From: XXguille AT XXiies DOT XXes (Guillermo Rodriguez Garcia) Newsgroups: comp.os.msdos.djgpp Subject: Re: Floating point..... I think.... Date: Thu, 17 Jun 1999 14:07:14 GMT Organization: Telefonica Transmision de Datos Lines: 37 Message-ID: <3768fca0.977979@noticias.iies.es> References: NNTP-Posting-Host: iies197.iies.es Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Newsreader: Forte Agent 1.5/32.451 To: djgpp AT delorie DOT com DJ-Gateway: from newsgroup comp.os.msdos.djgpp Reply-To: djgpp AT delorie DOT com El día Wed, 16 Jun 1999 13:30:19 +0300 (IDT), Eli Zaretskii escribió: >> What are you calling DFT and FFT? DFT is a transformation, and FFT is >> an algorithm to implement DFT, so how is it that you seem to have >> separate functions for DFT and FFT? > >FFT is an algorithm to perform a lot of DFT's simultaneously >in an efficient way. A single DFT is just the value of the >spectrum for a certain frequency, and doesn't need any algorithms >to compute. The DFT (Discrete Fourier Transform) is a mathematical transformation that maps a complex valued discrete function, f(n), in another complex valued function, F(m), which holds information about the frequency spectrum of the former. The resulting function, F(m), can be evaluated at exactly N points of the spectrum, where N is the number of data points of f(n). The FFT is an algorithm to efficiently compute the DFT which is mostly used when N is a power of 2. There are many other algorithms which also compute the DFT, like Goertzel's, but you can just compute the DFT directly, that is, by using its *definition*. You can compute all the N values of the DFT or just the ones you want, but in order to do this you need an algorithm as well, even when it can be as simple as traversing all the values of f(n) and multiplying them for the appropiate complex exponentials. If you say that you need no algorithms to compute the DFT, I may reply that you don't need one for the FFT either, as it can be also expressed as a weighted sums of complex exponentials, just like a DFT where most of the unneeded terms have been left out. Regards, GUILLE ---- Guillermo Rodriguez Garcia XXguille AT XXiies DOT XXes (ya sabes :-)