Optical v. USB Revisited

[tfarney]

"It is also worth mentioning that even Nordost, those well known purveyors of quality, value-for-money audio leads, can't find much to charge for!

 

Their glass optical lead retails at £100 for a 2m length - which in Nordost terms is equivalent to giving it away, I think!! The fact that they don't sell a plastic optical lead at all would seem to fit in with Mike's findings. The fact that they sell their glass one for £100 would suggest that, either way, there won't be much in it. From a company whose analogue interconnects run to nearly £3000 for .6m length, I think that says rather a lot."

 

$150 for an optical lead is absurd, even if what you charge for an interconnect is criminal. It says that much.

 

Tim

 

[tfarney]

 

...but I recently changed from digital coax to an unremarkable optical cable in my headphone system. Because I had no audible electrical noise in my system to be carried by the coax cable, the change was predictably transparent. I only did it as an experiment, because I have access to cables at cost, and because the cost of such cables, at least the "bargain"- priced ones (compared to the absurdities being discussed here) is less than 1/10th of retail. The evidence would indicate that more expensive cables enjoy more expansive margins.

 

I can't be bothered to compare my $60 (retail) cable to a $150 one. Why? Because there is no good reason to do so.

 

There seems to be an implication by some that it is closed-minded to not believe there can be a difference between optical cables without testing them. I think, rather, that it would be a foolish waste of time and money to try to hear analog signal characteristics in digital data. I don't need to carefully observe the behaviors of a wide variety of ducks to understand that they are not cows. YMMV.

 

There is only one thing here that possibly (though not probably) merits investigation: jitter. It is measurable. Those who wish to sell expensive optical cables, claiming they are more musical than others, simply need to publish the jitter measurements of their cables, compared to common optical cables. In the meantime, those of you who wish to pay hundreds of dollars to deliver exactly the same data that a $10 cable delivers, with no evidence of superiority, with no logical reason to believe it could possibly be better, will continue to do so. And I will continue to be unsurprised that you manage to hear a difference.

 

ABX testing would, of course, settle the issue. But it seems that most of those who "trust their ears" only trust them if their eyes, their foreknowledge and their expectations are fully engaged, making any kind of objective testing of their ears completely impossible.

 

Blind faith, no blind testing. It is the code of audiophile nervosa.

 

Tim

 

[BobH]

if that was a swipe, Tim, I obviously didn't express myself very well. I thought I was saying, from a different angle, exactly what you are saying!

 

I'll have another go. If a company with the reputation of Nordost is charging only £100 for its optical offering then it is unlikely that there will be much difference, if any, between all well made optical cables, whatever their cost.

 

The fact that Nordost do not make a plastic optical cable means that they are not happy with some aspect, and the way it fits in with their business model. That may, or may not, indicate that plastic is in some way inferior to glass but, even it only means that the connectors can be secured in a more robust way, then it may be worth a look at glass.

 

I think we agree, sort of.......

 

[Max]

One genuine difference between optical & other interconnects is that there is no electrical inter-connection. So no ground loops (low level hum) or possibility of conducted RFI, giving rise to noises off. But glass or plastic fibres wont make any difference

 

Max

 

[tfarney]

But at the seller of the $150 optical cable. And I suspect that the use of glass instead of plastic has less to do with performance than it does with the business model, which I suspect has to do with using the finest materials available instead of the finest materials that will make a difference, and charging a huge mark-up for their illogical behavior.

 

Tim

 

 

[BobH]

I don't think that is far wide of the mark, to be sure!

 

A pair of those 1m interconnects, at £6000, would buy two sets of Stax Omega SR-007 Reference System II's. Now that puts things into perspective!

 

[tfarney]

...it would seem, then, that you agree that "high-end" has snake oil vendors and gullible marks. Now if we can only bring the line a bit closer to the ground, I can lure you over to the dark side....bwahahahahahahahah!!!!!!

 

Tim

 

[glt]

Seems that all receivers (SPDIF and USB) have inherent jitter no better than 50 ps according to the data sheets. Is this the absolute minimum that can be passed to a DAC without a reclocker?

 

[Ashley James]

Dr. Chris Smith of Roke Manor sent us this. He's got nothing to sell us.

 

 

Reading through that CA thread on jitter, it occurred to me that no one had bothered to check out the science.

So here it is. The effect of jitter on a sampled signal may be expressed as a signal to noise ratio :-

 

SNR(jitter) = -20*log(2*pi*tor*freq) dB

 

Where

tor = the jitter in seconds.

freq = frequency of the sinewave test signal in Hz.

 

So the degradation is proportional to frequency, and the worst case would therefore be freq=20kHz. However, as the ear's sensitivity has rolled off by at least 20dB at 20kHz, a more realistic test would be ~4kHz.

