Re: Test report MBP built-in audio device
Re: Test report MBP built-in audio device
- Subject: Re: Test report MBP built-in audio device
- From: "Mikael Hakman" <email@hidden>
- Date: Tue, 26 Aug 2008 13:46:22 +0200
- Organization: Datakonsulten AB
He understands this very well since long, long time back because he is
working with digital signal processing for decades, long before there were
digital audio and Mac computers around. Digital signal processing had its
applications long before anybody used it for audio. These are hardly new
topics; some were known and used for centuries, some others for decades,
even if they may appear new to people in digital audio because digital audio
application itself is relatively new. Used methodology isn't.
If you look at the next page entitled "windowing II" you'll see a signal
reminiscent of a guitar or piano note decaying (top picture). After
windowing function you get the signal depicted on the next row. Which one do
you listen to when you listen to a guitar? Then, which one should be
analysed?
Furthermore, from the point of DFT, the spectrum on top row is a correct
representation of the signal, the spectrum on next row isn't - that is a
correct representation of the signal on the same row, which isn't the signal
you've got in the first place. What all this demonstrates is a basic
inability of DFT to represent signals in a way that we humans would like
them to be represented. We would like to describe the signal on the top row
using a fixed frequency and an envelope causing the decay, because this is
what we intuitively feel it is. DFT is unable to give us such a description.
You can distort the signal by a windowing function so that DFT gives more
pleasant and/or wanted results but this will be even more un-right.
Therefore I have developed a method of computing THD+N without using DFT
that separates amplitude and DC-offset (the envelope) from the frequencies
(the swinging) itself. In addition this new method of mine can be used on
only few signal samples, which DFT-based methods cannot. I have reported
this and the results in my report.
However, if for a constant amplitude input signal, a device produces output
signal like the one on the top row then you want to know this. DFT-based
THD+N methods without windowing will tell you that. The answer you get with
windowing will be false; it will describe the signal on the bottom row.
Regards/He
On Monday, August 25, 2008 9:54 PM, Matthew Xavier Mora wrote:
It might help him if he took a look at:
<http://www.bores.com/courses/intro/freq/3_window.htm>
On Aug 25, 2008, at 11:31 AM, Andy Peters wrote:
On Aug 25, 2008, at 11:27 AM, Mikael Hakman wrote:
On Aug 25, 2008, at 4:49 PM, Richard Dobson wrote:
Mikael Hakman wrote:
> None,
That would indicate the rectangular window, therefore.
This is one way to look at it - a rectangular window of height 1
extending from 0 to infinity. But then all signals not filtered in any
way could be said to be filtered by such a rectangular window, and then
not only once but infinite number of times. I am aware of this point of
view but in my taste this is too much of bending the reality to the
theory. I prefer the other way around. If the signal is not filtered
then it simply isn't filtered.
Wow, that is just ... wrong.
Do you understand why windows are used?
-a
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