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: Thu, 28 Aug 2008 18:38:31 +0200
- Organization: Datakonsulten AB
On Thursday, August 28, 2008 5:07 PM, Stephan M. Bernsee wrote:
There is one case where this scenario doesn't apply and where windowing
is actually harmful and this is when the period of your measuring signal
is exactly an integer multiple of the DFT transform size. In this special
case I would have to agree with Mikael that windowing will pollute the
measurement and you will lose resolution.
There are many cases where you don't want windowing. In general it depends
on the purpose of you taking the DFT. Then as you say there are additional
cases when the period of the signal is an exact integer _divisor_ of the DFT
transform size.
So it all depends on how the DFT is being used if windowing is actually
desired, or not. In my opinion it is necessary to include information on
how these measurements were obtained. IOW: if you're not using a window
you should mention this so people could interpret your graph in the right
context.
I have given the information that I'm using 4800 points DFT and an SR of
48000 samples/sec. Then how difficult can it be to infer that at 1000 Hz
(one of the charts in my report) there are exactly 100 periods in signal
segment submitted to DFT? And that at 10000 Hz (the other test frequency)
there are 10 periods? At least for someone pretending to know the subject
and having opinions what should or shouldn't be done? Do I have to write
every trivial and obvious detail on such person's nose?
Also, to go back to the original application, taking these measurements
to judge the quality of an audio algorithm may be misleading depending on
the nature of the algorithm. A time stretching algorithm may have a
perfect frequency response but may still sound horrible to human ears due
to transient smearing and other factors that will not show up in a DFT
graph.
It may be misleading but I don't think it is, and you have no information
pointing in another direction. A time stretching algorithm produces horrible
distortion, which is detected both by traditional steady-state and my new
method THD+N computations. This distortion will show up in a DFT graph if
you do measurement and analysis in a correct way.
Regards/Mikael
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