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 15:09:29 +0200
- Organization: Datakonsulten AB
Brian,
On Tuesday, August 26, 2008 8:13 AM, Brian Willoughby wrote:
On Aug 25, 2008, at 19:15, Mikael Hakman wrote:
Nonsense! Distortion measured under steady-state conditions is -75 dB at
1000 Hz. This is in Apple's spec and I have verified and reported this
in my report. Distortion at 1000 Hz measured right after the signal
starts (reaches the receiver) is not specified by Apple neither by codec
vendor. I have measured it to be around -46 dB or 0.5%. It is quite
common for DACs that they exhibit higher distortion at the very
beginning of a signal. Interesting issue is not whether there is higher
distortion but by how much, and how fast the device stabilizes. Use or
not use of windowing function in analysis has nothing to do with it.
Apple does not publish distortion specs for "right after a signal starts"
because the distortion spec does not change. It is sufficient for Apple
to report the steady-state distortion ratings and leave it at that.
Distortion spec is what it is - constant because it is written on paper.
Apple uses the same spec as all other guys - they spec steady-state
distortion at a constant frequency. Often they even don't bother with giving
distortion at a number of frequencies, not intermodulation distortion
either. Apple isn't better or worse than the others.
A DAC cannot have a stabilization time that lasts longer than one sample
period. You're talking about hundreds or thousands of samples before
your measurement code stabilizes, and that has nothing to do with the D/A
or A/D. Note that any filtering (analog or digital) in the signal path
may introduce ringing that lasts longer than one sample, but this will
appear at all times, not just with the first sample at the start of a
signal.
Distortion caused by a device (and possibly its driver etc.) itself, on the
other hand, varies with a number of external factors. From where did you get
that "a DAC cannot have stabilization time that lasts longer than one sample
period"? This is untrue. I say that DACs take time to stabilize.
Can you cite an independent source for your claim that it is quite common
for DAC to exhibit higher distortion at the very beginning of a signal?
I cannot cite independent source, because they don't publish this data,
probably because they don't know how to measure it (in the best case) or don't
want to (in the worst case) or simply because they don't do it. Format of
current specifications published by the vendors originates from times when
such measurements were made by analog measuring equipment. Using analog
technology you need some time (quite long compared to signal frequencies) in
order to do analysis. These devices were unable to give you distortion
during 1:st millisecond after applying a signal.
Windowing is used because the FT analyses a small part of a signal as
if that part repeated forever. Unless your D/A and A/D line up
perfectly, your windowless analysis is actually looking at a
completely distorted picture of the signal, and thus you'll see very
high distortion as an artifact of your custom tools, not due to any
problem in the Apple audio circuits.
Unless what? A/D doesn't know the signal is coming from another D/ A, it
could be a microphone. D/A doesn't know it sends output to an A/D, it
cold be a monitor. So if I do get distorted signal starting with pure
one then they must have distorted it, and this is precisely what I want
to know and measure.
I am saying that your distortion measurements are wrong. You blame the
results on the hardware, I blame it on your custom analysis tools which
go against the entire industry and skip windowing because of your own
personal aesthetic. I will let others comment further on this, but I
think you've really opened a can of worms by publishing a "Test report"
on the MacBook Pro and claiming that it's distortion levels jump above
Apple's advertised specs whenever your pure signals arrive. You've got a
hard sell ahead of you.
You are saying this without any proof. There is nothing wrong with my
analysis and the proof is that it gives correct answers if the signal does
not pass through a DAC. If my computations were causing distortion
variation, then this would be present also when not passing the signal
though a DAC, wouldn't it? The effect is there only if there is a DAC in the
path.
You have difficult to accept my results because you never before seen such
an analysis. Well, somebody has to be the first, in this case I'm.
Windowing or not has nothing to do with it. You are free to give me a spec
for a windowing function of your choice, including all its parameters and I
will repeat measurements using your function.
It isn't MacBook Pro distortion that varies but in there used codec. This is
normal.
In fact, this is worst case when you start the test signal after
silence, because the FT sees a repeating waveform that is very
complex - and this totally explains the high distortion you're seeing
in your custom tools.
Worst case!? Well, about as worst as saxophone starting to play a note
from silence or near silence. About as worst as an artist playing a slow
classic composition on acoustic guitar and letting every note decay to
low levels! What are you talking about?
I am talking about problems with your math and your unique approach. The
Fourier transform has limitations. It's very difficult to get around
them.
Because you don't know anything about my math, you couldn't possibly have
ideas about problems therein.
Your examples hint at the importance of the attack portion of any natural
sound, and you are correct that the human perception focuses on these
elements. However, it is a fact that linear time-invariant systems do
not treat the beginning of a signal differently than the middle or the
end. It is only Fourier analysis that has trouble with transitions, and
it has the most trouble when you skip windowing.
Who said that a DAC is a perfect linear time-invariant system? On the
contrary, all tests aim at assessing how far from this ideal a particular
device is.
If Fourier analysis had trouble with attack time then it would be having it
even without DAC in the path, wouldn't it?
Because what? I send pure simple sine wave and if it becomes very
complex signal then the device must have made it so. Very complex signal
from simple one = high distortion, which is precisely what I want to
know.
But you are using analysis which does not analyze the exact signal, it
analyzes a different signal which repeats a small subset of the true
signal. The distortion is not created by the device, it is a by- product
of the analysis. You are making assumptions about the true signal based
upon analysis tools which are not precisely dealing with the true signal.
No, you do not understand. Distortion is created by the device because
without the device in path there is no distortion.
There isn't any discontinuity; my specially designed signals guarantee
this. I start analysis when and at the very first test signal sample
reaches my receiver. If there were any discontinuity thereafter, my code
would throw an exception. I just designed that different solution you
are asking for and the results are put forward in my report.
The discontinuity comes before your specially-designed signal, not after.
Even if you try to be clever and throw away all incoming samples until
you reach some special sample that you're looking for, you still have a
discontinuity at the point when you begin your analysis - there is no way
to avoid it. There are always an infinite number of zero samples before
your signal. You can ramp up the amplitude of the incoming signal with
an envelope, in order to minimize this discontinuity at the beginning,
but that's a form of windowing.
No, there is no discontinuity. There is a normal situation where at first
there is silence and then a voice starts to play.
Regards/Mikael
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