Null Difference Testing

[fdegrove]

Transmitting.

Hi,

But interconects in audio and speakers cables don't work as transmission lines(if they work they will need matched impedances at the sending and receiving end)...

Matched impedances is not a requirement for a transmissionline to exist.
This is precisely one of the problems with audio design, you don't have a fixed standard for impedances other than a digital line as you point out.

A simplification of a model:

An externally hosted image should be here but it was not working when we last tested it.

Cheers,

 

[Tube_Dude]

The crossed lines....

Matched impedances is not a requirement for a transmissionline to exist.

The rule for transmission lines is that the load (and possibly the source) should present an impedance equal to the characteristic impedance of the line...
The line is then "matched"...

This is precisely one of the problems with audio design, you don't have a fixed standard for impedances

No itsn't ...because for audio the signal source should have a source impedance small compared with the impedance of the load being driven and that the load should present an input impedance large compared with the source impedance driving it!!!

Anyway...what have transmission line theory to do with a null test for audio???

 

[phase_accurate]

A transmissionline is a transmissionline and stays a transmissionline independant of source or load impedance. Though the proberties of the whole arrangement will change.

It works best however if it's own impedance (I don't have the correct expression on hand, in German it is "Wellenwiderstand"), the source impedance and the load are matched.

For our audio equipment it is indeed usual to have low source and high load impedances, wich is handled more easily by the electronics but is nonideal regarding signal transmission in cables.

If you want to compare cables then you HAVE to take the effects, generated by said cable's properties as transmissionline, into account.

Regards

Charles

 

[nw_avphile]

audio transmission lines

If you want to compare cables then you HAVE to take the effects, generated by said cable's properties as transmissionline, into account.

Except Tube_Dude is correct, the transmission line issues don't matter below 20khz! I challenge someone to show me a measurement of any cable artifact that can be reproduced with typical cables and equipment AND can be argued is audible (i.e. it's amplitude is greater than -70db below the signal).

I also challenge anyone to a blind listening test comparing any reasonable cables to the Radio Shack variety. You'll fail just like everyone else has.

 

[nw_avphile]

Yes, but on the other hand, if you cannot hear any differences even with the imperfect nulling test, as has been stated by some, what are the chances that there is a real difference, masked by the residual nulling error, that you CAN hear (the subtle cable differences) in a normal listening session?

My own tests show that a null of -50db is sufficient for most people not to hear a difference in blind listening tests. I extend that to -60db to allow for people with really exceptional hearing/listening skills. Bob Carver claimed he managed a -70db null in the Carver Challenge which fooled the ears of the Stereophile editors.

So somewhere between -50db and -70db null I believe we can safely say there are no audible differences to be heard. So, yes, if the amplifier is only managing -40db or -50db by itself, it may well mask differences in the cables. If it's closer to -70db, however, I'd have to say the null is sufficiently deep to argue the two things being compared will be indistinguishable in a blind listening test.

As was discussed a bit in the other thread, the more accurate way to do this would be to "weight" the null results (something like A weighting for S/N measurements) according to human hearing sensitivity. A poor null at 20khz is less likely to be audible than a poor null at 500hz.

 

[fdegrove]

TRANSMITTING.

Hi,

Except Tube_Dude is correct, the transmission line issues don't matter below 20khz!

If I recall correctly:

Weren't you the one claiming that some cable manufacturers were deliberately introducing cables that were "distorting" the signal by introducing a filter that made them roll off at 20KHz?

If you need to make such bold claims, can't you at least TRY to be consistant?

And transmissionline anomalies do cause havoc in even your audible range unless phase delays and other deviations from linearity are inaudible to you.

No offense, but please refrain from spreading nonsense.

Cheers,

Cheers,

 

[Tube_Dude]

The good and the bad guys....

And transmissionline anomalies do cause havoc in even your audible range unless phase delays and other deviations from linearity are inaudible to you.

So for you interconects must be considered transmission lines ...so for a load impedance of 47k the cable feeding it must be a 47kOhms intrinsec impedance...

