> wow, thanks for your reply, I'll have a look at it.
> Now I'm not taking care about the phase so I could discard those phase
> rotations I think. Since now I was just performing 8 FFTs of 64 blocks
> of consecutive data (not mixed as you said) and summing up them all,
> but probably that's not correct
>
Nope, the phase rotations are required as part of the FFT computation,
they are done between the smaller FFTs. That mixed radix algorithm is
simply a decomposition of the Fourier transform to allow you to compute
a larger FFT using smaller kernels. You need to do this in order to get
the spectra of a 512 point FFT. If you skip the phase rotations, you
are basically doing a 2D FFT on data in an 8x64 array.
Reply by Jerry Avins●February 16, 20072007-02-16
Al wrote:
...
> There's no way to get the same spectra by using consecutive data then,
> right?
Right.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by Jerry Avins●February 16, 20072007-02-16
Al wrote:
...
> thanks! and by using this algorithm, Am I preserving the resolution
> then or definitely not?
It's just a different way to calculate a 512-point transform. Compare
the data flow in radix 2 and radix 4 FFTs to get a general idea. (You
should be able to find those with Google.) Ray described a mixed-radix
(2 and 8) for you. In the end, you have the same numbers in the same bins.
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by Al●February 16, 20072007-02-16
On Feb 16, 5:27 pm, Jerry Avins <j...@ieee.org> wrote:
> Al wrote:
> > On Feb 16, 3:24 pm, Ray Andraka <r...@andraka.com> wrote:
> >> Al wrote:
> >>> Hi everybody! I have a question regarding FFT. I'm working with radar
> >>> data & matlab.
> >>> I have a block of data of 512 samples and I'm doing its FFT, but I
> >>> want to do the same with exactly the same block of data but in blocks
> >>> of 64 samples each, so I should do 8 FFTs right?
> >>> What I don't know is how to merge those 8 FFTs afterwards, to be able
> >>> to compare the two methods, that is, the FFT of 512 samples, or 8 FFTs
> >>> of 64 samples each.
> >>> Thank you!!
> >> If I understand you correctly, you want to use a 64 point FFT to perfo=
rm
> >> your 512 point transform? If so, then you want to look at the mixed
> >> radix algorithm. Basically, you take every 8th point and perform the =
64
> >> point FFTs on those. i.e. a 64 point of of points 0,8,16,.... one of
> >> points 1,9,17,... one of points 2,10,18,... Then you combine the
> >> results by doing a phase rotation and then 8 point FFTs across the sets
>
> >> you wind up doing 8 64-point FFTs, 512 phase rotations and then 64
> >> 8-point FFTs.
>
> > wow, thanks for your reply, I'll have a look at it.
> > Now I'm not taking care about the phase so I could discard those phase
> > rotations I think.
>
> No. You need them for the calculation. You can discard them later.
>
> > Since now I was just performing 8 FFTs of 64 blocks
> > of consecutive data (not mixed as you said) and summing up them all,
> > but probably that's not correct
>
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF
There's no way to get the same spectra by using consecutive data then,
right?
Reply by Al●February 16, 20072007-02-16
On Feb 16, 5:27 pm, Jerry Avins <j...@ieee.org> wrote:
> Al wrote:
> > On Feb 16, 3:24 pm, Ray Andraka <r...@andraka.com> wrote:
> >> Al wrote:
> >>> Hi everybody! I have a question regarding FFT. I'm working with radar
> >>> data & matlab.
> >>> I have a block of data of 512 samples and I'm doing its FFT, but I
> >>> want to do the same with exactly the same block of data but in blocks
> >>> of 64 samples each, so I should do 8 FFTs right?
> >>> What I don't know is how to merge those 8 FFTs afterwards, to be able
> >>> to compare the two methods, that is, the FFT of 512 samples, or 8 FFTs
> >>> of 64 samples each.
> >>> Thank you!!
