Do I need a window?
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From: Clay on 22 Mar 2010 13:04 On Mar 22, 9:35 am, "blackhermi" <dheeraj.iitm(a)n_o_s_p_a_m.gmail.com> wrote: > Hi > > I need to find out the frequencies present in the variation of a physical > quantity from a discretely sampled data of finite length. Which window (if > any) should I use? I think doing an fftn in MATLAB uses a rectangular > function by default. Please correct me if I am wrong. > > Also, is there a way to create a 2D version of a given window? > > Thanks and Regards Do you know apriori how many frequencies are in the data? I.e., do you have a physical reason for there being n frequnecies in the data? Are the frequencies constant in time? Clay
From: Rick Lyons on 28 Mar 2010 08:18 On Mon, 22 Mar 2010 07:14:08 -0700, Tim Wescott <tim(a)seemywebsite.now> wrote: >Rune Allnor wrote: >> On 22 Mar, 14:35, "blackhermi" <dheeraj.iitm(a)n_o_s_p_a_m.gmail.com> >> wrote: >>> Hi >>> >>> I need to find out the frequencies present in the variation of a physical >>> quantity from a discretely sampled data of finite length. Which window (if >>> any) should I use? >> >> It depends entirely on the data and what you attempt to do. >> If the SNR is large and the sinusoidals are well separated, >> then don't use any. If the SNR is low and you need to >> suppress side lobes, choose a window that can be tuned to >> your particular needs (the Kaiser window is just about the >> only one that fits that bill). Or avoid controversy by using >> the most popular window that everybody else use. I don't have >> any statistics, but the Hanning / Hann / von Hann window ought >> to end up among the top 3 in the popularity polls. > >Understanding the properties of the FFT would help, too -- that will let >you understand why you want to window the data, which in turn will help >you understand what window (if any) you want and why. > >While I could stand in front of a room of fellow engineers and defend a >choice of window by "it's popular _and_ it doesn't matter much anyway", >defending it with "'cause it's popular" would quite deservedly get me >laughed out of the room. Hi Tim, If someone asks me, "Why did you use the Hanning window in this application?", I typically answer with either, (1) "Because that's the way God wanted it to be.", or (2) "Because that's what the voices told me to use." See Ya', [Rick-]
From: Andor on 29 Mar 2010 03:16 On 28 Mrz., 14:18, Rick Lyons <R.Lyons@_BOGUS_ieee.org> wrote: > On Mon, 22 Mar 2010 07:14:08 -0700, Tim Wescott <t...(a)seemywebsite.now> > wrote: > > > > > > >Rune Allnor wrote: > >> On 22 Mar, 14:35, "blackhermi" <dheeraj.iitm(a)n_o_s_p_a_m.gmail.com> > >> wrote: > >>> Hi > > >>> I need to find out the frequencies present in the variation of a physical > >>> quantity from a discretely sampled data of finite length. Which window (if > >>> any) should I use? > > >> It depends entirely on the data and what you attempt to do. > >> If the SNR is large and the sinusoidals are well separated, > >> then don't use any. If the SNR is low and you need to > >> suppress side lobes, choose a window that can be tuned to > >> your particular needs (the Kaiser window is just about the > >> only one that fits that bill). Or avoid controversy by using > >> the most popular window that everybody else use. I don't have > >> any statistics, but the Hanning / Hann / von Hann window ought > >> to end up among the top 3 in the popularity polls. > > >Understanding the properties of the FFT would help, too -- that will let > >you understand why you want to window the data, which in turn will help > >you understand what window (if any) you want and why. > > >While I could stand in front of a room of fellow engineers and defend a > >choice of window by "it's popular _and_ it doesn't matter much anyway", > >defending it with "'cause it's popular" would quite deservedly get me > >laughed out of the room. > > Hi Tim, > If someone asks me, "Why did you use the > Hanning window in this application?", > I typically answer with either, (1) "Because that's > the way God wanted it to be.", or (2) "Because that's > what the voices told me to use." Hi Rick My knee-jerk answer would be: "The guy's name was von Hann!" Regards, Andor
From: Rick Lyons on 11 Apr 2010 19:29
On Mon, 29 Mar 2010 00:16:39 -0700 (PDT), Andor <andor.bariska(a)gmail.com> wrote: >On 28 Mrz., 14:18, Rick Lyons <R.Lyons@_BOGUS_ieee.org> wrote: >> On Mon, 22 Mar 2010 07:14:08 -0700, Tim Wescott <t...(a)seemywebsite.now> >> wrote: >> >> >> >> >> >> >Rune Allnor wrote: >> >> On 22 Mar, 14:35, "blackhermi" <dheeraj.iitm(a)n_o_s_p_a_m.gmail.com> >> >> wrote: >> >>> Hi >> >> >>> I need to find out the frequencies present in the variation of a physical >> >>> quantity from a discretely sampled data of finite length. Which window (if >> >>> any) should I use? >> >> >> It depends entirely on the data and what you attempt to do. >> >> If the SNR is large and the sinusoidals are well separated, >> >> then don't use any. If the SNR is low and you need to >> >> suppress side lobes, choose a window that can be tuned to >> >> your particular needs (the Kaiser window is just about the >> >> only one that fits that bill). Or avoid controversy by using >> >> the most popular window that everybody else use. I don't have >> >> any statistics, but the Hanning / Hann / von Hann window ought >> >> to end up among the top 3 in the popularity polls. >> >> >Understanding the properties of the FFT would help, too -- that will let >> >you understand why you want to window the data, which in turn will help >> >you understand what window (if any) you want and why. >> >> >While I could stand in front of a room of fellow engineers and defend a >> >choice of window by "it's popular _and_ it doesn't matter much anyway", >> >defending it with "'cause it's popular" would quite deservedly get me >> >laughed out of the room. >> >> Hi Tim, >> � If someone asks me, "Why did you use the >> Hanning window in this application?", >> I typically answer with either, (1) "Because that's >> the way God wanted it to be.", or (2) "Because that's >> what the voices told me to use." > >Hi Rick > >My knee-jerk answer would be: "The guy's name was von Hann!" > >Regards, >Andor Hi Andor, You are exactly correct. Regards, [-Rick-] |