What about a digital cable for interconect??...it was built for to work as a transmission line...well...but only with 75 Ohms load!!!

It's amazing!!!

No offense, but please refrain from spreading nonsense.

But who is in reality spreading nonsense here???

 

[nw_avphile]

Re: TRANSMITTING.

If I recall correctly:

Weren't you the one claiming that some cable manufacturers were deliberately introducing cables that were "distorting" the signal by introducing a filter that made them roll off at 20KHz?

If you need to make such bold claims, can't you at least TRY to be consistant?

And transmissionline anomalies do cause havoc in even your audible range unless phase delays and other deviations from linearity are inaudible to you.

No offense, but please refrain from spreading nonsense.

Fdegrove: Show me the proof of the "nonsense"? You've yet to do that despite the question being posed several times. So I could just as easily request you refrain from spreading unproven myth. At least the objective facts and many well documented references support my statements.

As for being consistent, the sorts of transmission line effects being discussed here (reflections, impedance matching, etc.) have nothing to do with the simple RC network roll-off I referred to earlier. They're apples and oranges. If you don't understand the difference, you might want to read up on the subject.

As for my "bold claims", I'd suggest those with an open mind check out the information at this link:

http://www.svconline.com/ar/avinstall_designer_cables_critical/

The link above discusses the very issues we're talking about. Please take particular note of the references at the end--something the subjectivists here like fdegrove are entirely lacking to support their arguments.

So... do we believe the opinions of non-blind listeners clouded with psychological bias and those who have a vested interest in maintaining the status quo in high-end audio (fdegrove appears to be one of them as he claims to be a high-end cable "designer"), OR do we believe the results of carefully run, documented and published blind listening studies and other objective evidence?

It's your choice.

 

[fdegrove]

MORE NONSENSE.

Hi,

If you must insist on the topic:

Anyway...what have transmission line theory to do with a null test for audio???

I'll repeat it for the last time:

You don't need matched source and receiver impedances to have a transmission line.

And yes you could use a cable designed for digital transmission as an audio cable.
Whether is would sound good is another matter, but work it will.

FWIW, a digital cable is nothing more than a constant impedance coax.
When hooked up in a digital system the termination is done inside the gear involved, a transport and a DAC for example.
If I'm not mistaken the impedance standard is 50 Ohm, I am not really into digital design so I may be off.

Ciao,

 

[fdegrove]

NULL.

Hi,

NW,

The link you inserted points to a commercial website which does not constitute scientific proof in my book.

Systems designed to produce subjectively pleasing and distinctive coloration appropriately belong in the recording studio where, like musical instruments, they can be manipulated by artists and producers to generate the desired effects.

These cables don't even belong in a studio either, an musician may use those if it pleases him.

I believe, however, that the ultimate goal of a music reproduction system is as much transparency and neutrality as science will allow.

Now here I fully agree.

Why you insist on labelling people as subjectivist is beyond me for I strongly believe audio is a science and the listening to it is a subjective experience.

Unfortunately a lot of the science that goes into it is not widely, or even not understood at all.

Cheers,

P.S. There is nothing artistic about designing good cables, unfortunately a lot of so called designers jumped on the bandwagon hoping to make a quick one.

 

[Tube_Dude]

To match or not to match...

You don't need matched source and receiver impedances to have a transmission line.

So for you any interconect ...at any load impedance...and at any frequency is a transmission line!!!!

Fine!!!

And yes you could use a cable designed for digital transmission as an audio cable.

Of course....it will work ...as a interconect but not as a transmission line!!!

a digital cable is nothing more than a constant impedance coax.

Now i agree!!!!

When hooked up in a digital system the termination is done inside the gear involved, a transport and a DAC for exemple.

Of course and the termination must be the some as the intrinsec impedance of the cable to minimise reflections...and then(at last) the cable behave as a transmission line...

If I'm not mistaken the impedance standard is 50 Ohm, I am not really into digital design so I may be off

No...the impedance standard in SPDIF norme is 75 Ohms and in the more pro norme AES/EBU is 110 Ohms...