 

If we set the SNR target due to jitter at 100dB at 4kHz, so as to be masked by the 16 bit quantisation noise, plugging the numbers into the equation above gives us the absolute minimum audible jitter = 400ps. This is way above the level deemed to be necessary by the armchair experts. Maybe you could run it past Martin.

 

I did and Mart agreed.

 

Ash

 

[BobH]

Oh! yes!! But you need to know I'm going nowhere without my NOS dac, 2w SET and open baffles (once I've built 'em, that is!). No sir, your work here is not yet done.

 

 

[tfarney]

You can bring the open baffles, but why would you want to feed them a clipping amp and a jittery DAC?

 

Never underestimate the power of 21st century technology, Luke, er...Bob. Not to mention the sound of an amp that is not clipping.

 

 

Tim

 

[cfmsp]

 

 

"One final point I'd make is that I stand to gain nothing for providing this information but some of you may save money as a result of taking heed of what I've said.

With sincere intent,

Ash"

 

WHile I do not doubt that you 'intend' sincerity, given the facts of your business as a manufacturer, it does NOT automatically follow.

 

As a manufacturer of a product which includes exactly the type of DAC implementation you argue vociferously here is as good as can be obtained, you DO have something to gain by 'providing this information'. Indeed, you would seem to have MUCH to gain by convincing computer audiophiles coming here with questions that it is not possible for anyone to detect a significant difference between this type of DAC implementation and those which are more expensive.

 

You argue that anything but the cheapest Toslink cable is a waste of money, and at the same time, point out that you modified the toslink input on your product to only accept the type of connectors found on the cheapest Toslink cables. IOW, the more expensive Toslink cables that you argue against will not even connect to your product. How could that sort of comment here NOT be seen as self-serving (to your business interest).

 

As with the engineers believing that the chip designers are the 'clever people', I would totally expect that you would support your business decisions (as to product design , feature, etc.) here, BUT, and this is a big but, asking us to believe that you have nothing to gain by providing your beliefs/opinions would NOT pass Hemingway's crap detector.

 

I intend no personal offense. I'm simply pointing out that your statements cannot be taken at face value given your business interest.

 

respectfully disagreeable,

clay

 

 

 

 

[AV-OCD]

This may be of interest.

 

"A recent paper describes practical research that found the lowest jitter level at which the jitter made a noticeable difference to be about 10ns rms. This was with a high level test sine tone at 17kHz. With music none of their subjects found jitter below 20ns rms to be audible.

 

The author developed a model for jitter audibility based on worst case audio single tone signals and including the effects of masking. This concluded: “Masking theory suggests that the maximum amount of jitter that will not produce an audible effect is dependent on the jitter spectrum. At low © N J Dunn March 1999 3 of 30 AES 106 May 1999 frequencies this level is greater than 100ns, with a sharp cut-off above 100Hz to a lower limit of approximately 1ns (peak) at 500Hz falling above this frequency at 6dB per octave to approximately 10ps (peak) at 24 kHz for systems where the audio signal is 120dB above the threshold of hearing.”

 

In the view of the more recent research cited above this may be considered to be over cautious. However the indication that jitter below 100Hz is more than 40dB less audible than jitter above 500Hz is useful when determining the properties of jitter attenuation devices."

 

http://www.nanophon.com/audio/1394_sampling_jitter.pdf

 

And this:

 

"When the (random) jitter size was 2 ms (r.m.s.), all listeners scored more than 75% correct. About 25% of the listeners detected jitter when its size was 500 ns. When it was 250 ns, however, no listener could discriminate the sounds.

 

http://amorgignitamorem.nl/Audio/Jitter/Detection%20threshold%20for%20distortions%20due%20to%20jitter%20on%20digital%20audio%2026_50.pdf

 

And this:

 

"We can see that the audibility threshold decreases from 500ns at low frequencies to as little as 20ps at 20kHz. Especially when using formats or converters with high sample-rate this will be a major issue. The high-frequency sample-jitter must be rejected vigorously. Fortunately, PLL-circuits, as used for clock extraction in AES-receivers and DACs, have a low-pass jitter transfer function. Still, good design of these is critical."

 

http://www.iet.ntnu.no/courses/fe8114/files/Report_audiodac.pdf

 

I'm still trying to make sense of all of this, but it seems that the audibility of jitter is dependant on the type and frequency. In the end though, since we as consumers generally aren't told what the jitter levels are of the devices we buy, nor are we told if the jitter is broadband or frequency dependant, we still have to use our ears and flawed, suggestable brains to decide whether or not we like the sound of said product. And whether or not a piece of digital gear sounds good may have nothing or everything to do with the level of jitter it produces.

 

[audioengr]

"It may be my horrid cold but I am having some difficulty getting my head around the concept that the material an optical cable is made of can affect the sound..."

 

Certainly it can. A plastic fiber cable will generally have less bandwidth than a glass-fiber and the glass will have lower loss. The result of this is less jitter because the optical receiver gets a crisper "edge". This means that the detection of the edge is more consistent and invariant, resulting in less jitter.

