Path: utzoo!attcan!uunet!microsoft!brianw
From: brianw@microsoft.UUCP (Brian Willoughby)
Newsgroups: comp.dsp
Subject: Re: Adjust-Speed CD player??
Message-ID: <7927@microsoft.UUCP>
Date: 2 Oct 89 02:45:56 GMT
References: <3441@abaa.UUCP> <6028@jpl-devvax.JPL.NASA.GOV> <89255.105143P85025@BARILVM.BITNET>  <7767@microsoft.UUCP> <89264.171306P85025@BARILVM.BITNET> <7813@microsoft.UUCP> <474@eedsp.gatech.edu>
Reply-To: brianw@microsoft.UUCP (Brian Willoughby)
Organization: Microsoft Corp., Redmond WA
Lines: 42

In article <474@eedsp.gatech.edu> jensen@bessel.eedsp.gatech.edu (P. Allen Jensen) writes:
>An FFT can be done in real-time if you assume that real-time can include
>a delay between input and output for startup.  Then you get the first set
>of samples, do the FFT and get the next set of samples in paralle.  The
>FFT must be done in less than or equal to the time to get the first set
>of samples.  You then have a pipeline doing FFT and getting the next set
>of samples with a delay of one window (frame) time.
>
>Anyone see any problems with that ?

I don't think that the delay would be the big problem, but IF the goal is
to subsequently compute an inverse transform for the purpose of creating
audio, then you must realize that you have lost information about the
original signal.  The problem is that the result of such a series of FFT
calculations is not a continuous picture of the frequency spectrum, but
an average over a relatively long (compared to the sample rate) period of
time.  It is true that the FFT probably has enough info to give you the
original signal back, but if you do any processing on the data in the
frequency domain, then you are very likely to come up with incorrect
(read: distorted) results because of the averaging effects of the FFT.

I'm still curious about whether the sliding-FFT, or some version of the
DFT, is capable of generating detailed enough frequency domain data at
the same rate as the sampled data for the purpose of accurately
reconstructing a slightly modified (but equal quality) version of the
incoming audio samples.

If you just want to display the data, you are fine.  In fact, the
HP spectrum analyser we had at NCSU had a noticable delay, but the
information on the display still updated faster that I could react to
what I saw.

>P. Allen Jensen
>Georgia Tech, School of Electrical Engineering, Atlanta, GA  30332
>USENET: ...!{allegra,hplabs,ihnp4,ulysses}!gatech!eedsp!jensen
>INTERNET: jensen@eedsp.gatech.edu

Brian Willoughby
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