Cheers

 

[nw_avphile]

Re: NULL.

The link you inserted points to a commercial website which does not constitute scientific proof in my book.

What commercial interest do they have? How do they make money by discouraging people from buying overpriced cables? But more important, look at the references and data presented. Those ARE what most consider "scientific proof." Go ahead, discredit all the articles, audio engineers, the AES, etc. if you want. It just makes your position look that much more desparate.

Why you insist on labelling people as subjectivist is beyond me for I strongly believe audio is a science and the listening to it is a subjective experience.

Unfortunately a lot of the science that goes into it is not widely, or even not understood at all.

Well if you'd ever provide us with some objective science that was applicable and verifiable to support your many claims, I'll gladly stop calling you a subjectivist. But so far, all you've provided is a bunch of sarcasm, hyperbole, myth, opinion and numerous personal attacks.

 

[fdegrove]

SPOOKY.

Hi,

Well if you'd ever provide us with some objective science that was applicable and verifiable to support your many claims, I'll gladly stop calling you a subjectivist. But so far, all you've provided is a bunch of sarcasm, hyperbole, myth, opinion and numerous personal attacks.

As far as I'm concerned, so far you just haven't convinced anyone around here.
The only one offending people here is you for you wouldn't even be able to recognize scientific proof even if it flew you in the face.

Naturally it makes life much easier to stop investigating into the why two components sound different.

What commercial interest do they have?

I don't care about how they sell their product.
Fact remains, they sell and that makes it commercial.

Call it reverse marketing if you like.

Go ahead, discredit all the articles, audio engineers, the AES, etc. if you want. It just makes your position look that much more desparate.

Five reference are given; one is a manufacturer, the next is someone expressing his view, the third is from some journal, four and five may be related but since I have no idea what they really talk about I'd rather not discuss it.

So, if that's the best you can do than we better leave it as it is.

Jorge,

You seem to be stuck on the idea that a transmission line needs to be ideal for it to behave like one.
It doesn't have to and as I said before in audio it rarely is an ideal transmission line which is precisely why cable will interact differently depending on how it is terminated.

Cheers,

 

[nw_avphile]

Re: SPOOKY.

The only one offending people here is you for you wouldn't even be able to recognize scientific proof even if it flew you in the face.

What "scientific proof" have I failed to recognize fdegrove? Please, reference the posts.

I don't care about how they sell their product.
Fact remains, they sell and that makes it commercial.

Call it reverse marketing if you like.

They're a magazine for sound and video installers. They're just providing well researched advice for their readership. Besides you claim to sell your audio services, does that mean we shouldn't listen to you?

Five reference are given;

So, if that's the best you can do than we better leave it as it is.

Well it's INFINITELY better than you've done as far as references!

There are another 23 references here supporting my views if you'd like more:

http://www.dself.demon.co.uk/subjectv.htm

 

[Thunau]

A little different approach to difference testing

A while back I did a test of a power cord, that seemed to improve the sound of my mid-fi CD player. For sanity check I ran a test the way I knew how- using my DAW ( digital audio workstation). My test showed not only that there are measurable differences in performance with and without said power cord, but it also let me see and hear how those differences relate to musical material.
You can read (and listen to) my test at:
MPC Test
Let me know what you think.

 

[nw_avphile]

Re: A little different approach to difference testing

A while back I did a test of a power cord, that seemed to improve the sound of my mid-fi CD player. For sanity check I ran a test the way I knew how- using my DAW ( digital audio workstation). My test showed not only that there are measurable differences in performance with and without said power cord, but it also let me see and hear how those differences relate to musical material.
You can read (and listen to) my test at:
MPC Test
Let me know what you think.

Interesting stuff! I have the following questions/comments:

First, if I read things correctly, you replaced the power cord of the ADC/DAC box, with the MPC one, correct?

I'm not sure how valid it is to replace the power cord of what, in this case, is your test equipment? It would be interesting to see what happens if you replaced the power cord of a pre-amp that was in that loop.

