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thedude
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 Posted:
Mon Dec 20, 2004 7:11 pm Post subject:
A better window than hanning |
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Hi
Im working with DSP for a school project.
We have made a sound compression program for wav files.
We uses a hanning window and makes 50% overlap on our sound samples,
before we use out fft.
Is there a window that uses less overlap with the same result? |
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Peter K.
Guest
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| Posted:
Mon Dec 20, 2004 7:43 pm Post subject:
Re: A better window than hanning |
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thedude wrote:
| Quote: | Im working with DSP for a school project.
We have made a sound compression program for wav files.
We uses a hanning window and makes 50% overlap on our sound samples,
before we use out fft.
Is there a window that uses less overlap with the same result?
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So the FFT does compression? What effect does the window have on the
compression? Why use a window at all?
You'll need to explain a little more clearly what you're doing and what
you hope to achieve.
Ciao,
Peter K. |
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Matt Timmermans
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| Posted:
Mon Dec 20, 2004 11:10 pm Post subject:
Re: A better window than hanning |
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"thedude" <gonzoz66@hotmail.com> wrote in message
news:5f5a61f6.0412200611.3da80e5d@posting.google.com...
| Quote: | Im working with DSP for a school project.
We have made a sound compression program for wav files.
We uses a hanning window and makes 50% overlap on our sound samples,
before we use out fft.
Is there a window that uses less overlap with the same result?
|
I assume that you're annoyed at having twice as many output samples as input
samples. Instead of messing with the windows, you should change the
transform. Google for information about the "modified discrete cosine
transform", or "lapped orthogonal transforms" in general.
--
Matt |
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Fred Marshall
Guest
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| Posted:
Mon Dec 20, 2004 11:20 pm Post subject:
Re: A better window than hanning |
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"thedude" <gonzoz66@hotmail.com> wrote in message
news:5f5a61f6.0412200611.3da80e5d@posting.google.com...
| Quote: | Hi
Im working with DSP for a school project.
We have made a sound compression program for wav files.
We uses a hanning window and makes 50% overlap on our sound samples,
before we use out fft.
Is there a window that uses less overlap with the same result?
|
I don't think that overlap and the window selection are related. But,
someone somewhere may have shown that it matters the tiniest bit. For what
you're doing I don't see how it can matter.
You can choose whatever overlap you like - including none. And, you can use
a von Hann / hanning window or some other window quite independent of the
overlap. It all depends on the algorithms you're using for compression.
Fred |
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George Bush
Guest
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| Posted:
Sat Dec 25, 2004 8:19 pm Post subject:
Re: A better window than hanning |
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In article <wL-dnQUXsZfUhVrcRVn-vA@centurytel.net>, "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote:
| Quote: |
I don't think that overlap and the window selection are related. But,
someone somewhere may have shown that it matters the tiniest bit. For what
you're doing I don't see how it can matter.
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It does if you go back to the time domain. A Hann window applied in the time
or frequency domain and fifty percent overlap will not have artifacts when
converted to the time domain. Fred Harris (fred harris) has a paper on this
subject from the 70s. Kaiser-Bessel and 75 percent overlap have the same
properties.
The only window that has this property and less overlap than 50% is the boxcar
window. At least as far as I know. |
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Greg
Guest
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| Posted:
Sun Dec 26, 2004 2:10 am Post subject:
Re: A better window than hanning |
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On Sat, 25 Dec 2004 15:19:37 GMT, george.w.bush@whitehouse.com
(George Bush) wrote:
| Quote: | It does if you go back to the time domain. A Hann window
applied in the time or frequency domain and fifty percent
overlap will not have artifacts when converted to the time
domain. Fred Harris (fred harris) has a paper on this
subject from the 70s. Kaiser-Bessel and 75 percent overlap
have the same properties.
|
I was unaware that Kaiser-Bessel had this property. (For which
value of Harris' alpha parameter is this true?)
Windows that sum-to-constant include:
Triangular, with 50% overlap
Cosine-Squared (Raised-Cosine; Hann), with 50% overlap
Cosine-Fourth, with 75% overlap
de la Vallé-Poussin, with 75% overlap
| Quote: | The only window that has this property and less overlap than
50% is the boxcar window. At least as far as I know.
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The only other named window that I know of that has the
sum-to-constant property with less than 50% overlap is the Tukey.
However, one could construct other windows in a manner similar to
the Tukey, that also sum-to-constant.
Greg Berchin |
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John E. Hadstate
Guest
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| Posted:
Sun Dec 26, 2004 3:52 am Post subject:
Re: A better window than hanning |
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"George Bush" <george.w.bush@whitehouse.com> wrote in message
news:dyfzd.36958$Ew6.27578@twister.socal.rr.com...
| Quote: | Fred Harris (fred harris) has a paper on this
subject from the 70s.
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He surely does. See the following citation:
========================
Harris, Fredric J, "On the Use of Windows for Harmonic Analysis with the
Discrete Fourier Transform", Proceedings of the IEEE, Vol. 66 No. 1, January
1978 pp 51-83
========================
On the subject of inverse-transforming to the time-domain, I'm under the
impression that any non-rectangular windowing will pretty well destroy the
possibility of getting back the original time-domain data. When you IDFT
the spectrum, you'll get back to the windowed time-domain data, not the raw
time-domain data.
Also, if you plan to overlap the time domain samples under a rectangular
window, I believe you have to adjust the phase relationships in the
frequency domain before you add frequency components. For example, with 50%
overlap all the odd sine and cosine components of the overlap piece will
have to be negated with respect to the previous piece in order to preserve
proper phase relationships. |
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