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 15:19:54 +0200
- Organization: Datakonsulten AB
Brian,
On Thursday, August 28, 2008 3:39 AM, Brian Willoughby wrote:
Mikael,
There is a very simple test for the things you are interested in, and it
is quite well established. It's called an Impulse Response. An impulse
response tells you everything about a linear time-invariant system.
The very first chart in my report at
http://www.dkab.net/Realtek HDA report.pdf shows you Impulse Response
for the actual device. It is entitled Single Pulse Response because this
name is easier to understand for people I presume will read that report. The
name Impulse Response is used in signal processing. In even more
mathematically oriented papers it is often called Transfer Function.
It is correct that for an ideal linear time-invariant system, its Transfer
Function tells you all, in mathematical sense that is. This means that given
any kind of input and system's transfer function, you can compute system's
response (output). This is done by convolution of input and transfer
function.
However, from this doesn't follow that a human looking at transfer function
charts for 2 or more devices can tell which of the devices is better (closer
to ideal). Neither do I know of any existing methods for deriving a single
(or few) number(s) from system's transfer function whose magnitude would
tell you which of the systems is better. I'm working on such method but I'm
not ready yet. This is why we use magnitudes of entities derived by other
tests, methods, and experiments in our attempts to assess (re)produced audio
quality of various devices and algorithms. Steady-state THD+N is one such
entity, which we both agree isn't sufficient.
Furthermore, statement "an impulse response tells you everything about a
system" requires system in question to be an ideal linear time-invariant
system. This is not the case when working with real systems. While a good
(pre)amplifier can be very close to this ideal, current DAC/ADC circuits are
a bit farther. Good systems are close to this ideal but all real systems
deviate from ideal to some degree. This deviation contributes to the
distortion produced by the system. When assessing audio quality (re)produced
by a device or an algorithm we are interested in magnitude and variation of
distortion added by the system to our signal. We do not care why the system
distort signal, only how much. The question of "why" is for device
implementers to study.
You are correct that the attack portion of natural sounds are a very
important aspect of our human perception of those sounds. You are also
correct that steady-state test signals cannot measure this. You are
incorrect when you state that the distortion characteristics of a system
like the MacBook Pro built-in audio change when your special test signal
arrives - what you're seeing there are mathematical artifacts of your
algorithm. One problem is that you're generating a sine wave and hoping
to measure impulse response.
I don't understand why you constantly refer to "a system like the MacBook
Pro built-in audio". Our discussion has nothing with Apple computers to do.
It is about an audio processing device or algorithm. Some Apple computers
have such device built-in but this is irrelevant for the discussion. In this
respect, Sysadmin could ask us to move our thread somewhere else. I started
this thread because my report at
http://www.dkab.net/Realtek HDA report.pdf reports results for Apple
MacBook Pro built-in audio device, which of course should be relevant and
interesting to members of this forum.
You are again putting words in my mouth. I never stated that "distortion
characteristics change when my special test signal arrives". Distortion
characteristics are as they are. I'm merely measuring their variation in
time under circumstances like those that arise during attack time of an
instrument. I'm not seeing mathematical artefacts of my algorithm - this is
your own and wrong conclusion for which you don't have any theoretical or
experimental support whatsoever. Then again you tell me what I'm hoping to
measure. I don't. I feed in sine wave and I record the resulting wave and I
compare the two and differences give me actual distortion.
If you continue in this way telling me what I said and what I think and hope
then I won't reply you anymore.
However, there are unavoidable artifacts of FT analysis, and thus you
cannot prove that your results reflect only the system under test.
Yes I can and did and I described this in informal way and to some extent
earlier on this thread. This is however not the right forum to present a
stringent proof. You have to wait until proper paper appears in
peer-reviewed scientific press. Then you will have a chance to disprove me
in the same stringent way.
You've not stated how you produce a perfect sine wave on the sending
side, nor how you produce a perfect sine wave on the receiving side for
comparison.
Yes I did. Please read my earlier answers on this thread again.
Another claim you've made is that you've created very special test
signals, which must obviously exist in the digital realm for your MacBook
Pro tests, and yet you also claim that you have performed the same tests
without a DAC - and I don't see how you can guarantee that you can
produce a special test signal in both the analog and digital realms and
be 100% certain that they are identical signals.
In both cases original output (test signal) and final input (response) is
digital. The same test software is used in both cases. In the first case
(presented in the report) there is device's DAC/ADC in the path. The report
shows cumulative effect of device's DAC/ADC, which is clearly stated in the
report. In the second case there isn't any DAC/ADC in the path, only device's
S/PDIF trans- and receiver. I'm sorry that you have difficulties seeing such
obvious and trivial experimental arrangements. For me, as scientific
experimenter, this is a standard procedure.
In other words, your algorithm has too many secrets for anyone with a
math background to confirm your results. You keep promising us that your
new methods are flawless, but I guess you'll just have to accept the fact
that many educated people will not believe you. I am not trying to
change your mind, just to let you know why you are meeting with such
resistance.
I do not disclose all the details because I don't want to. You'll see some
of the details in the forthcoming proper paper if you read that kind of
press that is. You won't see all the details until there is valid patent in
place.
Regards/Mikael
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