In this link:

http://www.thuneau.com/MPC/minus_10_difference.gif

I assume green is one channel and red is the other? If I read it correctly, you measured a peak of -48db of difference in the worst case channel? What was happening at that peak versus the rest of the time?

The peak was very brief--the whole "disturbance" you recorded appears about 25ms long. This is consistent with a power line disturbance (a single cycle at 60hz is 16ms). What I suspect you're seeing is the effect of some random noise--possibly power line noise or possibly just noise picked up in other parts of the signal chain?

The rest of the difference measurement appears to be much lower in level--close to the noise floor of your "control" measurment. Was that the case for the entire song?

To rule out random noise, or other chance occurances causing the differences, I'd prefer to see a few more runs with each power cord. If the differences are consistent, you may be onto something. If they exceed -60db, they may even be audible.

But, from what you've posted so far, I'd say the MPC cord *might* have attentuated some random power line noise. From an audible standpoint, what I see *might* have been heard as a very very soft "pop". What you've posted would not explain the many other (subjective) benefits often attributed to power cords, power conditioning, etc.

 

[Thunau]

disturbance

The "disturbance" you reffer to is just one of many peaks that appear in the difference file. I zoomed in on one to show it more clearly. The peaks in the difference file appear at more "noisey" moments in the musical sample. Especially snare hits and hard accoustic guitar strums. Stuff that has a lot of unrelated harmonics. If you listen to the difference files that I posted you will clearly hear it.

 

[nw_avphile]

Re: disturbance

The "disturbance" you reffer to is just one of many peaks that appear in the difference file. I zoomed in on one to show it more clearly. The peaks in the difference file appear at more "noisey" moments in the musical sample. Especially snare hits and hard accoustic guitar strums. Stuff that has a lot of unrelated harmonics. If you listen to the difference files that I posted you will clearly hear it.

Interesting. I have also done some "digital subtraction" tests and found them to yield roughly the same results as analog null tests. They're taken less seriously by the GoldenEars, however, because they argue the A/D and D/A process is masking the very differences you're trying to measure.

Your test, however, is somewhat different than what I've done. I have a friend with a multitrack box similar to the Alesis you used. I may see if I can get my hands on a few esoteric power cords and pay him a visit sometime. You have my curiosity up!

One thing that's a bit puzzling to me, however. Looking at your "analysis" screen shots, I see only the slightest of differences between the analysis of the regular files (in fact most are identical out to the precision of the measurements). In the difference files, sampling as fine as 10ms, it only shows a peak (worst case) difference level of -55db and the average is around -75db. That's pretty low (lower than the zoomed example would imply).

Did you amplify the difference MP3 track? In my experience, I shouldn't be able to hear a -75db average level signal that's been MP3 encoded when I set the gain to listen to a 0db signal at a comfortable level. So something fishy is going on here? Did you boost the level so it would survive being encoded?

The other fishy thing here is your test implies the Alesis is coming nowhere near to meeting its specs if your test is correct. The signal to noise and distortion measurements done with loop tests in reviews (obviously with the stock power cord) should reveal a problem. Did anyone bring up jitter in the original discussion as a possible source of error? I realize you're using a master clock, but there can still be clock phase shift--especially betweeen two chips. I've found it to be an issue in my tests--the question is, why does the MCP cord reduce it?

If the power cord test really stands up to multiple trials, I have to wonder if power line noise is somehow inducing jitter into the clock? If so, you've documented a real benefit of the power cord, but given the circumstances, I'm not sure if it would make an audible difference during *only* recording *or* playback of the signal (there's a lot of debate over the audible effects of jitter--see my note below). And it likely wouldn't make any difference at all in an analog component.

As I said, it would be interesting to repeat the test with a pre-amp in the loop, and swap the cord out on the preamp instead. It would also be interesting to repeat it with the DAC/ADCs on a single chip if possible. This should reduce any jitter effects.

