FFT phase

robert bristow-johnson wrote:
> dbd wrote:
> >
> >
> > The circularity of the DFT is a powerful concept that should only be
> > used for good.
> >
>
> oh, i guess that leaves me out.  i want to use the inherent circularity
> of the DFT for evil.
>
> Dale, how might i use the DFT for something not good?

By giving some poor undergrads an fft which references phase
to the circular discontinuity in the fft window, and then asking
them to interpolated the frequency and phase of a longer (and
thus windowed) non-synchronous sinusoid?  :-)


-- 
rhn A.T nicholson d.0.t C-o-M

robert bristow-johnson wrote:
> dbd wrote:
>>
>> The circularity of the DFT is a powerful concept that should only be
>> used for good.
>>
> 
> oh, i guess that leaves me out.  i want to use the inherent circularity
> of the DFT for evil.
> 
> Dale, how might i use the DFT for something not good?
> 
> just curious.
> 
> r b-j
> 
I guess you haven't worked in defence, or you'd know lots of really evil 
ways to use DFTs.

Steve

robert bristow-johnson wrote:
> dbd wrote:
> >
> >
> > The circularity of the DFT is a powerful concept that should only be
> > used for good.
> >
>
> oh, i guess that leaves me out.  i want to use the inherent circularity
> of the DFT for evil.
>
> Dale, how might i use the DFT for something not good?
>
> just curious.
>
> r b-j

I've always been concerned about the misuse of windows. This is a
concern that echos the thoughts of some of the great philosophers of
our age:

If you asked me to name the three scariest threats facing the human
race, I would give the same answer that most people would: nuclear war,
global warming and Windows.
Dave Barry

Windows are an often misunderstood and misapplied stepchild of spectral
analysis. Many windows are used in machine processing, not because they
are in some way superior, but rather because they are easy to
visualize; or are simply generated; or someone thinks that they perform
well; or "we've always done it this way".
fred harris  1976


Dale B. Dalrymple
http://dbdimages.com

Jeff Caunter ha scritto:

> john john wrote:
>
> >I can't understand the phrase "....this will be modified
> >if window function used.". Why?
>
> The magnitude in the spectrum will be reduced because the input frame is
> multiplied by a vector whose elements have a mean amplitude less than
> unity.
>
> Jeff

Do you know any C++ code about how to find and calculate the correct
phase and magnitude using hamming window too?

Not FFT or FFTW only pls. I have seen they a lot of time but nothing I
need really. I need a code to find correct phase value. In the Net
there isn't nothing about it. Incredible. Only an incredible amount of
theory. Blah. ....and no more with matlab pls :-( . Not all the people
knows matlab. My question is different than how to use matlab and its
functions.

Jeff Caunter ha scritto:

> john john wrote:
>
> >I can't understand the phrase "....this will be modified
> >if window function used.". Why?
>
> The magnitude in the spectrum will be reduced because the input frame is
> multiplied by a vector whose elements have a mean amplitude less than
> unity.
>
> Jeff

Do you know any C++ code about how to find and calculate the correct
phase and magnitude using hamming window too?

Not FFT or FFTW only pls. I have seen they a lot of time but nothing I
need really. I need a code to find correct phase value. In the Net
there isn't nothing about it. Incredible. Only an incredible amount of
theory. Blah. ....and no more with matlab pls :-( . Not all the people
knows matlab. My question is different than how to use matlab and its
functions.

Jeff Caunter ha scritto:

> john john wrote:
>
> >I can't understand the phrase "....this will be modified
> >if window function used.". Why?
>
> The magnitude in the spectrum will be reduced because the input frame is
> multiplied by a vector whose elements have a mean amplitude less than
> unity.
>
> Jeff

Do you know any C++ code about how to find and calculate the correct
phase and magnitude using hamming window too?

Not FFT or FFTW only pls. I have seen they a lot of time but nothing I
need really. I need a code to find correct phase value. In the Net
there isn't nothing about it. Incredible. Only an incredible amount of
theory. Blah. ....and no more with matlab pls :-( . Not all the people
knows matlab. My question is different than how to use matlab and its
functions.