> >> If I understand you correctly, you want to use a 64 point FFT to perfo=
rm
> >> your 512 point transform? If so, then you want to look at the mixed
> >> radix algorithm. Basically, you take every 8th point and perform the =
64
> >> point FFTs on those. i.e. a 64 point of of points 0,8,16,.... one of
> >> points 1,9,17,... one of points 2,10,18,... Then you combine the
> >> results by doing a phase rotation and then 8 point FFTs across the sets
>
> >> you wind up doing 8 64-point FFTs, 512 phase rotations and then 64
> >> 8-point FFTs.
>
> > wow, thanks for your reply, I'll have a look at it.
> > Now I'm not taking care about the phase so I could discard those phase
> > rotations I think.
>
> No. You need them for the calculation. You can discard them later.
>
> > Since now I was just performing 8 FFTs of 64 blocks
> > of consecutive data (not mixed as you said) and summing up them all,
> > but probably that's not correct
>
> Jerry
> --
> Engineering is the art of making what you want from things you can get.
> =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=
=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF
thanks! and by using this algorithm, Am I preserving the resolution
then or definitely not?
Reply by Jerry Avins●February 16, 20072007-02-16
Al wrote:
> On Feb 16, 3:24 pm, Ray Andraka <r...@andraka.com> wrote:
>> Al wrote:
>>> Hi everybody! I have a question regarding FFT. I'm working with radar
>>> data & matlab.
>>> I have a block of data of 512 samples and I'm doing its FFT, but I
>>> want to do the same with exactly the same block of data but in blocks
>>> of 64 samples each, so I should do 8 FFTs right?
>>> What I don't know is how to merge those 8 FFTs afterwards, to be able
>>> to compare the two methods, that is, the FFT of 512 samples, or 8 FFTs
>>> of 64 samples each.
>>> Thank you!!
>> If I understand you correctly, you want to use a 64 point FFT to perform
>> your 512 point transform? If so, then you want to look at the mixed
>> radix algorithm. Basically, you take every 8th point and perform the 64
>> point FFTs on those. i.e. a 64 point of of points 0,8,16,.... one of
>> points 1,9,17,... one of points 2,10,18,... Then you combine the
>> results by doing a phase rotation and then 8 point FFTs across the sets
>>
>> you wind up doing 8 64-point FFTs, 512 phase rotations and then 64
>> 8-point FFTs.
>
> wow, thanks for your reply, I'll have a look at it.
> Now I'm not taking care about the phase so I could discard those phase
> rotations I think.
No. You need them for the calculation. You can discard them later.
> Since now I was just performing 8 FFTs of 64 blocks
> of consecutive data (not mixed as you said) and summing up them all,
> but probably that's not correct
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by Al●February 16, 20072007-02-16
On Feb 16, 3:24 pm, Ray Andraka <r...@andraka.com> wrote:
> Al wrote:
> > Hi everybody! I have a question regarding FFT. I'm working with radar
> > data & matlab.
>
> > I have a block of data of 512 samples and I'm doing its FFT, but I
> > want to do the same with exactly the same block of data but in blocks
> > of 64 samples each, so I should do 8 FFTs right?
> > What I don't know is how to merge those 8 FFTs afterwards, to be able
> > to compare the two methods, that is, the FFT of 512 samples, or 8 FFTs
> > of 64 samples each.
> > Thank you!!
>
> If I understand you correctly, you want to use a 64 point FFT to perform
> your 512 point transform? If so, then you want to look at the mixed
> radix algorithm. Basically, you take every 8th point and perform the 64
> point FFTs on those. i.e. a 64 point of of points 0,8,16,.... one of
> points 1,9,17,... one of points 2,10,18,... Then you combine the
> results by doing a phase rotation and then 8 point FFTs across the sets
>
> you wind up doing 8 64-point FFTs, 512 phase rotations and then 64
> 8-point FFTs.
wow, thanks for your reply, I'll have a look at it.
Now I'm not taking care about the phase so I could discard those phase
rotations I think. Since now I was just performing 8 FFTs of 64 blocks
of consecutive data (not mixed as you said) and summing up them all,
but probably that's not correct
Reply by Ray Andraka●February 16, 20072007-02-16
Al wrote:
> Hi everybody! I have a question regarding FFT. I'm working with radar
> data & matlab.
