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: "James Chandler Jr" <email@hidden>
- Date: Wed, 27 Aug 2008 09:54:04 -0400
----- Original Message -----
From: "Mikael Hakman" <email@hidden>
I'm not physically, electronically or even logically switching the device on.
The device is on for a long period of time and it is feed by a continuous
signal. The signal consists of silence (zeros) up to a certain time. At that
time the signal becomes pure sine wave with phase 0 (i.e. first sine sample is
sin(0), next is sin(dt) etc). I continuously record output from the device and
I know the exact delay between my output into device and device's output into
my program. Therefore I know which of recorded samples that corresponds to
the first sine wave sample. I start my analysis from that sample. When I say
that I know the exact delay I mean the exact number of samples, which of
course vary a little between the runs but is measured at the very beginning of
each run, earlier (before silence and sine wave) in the test signal so to
speak.
The reason I'm using such a test signal is that it is a crude simulation of
what happens when a musical instrument is played. Because proper reproduction
of signal during instrument's attack time (first few milliseconds after you
hit a string, start blowing etc) has been shown to be the second most
important factor after harmonic content to our perception of timbre, I decided
to measure distortion in experiments that are close to this musical reality.
Perhaps such experiments could explain why 2 audio devices having the same or
very close specs, may sound so differently, one sounds right, the other
doesn't.
I know very little about it, but if you for instance duplicate this experiment,
feeding your signal (silent head + sudden onset sine wave)-- If you process your
test signal thru a simple IIR digital HiPass DC Blocker filter--
Visually examine the output, and the signal onset will not look like a clean
sine wave for quite awhile after the signal onset, until the DC Blocker settles.
You can see the same thing with about any kind of filtering, including the
low-pass anti-alias filtering on ADC inputs and DAC outputs (though those
filters settle quicker than a DC Blocker). A sudden onset interacts strongly
with the impulse response of the filter.
Dunno what an FFT of that first sudden-onset 'odd-looking' DC Blocker sine cycle
would show. A short un-windowed FFT on that first wave cycle might make you
think it indicates distortion. Dunno. Never tried it.
But such linear filters usually don't make harmonic distortion, only 'time
distortion' contributed by the impulse response.
Maybe you are looking at some other artifact, and have a way of ignoring linear
filter settling artifacts?
Audio DAC outputs and ADC inputs, will almost invariably have high-pass, DC
Blocking characteristic. A short-term harmonic distortion test should have some
way of ignoring the time-distortion of such filters' impulse responses.
That is one advantage of a windowed steady-state test-- In that case any linear
impulse responses in the system have presumably settled, and so they will not
confuse the distortion measurement?
jcjr
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