Anyone have Scilab 2.7 or 3.0 up and running under WinXP? I'm trying to plot magnitude of FFT of rectangular pulse/window. My code is a(1:899)=1; a(900:1000)=0; plot2d( [1:1000], abs(fft(a,-1)), logflag="nl") My output does not resemble what Mr. Lyons gives in Figure 3-25c. Multiple sources agree with Rick ,)

# Any DSPers using Scilab [ NEWBIE in trouble ; ]

Started by ●September 4, 2004

Posted by ●September 4, 2004

Hi Richard, I've got scilab but I'm not a DSper and I haven't got Mr Lyons book ( although I've read good things about it). All I can say is that the output from scilab looks good to me : the first plotted point is the DC offset then it moves through the positive frequencies in increasing frequency , steps to the most negative frequency bin and plots those in increasing prequency (i.e. thats the way the FFT output is organised). Best of luck - Mike "Richard Owlett" <rowlett@atlascomm.net> wrote in message news:10jkl2umfkcu3f3@corp.supernews.com...> Anyone have Scilab 2.7 or 3.0 up and running under WinXP? > > I'm trying to plot magnitude of FFT of rectangular pulse/window. > > My code is > > a(1:899)=1; > a(900:1000)=0; > plot2d( [1:1000], abs(fft(a,-1)), logflag="nl") > > My output does not resemble what Mr. Lyons gives in Figure 3-25c. > > Multiple sources agree with Rick ,) > >

Posted by ●September 4, 2004

As a guess - maybe you are trying to plot an approximate power spectral density? If so using abs()^2 is better, Mike "Richard Owlett" <rowlett@atlascomm.net> wrote in message news:10jkl2umfkcu3f3@corp.supernews.com...> Anyone have Scilab 2.7 or 3.0 up and running under WinXP? > > I'm trying to plot magnitude of FFT of rectangular pulse/window. > > My code is > > a(1:899)=1; > a(900:1000)=0; > plot2d( [1:1000], abs(fft(a,-1)), logflag="nl") > > My output does not resemble what Mr. Lyons gives in Figure 3-25c. > > Multiple sources agree with Rick ,) > >

Posted by ●September 4, 2004

Richard Owlett wrote:> Anyone have Scilab 2.7 or 3.0 up and running under WinXP? > > I'm trying to plot magnitude of FFT of rectangular pulse/window. > > My code is > > a(1:899)=1; > a(900:1000)=0; > plot2d( [1:1000], abs(fft(a,-1)), logflag="nl") > > My output does not resemble what Mr. Lyons gives in Figure 3-25c. > > Multiple sources agree with Rick ,) > >It looks like exactly what you'd expect from the given code -- I don't have the book, so I can't comment on the figure. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com

Posted by ●September 5, 2004

Richard Owlett <rowlett@atlascomm.net> wrote in message news:<10jkl2umfkcu3f3@corp.supernews.com>...> Anyone have Scilab 2.7 or 3.0 up and running under WinXP? > > I'm trying to plot magnitude of FFT of rectangular pulse/window. > > My code is > > a(1:899)=1; > a(900:1000)=0; > plot2d( [1:1000], abs(fft(a,-1)), logflag="nl") > > My output does not resemble what Mr. Lyons gives in Figure 3-25c. > > Multiple sources agree with Rick ,)The figure start indexing from -(M-1)/2, not 0. Matlab has some function called "fftshift" to 'swap' the higher and lower ends of the spectrum, so that the DC component appears in the middle of the plot and not in the first sample. Perhaps scilab has a similar function. Rune

Posted by ●September 5, 2004

Mike Yarwood wrote:> As a guess - maybe you are trying to plot an approximate power spectral > density? If so using abs()^2 is better, > > Mike >I was trying to reproduce his figures. It's part of a sequence he uses to explain DFT's of rectangular functions.

Posted by ●September 5, 2004

Rune Allnor wrote:> Richard Owlett <rowlett@atlascomm.net> wrote in messagenews:<10jkl2umfkcu3f3@corp.supernews.com>...> >>Anyone have Scilab 2.7 or 3.0 up and running under WinXP? >> >>I'm trying to plot magnitude of FFT of rectangular pulse/window. >> >>My code is >> >>a(1:899)=1; >>a(900:1000)=0; >>plot2d( [1:1000], abs(fft(a,-1)), logflag="nl") >> >>My output does not resemble what Mr. Lyons gives in Figure 3-25c. >> >>Multiple sources agree with Rick ,) > > > The figure start indexing from -(M-1)/2, not 0. Matlab has some > function called "fftshift" to 'swap' the higher and lower ends > of the spectrum, so that the DC component appears in the middle > of the plot and not in the first sample. Perhaps scilab has a > similar function. > > RuneThat's not the problem. I'm used to making that swap mentally. I tried reducing number of points by a factor of 10. ie a(1:89)=1; a(90:100)=0; plot2d( [1:100], abs(fft(a,-1)), logflag="nl") That plot closely resembled Rick's. I went back and plotted the original 100 points at a time. The individual sections resembled Rick's. Question for all: Is there somewhere I can post my 100 point and 1000 point plots ( jpg files)?

Posted by ●September 5, 2004

Richard Owlett wrote: ...> Question for all: > Is there somewhere I can post my 100 point and 1000 point plots ( jpg > files)?Unless I missed something useful, alt.binaries seems to be entirely given over to porn. Not pictures, but lists of web sites. I suppose you can post there anyway, like old times. You can email them to me; I'll give them a URL of their own. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������

Posted by ●September 5, 2004

Jerry Avins wrote:> Richard Owlett wrote: > > ... > >> Question for all: >> Is there somewhere I can post my 100 point and 1000 point plots ( jpg >> files)? > > > Unless I missed something useful, alt.binaries seems to be entirely > given over to porn. Not pictures, but lists of web sites. I suppose you > can post there anyway, like old times. You can email them to me; I'll > give them a URL of their own. > > JerryThanks, I've sent you copies. At least I think I have. I'm not getting incoming mail but outgoing apparently works.

Posted by ●September 5, 2004

Richard Owlett <rowlett@atlascomm.net> wrote in message news:<10jkl2umfkcu3f3@corp.supernews.com>...> Anyone have Scilab 2.7 or 3.0 up and running under WinXP? > > I'm trying to plot magnitude of FFT of rectangular pulse/window. > > My code is > > a(1:899)=1; > a(900:1000)=0; > plot2d( [1:1000], abs(fft(a,-1)), logflag="nl") > > My output does not resemble what Mr. Lyons gives in Figure 3-25c. > > Multiple sources agree with Rick ,)Here's my attempt to plot the spectrum of a sinc pulse. Code in matlabese, as always: %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% N=64; % Length of frame M=16; % Length of rectangular pulse s=zeros(N,1); % Allocate initial memory for signal s(1:M)=ones(M,1); % Set non-zero samples plot(fftshift(abs(fft(s)))) % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Rune