>
> I have a block of data of 512 samples and I'm doing its FFT, but I
> want to do the same with exactly the same block of data but in blocks
> of 64 samples each, so I should do 8 FFTs right?
> What I don't know is how to merge those 8 FFTs afterwards, to be able
> to compare the two methods, that is, the FFT of 512 samples, or 8 FFTs
> of 64 samples each.
> Thank you!!
>
If I understand you correctly, you want to use a 64 point FFT to perform
your 512 point transform? If so, then you want to look at the mixed
radix algorithm. Basically, you take every 8th point and perform the 64
point FFTs on those. i.e. a 64 point of of points 0,8,16,.... one of
points 1,9,17,... one of points 2,10,18,... Then you combine the
results by doing a phase rotation and then 8 point FFTs across the sets
you wind up doing 8 64-point FFTs, 512 phase rotations and then 64
8-point FFTs.
Reply by Al●February 16, 20072007-02-16
On Feb 16, 1:10 pm, cincy...@gmail.com wrote:
> On Feb 16, 6:54 am, "Al" <albertzur...@gmail.com> wrote:
>
> > Hi everybody! I have a question regarding FFT. I'm working with radar
> > data & matlab.
>
> > I have a block of data of 512 samples and I'm doing its FFT, but I
> > want to do the same with exactly the same block of data but in blocks
> > of 64 samples each, so I should do 8 FFTs right?
> > What I don't know is how to merge those 8 FFTs afterwards, to be able
> > to compare the two methods, that is, the FFT of 512 samples, or 8 FFTs
> > of 64 samples each.
> > Thank you!!
>
> The issue here is that if you do 8 FFTs of length 64, you don't get
> data that you can directly compare to the 512-length FFT. When you do
> a FFT on a set of data, the spacing of the frequency bins is (2*pi)/N.
> So, each of your 64-length FFT outputs will have a frequency spacing
> of pi/32, while your 512-length FFT will have a frequency spacing of
> pi/256. You could average your 8 64-length blocks, then decimate the
> larger FFT's output to do a direct comparison between like frequency
> bins, but I'm not sure what that buys you. In this sense, it's not
> like block filtering, where you can break the operation up into
> smaller chunks; if you shorten the length of the FFT, you get coarser
> frequency spacing on the output.
>
> Jason
mmm yes you're right.but i'm not reusing data, so that for the first
FFT of 512 I'm using 512 samples, and for the other FFT's I'm using
1:64, 65:128, 129:192 and so on. I know that the resolution will be
still worse but i wanted to compare the two methods just to see if
there are 'fake' spikes or other modes in the signal when using the
complete 512 FFT
Reply by ●February 16, 20072007-02-16
On Feb 16, 6:54 am, "Al" <albertzur...@gmail.com> wrote:
> Hi everybody! I have a question regarding FFT. I'm working with radar
> data & matlab.
>
> I have a block of data of 512 samples and I'm doing its FFT, but I
> want to do the same with exactly the same block of data but in blocks
> of 64 samples each, so I should do 8 FFTs right?
> What I don't know is how to merge those 8 FFTs afterwards, to be able
> to compare the two methods, that is, the FFT of 512 samples, or 8 FFTs
> of 64 samples each.
> Thank you!!
The issue here is that if you do 8 FFTs of length 64, you don't get
data that you can directly compare to the 512-length FFT. When you do
a FFT on a set of data, the spacing of the frequency bins is (2*pi)/N.
So, each of your 64-length FFT outputs will have a frequency spacing
of pi/32, while your 512-length FFT will have a frequency spacing of
pi/256. You could average your 8 64-length blocks, then decimate the
larger FFT's output to do a direct comparison between like frequency
bins, but I'm not sure what that buys you. In this sense, it's not
like block filtering, where you can break the operation up into
smaller chunks; if you shorten the length of the FFT, you get coarser
frequency spacing on the output.
Jason