again, even getting Jerry Avins to respond in his usual (immature?) style.
Reply by ●December 13, 20062006-12-13
DSP Guru skrev:> "jeff227" <rocksonics@earthlink.net> wrote in message > news:8cidnVYW8O0zR-PYnZ2dnUVZ_hm3nZ2d@giganews.com... > > OK, I'm going to really stick my naive neck out on this one... > > > > > > Why isn't the impulse response of a perfect LPF a flat line (i.e., zero)? > > > > For example, an infinitely large mass (the filter) hit by an infinitely > > small particle (the impulse) would not move at all because of its inertia > > (acceleration goes to zero as masses go to +/-infinity). > > > > So why is the impulse response of an infinitely long (perfect) LP FIR a > > "sinc" function rather than zero? > > Nice troll. > > Welcome back, Airy.R.Bean! > > (Where have you "bean" all this time?) > > I've read through the thread and you've certainly got them > all going again, even getting Jerry Avins to respond in his usual > (immature?) style.The only person I have seen use the word "immature" about others here on comp.dsp, is the very Mr Bean himself... Rune
