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: Richard Dobson <email@hidden>
- Date: Mon, 25 Aug 2008 15:49:01 +0100
Mikael Hakman wrote:
> Brian,
>
> None,
That would indicate the rectangular window, therefore.
in fact windowing function free methods were one of the reasons
> I'm developing my own set of tools. However, this often but not always
> requires special test signals to be generated and feed into the
> device/software under the test.
>
> Not that I want to argue against use of windowing function but IMHO the
> only good thing multiplying the signal by a windowing function does is
> to make result presentation (the charts) more pleasant to look at. It
> doesn't increase the precision of the results, rather the opposite. It
> "smears" out computed frequency spectrum.
A Hann or similar tapered window is a low-pass filter. It reveals
lower-level components by suppressing the strong sidebands of the
rectangular window spectrum. Nothing to do with "precision" as such
(that would be determined more by the window length, which appears to be
unspecified in your report). But very much to do with detecting a wide
dynamic range of signal components, essential for tasks such as
detection of alias and distortion components.
In fact windowed DFT of a pure
> sine wave results in a DFT of the windowing function itself, not in a
> single frequency spike as it should.
It results in a convolution of the FT of the window with that of the
signal. Even with a rectangular window, you will only get a single
spectral line if the period of the signal matches that of the window
~exactly~. Even with a DFT, that will not happen unless the signal's
period is an exact integer number of samples. In all other cases,
sidelobes will appear.
Spectrogram of an ideal device
> becomes a ridge of hills on the diagonal instead of single one pixel
> wide diagonal line on otherwise black background. (Sorry for this
> geodetic metaphor - I don't know how to express this otherwise).
>
Those are the DFT/window side-lobes - might as well use the established
technical language! Are you actually using a raw DFT algorithm or an
FFT? What length in samples?
Also, one of your plots in that report shows a chirp signal. Rectangular
windowing will not help you there; rather, it will help to conceal
lower-level components, whether from harmonics in the signal itself, or
generated by the system under test. Several are visible in the plot
(where the ascending line is reflected downwards from Nyquist); with a
low-pass window those alias components (and the extent of them) would be
very much more visible.
But to see even that amount of aliasing with a rectangular window
indicates either that the input sinusoid itself has a high proportion of
harmonics (i.e. distortion), or the system does. Including a plot of the
spectrum of the input would help - describing it simply as a
"specially designed test signal" is not really very informative. How do
you generate it?
Also, all physically realisable audio systems have a finite frequency
response (and all digital ones of course, +- Nyquist). Both the ADC and
the DAC will have more-or-less effective anti-alias filters
(more-or-less brickwall), such that any "ideal" pulse will of course be
converted to a sinc-like waveform. In this sense, there can be no such
thing as a "perfect device".
I would suggest including more source-response comparisons in your
reports; and allowing those reading it to form their own conclusions by
including both rectangular and low-pass windowed measurements, and
including a detailed description of the test signal.
Richard Dobson
..
> On Sunday, August 24, 2008 8:20 PM, Brian Willoughby wrote:
>
>
>> Mikael,
>>
>> Which kind of windowing do you apply to the signal before the DFT
>> analysis? I do not see this information documented anywhere in your
>> paper.
>>
>> Brian Willoughby
>> Sound Consulting
>>
>>
>> On Aug 24, 2008, at 10:39, Mikael Hakman wrote:
>>
>>> I have ported some of my precision tools for assessing quality of
>>> audio devices and algorithms from Windows to OS X. I have now used
>>> the ported tools to investigate MBP built-in audio device. The test
>>> report is at http://www.dkab.net/Realtek HDA report.pdf.
>>>
>>> Regards/Mikael
>>>
>
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