A misconception about measurements

Quote:

 

This is sound science, you're going to have to cite what we're measuring wrong.  We're not trying to gauge individual perception, just whether something makes a measurable difference most of the time.  No one can tell you how you perceive distortion as it will vary from person to person, just to what level it exists and to what level of accuracy equipment is following the signal.  I suggest you look at the mathematical definition of accuracy, because I doubt other definitions are used here often.

Quote:

 

We already have our own perceptions, and perceptions will vary as they are based mostly on opinion.

 

 

Quote:

 

 

If we want to listen to music (a signal) accurately then it's kind of important.

 

 

 

Quote:

 

 

Accuracy refers to precision that's quantifiable in a mathematical context.

I'm not sure what could possibly be going on that cannot be measured or quantified. There are three electrical qualities that define cables: resistance, inductance and capacitance. They interact with each other in elegant, simple equations. Not only that, but they've been backed up with nearly 150 years of applied and theoretical practice.

If you're saying that three feet of wire possess qualities that have gone unnoticed in tens of millions of experiments, real-world use and theory, well, that's remarkable.

Whatever this fourth force is would have to have no effect whatsoever on known values, which is also remarkable. Possible, yes, but it would completely rewrite what's known. That has happened before, but it is difficult to believe that it is possible.

To me, the claims are like getting a phone call from a telemarketer promising me guaranteed returns of 30% a year on an investment with no risk or downside. Possible, yes, but it throws up every red flag and leads me to start asking a lot of questions.

@mike1127

 

Great post!

 

@Uncle Erik

 

Great post as well.  However, it deviates from the topic at hand by restricting itself to cables.  It is a great response to the notion of accuracy and the collection of evidence for sure.  But, w/ respect to the OP, it turns his argument into a bit of a straw man because how an earphone sounds to us is a far more complex system to examine.  I will never be of the school that any measured data is useless.  I do prefer it to be taken in context w/ an appropriate level of scientific humility.  

I think the point being made is this:

(1) headphones/speakers are not perfectly linear

(2) measurements are very precise and accurate at what they are measuring, but because of (1) only actually give "perfect" information about what they are measuring -- extrapolation works mostly but not absolutely

(3) there are many different kinds of audio signals (i.e. many possible inputs), so they are not all described perfectly by a given finite set of measurements performed

 

e.g. put in an input of a 60 Hz + 10000 Hz signal and see if there is any measurable IMD on the output.  Now you know the IMD very precisely given those inputs.  Now, can you predict exactly what will happen if you do 125 Hz + 8000 Hz + 9000 Hz?  (Not really.)  Granted, the original measurement probably gives a decent idea of the behavior of the device.

 

 

As for the other part about accuracy, I am confused.  Accuracy is closeness to the actual or intended value.  As a serious question, what else do you think it means?  Or are you talking about perceived accuracy?

Edited by mikeaj - 8/29/10 at 11:20pm

I think I see what you're saying now, thanks.  You're talking about perceived accuracy, and in the example, the relative difference between two audio experiences.  First there is listening in the room and then there is the recording.  But here we get into issues of recording/microphone placement/mastering and the fact that the sound actually going into your ears is dependent on exactly where you're sitting in the auditorium or other space.  However, those recording issues are not relevant to measuring playback devices.  The goal of audio (playback) may be to reproduce an experience, but the goal of audio equipment is to reproduce what the input is.  The input is what's on the recording--in a CD, already stored in bits.

 

As a pretty decent student musician (no, it wasn't my major--EE is) that knows many musicians, I doubt that aural experiences are so easy to remember exactly, even for a musician.  e.g. please hum an A 440 within 1 cent without any reference.**  Or if humming is too hard, could you pick out which one was the correct 440 Hz if 438.5, 439, 439.5, 440, 440.5, etc. were all played and you had no reference other than past memories?  It's something you've heard countless times before, right?  I'm sure there are many other details that are forgotten or misremembered slightly.  If you're listening for certain details as opposed to others, what you remember will change.  The way you listen impacts the perception of the experience.

 

**Actually, some people can do this probably, but definitely not all professional musicians, even good ones.

 

edit: I now just skimmed through the last couple pages of the cable thread and am now confused again by what you mean.  I also didn't realize that some of what I stated about accuracy was a rehash of others' statements.

Edited by mikeaj - 8/30/10 at 12:06am

 

 

 

The problem is not that measurement are incomplete. They are complete. The problem is that, willingly or nor, no one performs the complete measurements.

 

According to Ethan Winer [1], audio measurements can be divided into four categories :

 

-Frequency response

-Distortion - THD, IMD, aliasing "birdies"

-Noise - hiss, hum & buzz, vinyl crackles

-Time-Based errors - wow, flutter, jitter

 

Let's add that phase response can be included into frequency response, and that reverberation fits in the time-based errors.

To be complete, measurements should include THD at all frequencies (2D plot) (for a speaker, it depends strongly on the frequency), and IMD at all pairs of frequencies (3D plot).

 

Adding on top of that that for amplifiers and speakers, measurements should be done at various levels, this is not very convenient.

 

However, there is one measurement that encompass all of them : signal cancellation. Take the output and substract it from the input. What's left is the difference between the two.

 

[1] http://www.youtube.com/watch?v=BYTlN6wjcvQ

 

Select the text in the quote box from the end to the beginning. Go on outside the quote box until the "Quote" word. Copy that into your clipboard. Insert some carriage returns, and paste your clipboard in the middle of the carriage returns. Then, remove from the pasted quote boxes the text that you don't want to quote.