A note on jitter: Many years ago Stereophile started making jitter measurements of the digital gear they tested. Interestingly, after all these years, they're still hard pressed to correlate what the GoldenEars hear with the jitter measurements. There's a lot of other debate on this subject and it's complex as there are many kinds of jitter. I'm not convinced one way or the other of the audible effects of jitter but I can safely say that some gear with obscene amounts of measured jitter supposedly sounds lovely according to the GoldenEars.

 

[Thunau]

No, Nothing boosted in the MP3 file. What you hear is the difference file as it relates to the original.
Yes, jitter is possibly the reason why one recording is different from the other. Another explanation is a lack of RF oscilation in the power chord itself. As you know inaudible oscilation can be detremental to audio. The maker of this particular power cord said that he based his product on some 80 year old GE patents relating to power lines. He wouldn't say exactly which.

Anyway, The Alesis piece is close to the spec even with the regular cord. Keep in mind that we are reading DA and AD performance in series, so the error is multiplied.

And I assure you there is nothing doctored in the test. Maybe fudged, because after all I'm not an EE, but not doctored on purpose. I'm a recording engineer for last 9 years and lately a systems integrator for Clair Brothers. The test was done just to see if I was a victim of suggestion. I never believed that a power cord could make a difference, until I plugged this one in.
I'm absolutely open to suggestions how to improve on my test procedure and get more reliable results.

 

[nw_avphile]

Digital difference testing continued...

No, Nothing boosted in the MP3 file. What you hear is the difference file as it relates to the original.

Hmmm... Even more interesting!

Yes, jitter is possibly the reason why one recording is different from the other. Another explanation is a lack of RF oscilation in the power chord itself. As you know inaudible oscilation can be detremental to audio.

Jitter remains the most likely explanation. I can't imagine how the music signal is creating RF oscillations in the power cord? If the cord was simply reducing the background noise, the RF argument might make more sense. But the cord appears to be literally making the A/D and/or D/A process more accurate as the error signal is only significant on musical transients.

The maker of this particular power cord said that he based his product on some 80 year old GE patents relating to power lines. He wouldn't say exactly which.

Yes, well for reasons already discussed, we can't put much weight in what the person selling these things has to say There are all SORTS of snake oil explanations out there that don't stand up to much scrutiny. But your results merit more investigation.

The cords appear to have large ferrite cores on them. Those will certainly reduce RF traveling in to or out of the Alesis box. Again, intuitively at least, I'd guess this would (at best) result in a lower background noise.

Anyway, The Alesis piece is close to the spec even with the regular cord. Keep in mind that we are reading DA and AD performance in series, so the error is multiplied.

-75db isn't close to spec--especially running in 24 bit mode.

And I assure you there is nothing doctored in the test. Maybe fudged, because after all I'm not an EE, but not doctored on purpose.

I'm not questioning your honesty. What you posted appears to be sincere. It's just that the results and the mp3 thing are still a bit weird. My experience is mp3 encoders tend to discard stuff that's at that low of a level as noise but I suspect it depends on the encoder, bitrate, etc.

I am an EE, so for me it's a bit more of a puzzle. But I'm also quick to admit this is exactly what null/difference testing is good for! It helps us explore perceived differences. Blind listening tests really serve as the ultimate judge of differences. If the power cord survives a well conducted blind test, there's little doubt it's doing something.

I'm absolutely open to suggestions how to improve on my test procedure and get more reliable results.

Well, perhaps the easiest thing would be to see if you can repeat your measurements? I'd also suggest using the channels that are on the same chip for the reasons I suggested earlier. And most important: Make CERTAIN, you're not changing any other variables besides the power cord.

You said you have a Sony CD player and a Halfer amp? Do you drive the power amp direct from the CD player directly? I doubt you have a preamp with a detachable power cord?

Finally, if you have a friend who's willing to swap the cord(s) in and out of your system in a way you can't tell which are installed, it might be interesting to see if you can still hear a statistically significant difference. Especially given that it's been since 2001, the blind test might be the easiest thing to do.