 

With all USB devices, even the ones using good chips, the power source, implementation and clock quality will affect the performance. Nothing is apples to apples unless maybe it's from the same manufacturer.

 

Steve N.

 

[mpmct]

 

"The result of this is less jitter because the optical receiver gets a crisper "edge". "

 

What, exactly, is "edge"?

 

[shenzi]

Speaking as someone who isn't a customer, I find AVI's business model interesting.

 

They have taken a difficult path, similar to that adopted by Quad in the 70s and early 80s. Namely, try to educate the public to appreciate the sane, rational engineering approach they are taking, rather than take the easy route and produce crap at the highest price the market will stand. (No specific criticism intended by the last category.)

 

I find that refreshing and Ash's contributions here invaluable. I don't think he has a specific agenda other than kicking against the crap.

 

[glt]

Most likely the bandwidth limitations are the toslink transmitter and toslink receiver as they are photodiodes that activate above a certain threshold. The photoreceptor in the receiver will operate based on the intensity of the receiving light. It will be "on" if the intensity is above a certain level and "off" if below certain level, So you have to loose a lot of light before the photoreceptor is unable to operate (this is why it is either working or not working, but it can't just loose a bit here and there). The toslink receiver also has a waveshaping circuit to make the waveform completely square...

 

[mpmct]

So with a ~3 meter toslink, would this ever be an

issue in terms of *audibility*? I've been using a 2-bit plastic toslink

cable of that length for a year or so, and I've never heard

a drop-out, that I can tell. I'm asking then: what matters

in *practical terms*, terms that I think Max had alluded to earlier in the thread?

If it's much ado about nothing, or not very much, or only exceptionally ...

well, I'm a busy guy -- with lots of other fish to fry! ;-)

 

[mpmct]

"I find that refreshing and Ash's contributions here invaluable. I don't think he has a specific agenda other than kicking against the crap."

 

+1 !

 

[Mike19]

No matter what conclusions I may draw from this very informative thread, I can't buy any more cables becuase I'm saving up for a Mapleshade 1 1/2" platform and a set of Black Diamond Racing Cones to put underneath my desktop.

 

It is a well established principle among many audiophiliacs that vibration reduction and/or isolation of the source component is a must (along with power cords of at least 10 gauge).

 

I'm wearing a flack jacket and a steel helmet so have at it!!!

 

[glt]

I've tried pastic and glass, thick and thin an can't really tell a difference. Based on the experiences on the web, it seems that you really have to concentrate to hear these minute differences. I usually give up if it is not clearly apparent to me. Form a system/engineering perspective, it seems to me that the transport medium (the optical cable) is not the weakest link because it merely reflects the photons from one end to the other (but this is the only component that people can swap). The quality of the toslink transmitter and receiver (e.g. how good it can create and respond to the light pulses), the clock used to create the spdif stream and how the spdif receiver reconstructs the clock and data are more important in the big picture (but often there is nothing you can do about them). In the end, the entire spdif/optical system may not matter at all if the spdif receiver is capable of rejecting this jitter, leaving just intrinsic jitter of the receiver which you still cannot do anything about.

 

But if your DAC has a reclocker, then the original clock signal as retrieved by the spdif receiver is not even used, but the reclocker generates a new clock (typically based on a very low jitter, high quality clock) which is fed to the DAC. This is why some say that the jitter can be completely eliminated at this point. In practice, there is some jitter (very small) but this is white noise jitter which translate to SNR numbers which may be completely under by the noise floor of your 16-bit source material.

 

So if I look into the entire chain where digital data in a PC starts its journey to a DAC, the optical cable is probably the strongest link.

 

[glt]

A cone system will couple to the ground, so all vibrations in the ground will transmit easily. If you want to isolate your component from ground vibrations, you have to suspended it in air or put some soft material on the bottom. If you want to isolate from airborne vibration, you can fill some bags with sand and put on top or glue some rubber sheets like dynamat. Basically you are changing the resonant frequency of the equipment case. But nothing will match the beauty of the racing cones :-)

 

[The Computer Audiophile]

Don't the racing cones turn vibrations into heat? Vague memory coming back here.

 

[audioengr]

""The result of this is less jitter because the optical receiver gets a crisper "edge". "

 

What, exactly, is "edge"?"

 

Edge refers to a digital signal transition, either a rising edge (0->1 transition) or a falling edge (1->0 transition). This is common terminology in digital design.

 

Steve N.

Empirical Audio

 

[glt]

Eventually vibration will turn to heat in any material, even water.

 

My only practical experience is isolating hard drives in computers. Usually I just suspend them with rubbery type of material (or rubber bands) and the reduction is vibration is very substantial. Every dampening mechanism I've seen (inside CDROMS, Table Saws, car engines) is made out of rubber and/or springs. I know from experience that if you use something rigid to couple a vibrating thing, the vibration will just transfer...