 

Indeed cancellation might not work every time. For example if a system aliases past 18 kHz, and the test song has no frequency over 18 kHz.

But the other measurements cover all that can happen in a real-life system. Aliasing will appear in the IMD measurements with pairs of frequencies above 18 kHz.

On the other hand, don't take the above list as a guide. I'm no expert, and I can see that in some cases, special measurements should be performed. For example, to detect quantization noise, noise should be measured with a signal.

And mp3 encoders are an example of extremely non-linear devices. They can be tweaked in order not to alter frequency response, and still produce very distorded results. In this very special case, a measurement that can tell if the sound is distorded is the signal cancellation.

 

Despite these examples, I stand with my initial view : saying that measurements -in general- are only a partial description of the behaviour of a system is very far from reality.

The problems lies in

-Manufacturers don't perform or don't publish relevant measurements.

-Sometimes measurements show that the system is not perfect, but give no useful information : headphone frequency response, mp3 signal cancellation...

-Sometimes very special measurements are needed : aliasing, loose connection, piece of hair rattling inside a headphone driver...

 

As a conclusion, I would say that measurements can completely describe the behaviour of an audio device, but that measuring completely an audio device is sometimes far from easy.

 

 

The case where a cable is not time-invariant is when there is a loose connection. Sometimes it works, sometimes not. Of course, it can happen extremely fast, so that the cable transmits a crackling sound.

We may mention another case : heating. But for that to be significant, the cable should become bright red. This occurs in speaker coils, but not in inteconnects or even speaker cables.

 

Wether a cable is linear or not depends on the definition of the input. If you include external electromagnetic interference as an input, then a cable is completely and totally linear. If you don't include EMI as an input, then sensitivity to EMI is a non-linearity of the cable, as an audio device.

 

I recall the definition of linearity : output is an algebraic linear application of the input. An equalizer is a linear device. A reverberation device too. A dynamics compressor is not.

 

 

On the contrary, R, L and C are a bit difficult to handle at audio frequencies, while they become a more effective way of describing the cable behaviour as a transmission line. They become ineffective at hyperfrequencies, above several GHz, when copper ceases to behave as a conductor.

 

 

This is a misconception. Science always builds something new on top of something old.

Sometimes, the new model is based on completely different ideas than the old. That's what is called a paradigm change. But it never contradicts the old model.

 

The old Newtonian gravitation model said that all massive objects attract each others. As a result, on earth's surface, objects fall down with an acceleration of 9.8 m/s2.

The new general relativity says that mass and energy curve space-time. As a result, on earth's surface, objects fall down with an acceleration of 9.8 m/s2.

 

Today, the output of interconnects cancels the input down to -110 dB. Later, if a new theory of cables is found, that won't change the fact that the output of interconnects cancels the input down to -110 dB.

 

 

By the way, Mike, some time ago, you started in-depht long-term blind listening tests. What was the outcome ? Are you still working on this ?

Edited by Pio2001 - 8/30/10 at 7:00am

Shouldn't there be a 4D IMD graph with sets of three frequencies, and a 5D IMD graph with sets of four frequencies, etc.?  And you need each graph at different volume levels too.  And how about transient intermod?  Non-transient IMD when there are also other transients occuring at the same time?  If we agree that a system is nonlinear and you don't have a complete formula that perfectly describes the output for any input, there are still some things you do not know even given full 2D IMD, full THD, etc. graphs.  Whether or not these effects are audible or worth considering down to a certain point is a different matter.

Well, if we're talking about standard interconnects, those behave linearly in the conditions we are concerned with at least down to the point of the noise floor of the best measuring equipment.  I agree that If cancellation works, there is no measurable difference in actual music and thus you can be sure that there is no difference to hear.  Differences may still be heard, but that's a property of the listener.  I originally thought that the argument was intended only to cover those devices that are known to be significantly nonlinear.

Measuring significantly nonlinear systems is tricky, so there is a lot of research on the subject.  Headphones and speakers are even more difficult to measure than nonlinear electronics where you're just dealing with input and output voltages and currents, but this has to do with the practicalities of recording sound waves.  However, very good descriptions can still be made even with these kinds of devices. 

Granted, I'm neither an expert in measure theory nor instrumentation, so I will leave it at that for now.  Please correct or debate any points you take issue with.

Maybe you'll find this relevant, maybe not.  Just happened on an article that did a great job of discussing the difficulty of perceiving accuracy.  The article revealed how a set of Stax Signatures were more accurate is revealing the recorded source while the Koss ESP950s sounded more accurate to the actual live Piano.

 

http://www.stereophile.com/headphones/koss_esp950_electrostatic_stereophones/index3.html

 

I think its a good read for many on Head-Fi to get a better understanding of what you personally should be looking for and understanding what others might be searching for.  Neutrality and accuracy usually get bandied about as this universal monolithic dogma without any context or point of reference.  Sometimes coming at odds w/ musicality in certain cases. 

 

Is IMD with three frequencies more than the sum of IMD for each pair of frequencies, and if so, does it make a significant difference, or is it just 0.1 % of 0.1 %, and IMD with four frequencies 0.1 % of 0.1 % of 0.1 % ?

 

Transient IMD asks the same question. If you measure IMD on a fixed frequency sine plus a transient burst, will you get more than the total IMD for all frequencies contained in the burst associated with your sine ? If so, is it significantly more, or just 0.1 % more ?

 

I don't know.