| Re: Discrete sine transform [message #30553 is a reply to message #30545] |
Thu, 02 May 2002 08:36   |
mvukovic
Messages: 63
Registered: July 1998
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"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
>>
>> --
>> Dr. M. J. Angling
>> Centre for RF Propagation and Atmospheric Research
>> QinetiQ Malvern
>> mjangling@QinetiQ.com
>>
>>
>>
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
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