Jeff Caunter ha scritto:

> john john wrote:
>
> >I can't understand the phrase "....this will be modified
> >if window function used.". Why?
>
> The magnitude in the spectrum will be reduced because the input frame is
> multiplied by a vector whose elements have a mean amplitude less than
> unity.
>
> Jeff

Do you know any C++ code about how to find and calculate the correct
phase and magnitude using hamming window too?

Not FFT or FFTW only pls. I have seen they a lot of time but nothing I
need really. I need a code to find correct phase value. In the Net
there isn't nothing about it. Incredible. Only an incredible amount of
theory. Blah. ....and no more with matlab pls :-( . Not all the people
knows matlab. My question is different than how to use matlab and its
functions.

john john wrote:
> I need a code to find correct phase value.

The phase of what with reference to what?
Midnight 1/1/2007 GMT?  Or some other point in time?


IMHO. YMMV.
-- 
rhn A.T nicholson d.0.t C-o-M

Ron N. ha scritto:

>
> The phase of what with reference to what?
> Midnight 1/1/2007 GMT?  Or some other point in time?
>
>
> IMHO. YMMV.
> --
> rhn A.T nicholson d.0.t C-o-M

We have a waveform from a certain number of data, tell 1024. From this
waveform you can have a lot of different frequencies. Right? Ok. No
think at magnitude. Is not important now.  Every frequency has a proper
phase. I need to reference this phase with something? Can you show me
how? In my calculation I find negative frequecies. Possible? In this
days I've searched this informations in the web but I've seen that is
impossible to find a code (c, vb, fortran etc) that explain it. I can't
believe at this.

Look here at my results:

Triangular waveform

N   Magnitude      Phase
0)  0.000000   0.000000
1)  103.753218   -90.045649
2)  0.000000   0.000000
3)  11.528425   90.045649
4)  0.000000   0.000000
5)  4.150441   -90.045649
6)  0.000000   0.000000
7)  2.117732   90.045649
8)  0.000000   0.000000
9)  1.281225   -90.045649
10)  0.000000   0.000000
11)  0.857788   90.045649
12)  0.000000   0.000000
13)  0.614248   -90.045649
14)  0.000000   0.000000
15)  0.461450   90.045649
16)  0.000000   0.000000
17)  0.359332   -90.045649
18)  0.000000   0.000000
....................
1023).............

Square wave

N   Magnitude      Phase
0)  0.000000   0.000000
1)  318.310386   -89.869779
2)  0.000000   0.000000
3)  106.104793   -89.518038
4)  0.000000   0.000000
5)  63.664474   -89.166297
6)  0.000000   0.000000
7)  45.476336   -88.814556
8)  0.000000   0.000000
9)  35.372260   -88.462816
10)  0.000000   0.000000
11)  28.942756   -88.111075
12)  0.000000   0.000000
13)  24.491869   -87.759334
14)  0.000000   0.000000
15)  21.228151   -87.407593
16)  0.000000   0.000000
17)  18.732602   -87.055852
18)  0.000000   0.000000
19)  16.762643   -86.704112
20)  0.000000   0.000000
21)  15.168105   -86.352371
22)  0.000000   0.000000
23)  13.851052   -86.000630
24)  0.000000   0.000000
25)  12.744888   -85.648889
26)  0.000000   0.000000
27)  11.802748   -85.297148
28)  0.000000   0.000000
29)  10.990697   -84.945407
30)  0.000000   0.000000
........................
1023)....

what about.

john john wrote:
> Every frequency has a proper
> phase. I need to reference this phase with something?

Yes.  For instance, a weekly cosine wave starting with phase 0
at Sunday midnight will not be the same as a cosine wave starting
with phase 0 at Monday noon.  A phase of 0 tells you little without
a reference point.  You will also need a direction if you have a
time machine which can go in either direction (past or future).