| Re: Discrete sine transform [message #30545] |
Fri, 03 May 2002 00:41  |
Matthew Angling
Messages: 5
Registered: May 2002
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Junior Member |
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Mirko Vukovic <mvukovic@taz.telusa.com> wrote in message
news:d96c8f7c.0205020736.6eceab0d@posting.google.com...
> "Dominik Paul" <dpaul@ukl.uni-freiburg.de> wrote in message
news:<aar135$hqq$1@n.ruf.uni-freiburg.de>...
>> I am not 100% sure about it, but I think the sin transform is just
the
>> imaginary part of the Fourie Transformation.
>>
>> Dom
>>
>> "Matthew Angling" <mjangling@QinetiQ.com> schrieb im Newsbeitrag
>> news:aaqvkg$85k$1@hamble.qinetiq.com...
>>> Hello all,
>>>
>>> I'm another lurker drawn out to ask for help. I think this is
more of
>>> a maths question rather than IDL, but as I'm working with IDL
and this
>>> seems to be the most friendly newsgroup I know of, I thought I'd
try
>>> here!
>>>
>>> Does anybody have the algorithm for the discrete sine tranform
coded
>>> in IDL. If I understand correctly it can be done (with some
>>> redundency) by using the FFT - but I'm not sure how!
>>>
>>> Thanks in advance for your help,
>>>
>>> Matt
>>>
>>> --
>
> nope! Funny, I was just leafing Numerical Recipes on that topic
> yesterday. By way of explanation (and probably quoting the Book and
> other authorities), the sine transform is a ``complete'' transform
of
> the data, i.e., you can get the data back by doing an inverse
> transform. If you were just to take the imaginary part of your FFT,
> you would loose half the information, and could not recover it when
> doing the inverse.
>
> The Book does provide an algorithm to do it using the FFT. You have
> to make an anti-symmetric series out of your data by extending the
> data series. The first N samples stay the same. The other N
samples
> are anti-symmetric about N+1 where the N+1 sample must be zero.
I've
> never done it, and advise going to the Book itself to make sure you
> got everything right. The topic is covered in Ch. 12 of the second
> edition.
>
> Mirko
Hi!
Thanks for everybody's comments. If anybody is interested I have
attached my dst function below. Its a translation of the matlab dst
routine.
Matt
--
Dr. M. J. Angling
Centre for RF Propagation and Atmospheric Research
QinetiQ Malvern
Tel: +44 (0)1684 896460
Fax: +44 (0)1684 894657
email: mjangling@QinetiQ.com
------------------------------------------------------------ ----------
--------
Function mldst,a
;function to calculate the discrete sine transform
;translated from the matlab routine dst
;normalisation is split between the forward
;and inverse transforms
;i.e. mldst(mldst(test) is equal to test
;
;NB to take advantage of efficient FFTs the input vector
;should be length 2^m-1
n=n_elements(a)
i=complex(0,1)
;set up antisymmetric vector
y=[0,a,0,-1.*reverse(a)]
;do fft and normalise
yy=2.*sqrt(2*(n+1))*fft(y)
out=yy[1:n]/(-2.*i)
;check type
type=size(out,/type)
;return real part if input was real
if type ne 6 and type ne 9 then out=float(out)
return,out
end
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