Topic: Digital Cable Minimum length?

[SparkE!]

Okay, so it seems this is an explanation of one potential cause of jitter.  It seems a simple buffer would solve the jitter problem.  Do not quality DACs have a buffer in which to store the 0s and 1s as they arrive, re-clocking them to the proper timing upon playback?  Seems (to me) like a simple solution, regardless of the cable used.

Ding, ding, ding, ding! We have a winner!  Yes Brian, properly designed playback systems don't use their input clock do directly drive their sample output clock.  Jitter on the input does not directly translate to jitter on the output, though input clock jitter can affect your circuit's ability to create a low jitter output clock.  A well designed output clock circuit will average out most of the received jitter by using a phase locked loop whose loop bandwidth is many, many clock cycles.

Something else to remember... If reflections from an improperly terminated cable actually come back during the rise time, they will serve to hasten the rise of the signal.  Not only that, but they will hasten that rise by exactly the same amount of time every time.  (The cable is not changing lengths from bit to bit, nor is the speed of propagation on the cable changing.)  So even if there is an effect on the rise time, it's the same for each and every bit transmitted.  That simply cannot cause an increase in jitter.  Remember that jitter is a variation in bit timing.  Since the effect is always the same, there is no increase in the variation in bit interval that is due to cable reflections.

And again, my main point in my previous post is that THERE ARE NO REFLECTIONS if your cable impedance is the same as the input impedance of the electronics that are connected at the destination end of the cable.  It simply does not matter how long the cable is unless you have an impedance mismatch.

[cshepherd]

I don't think improperly designed playback systems or cheap dacs are really the issue here.  The claim is that it's impossible to have signal path components that are perfectly matched for impedance...and that reflections have to occur because of this. 

In the end, no matter which theories get debated, the 1.5 meter cables rendered a better sound when two lengths of the same cable were compared.  This same result happened with three different quality cables.  I think it's pretty interesting.

Chris

[Brian Skalinder]

I don't think improperly designed playback systems or cheap dacs are really the issue here.  The claim is that it's impossible to have signal path components that are perfectly matched for impedance...and that reflections have to occur because of this.

Agreed, that's not the point of the article.  But properly designed playback systems (mainly DAC) are a solution to jitter.  Properly designed DAC = doesn't matter whether your cables length / brand / composition minimizes jitter.  So why worry about cables (other than if you want to sell them), better to address the problem with a broader solution.

[JasonR]

In the end, no matter which theories get debated, the 1.5 meter cables rendered a better sound when two lengths of the same cable were compared.  This same result happened with three different quality cables.  I think it's pretty interesting.

Chris

Double-Blind A-B-X testing, of course, right?

- Jason

[cshepherd]

In the end, no matter which theories get debated, the 1.5 meter cables rendered a better sound when two lengths of the same cable were compared.  This same result happened with three different quality cables.  I think it's pretty interesting.

Chris

Double-Blind A-B-X testing, of course, right?

- Jason

Yeah, three people at UHF Magazine testing two lines of Atlas cables ($93 & $278 for 1m), testing both with 1m and 1.5m lengths.  One person in the know, two people are not.  The more expensive 1.5m Opus All Cu wound up staying in their reference system.

Our Atlas rep did a comparison with Harmonix digital cables (~ $1700 for 1.5m) at the same lengths upon hearing about the Atlas comparison.  He came to the same conclusion.  Both UHF and our Atlas distributor are returning all 1m digital cables.

Okay, so it seems this is an explanation of one potential cause of jitter.  It seems a simple buffer would solve the jitter problem.  Do not quality DACs have a buffer in which to store the 0s and 1s as they arrive, re-clocking them to the proper timing upon playback?  Seems (to me) like a simple solution, regardless of the cable used.

Ding, ding, ding, ding! We have a winner!  Yes Brian, properly designed playback systems don't use their input clock do directly drive their sample output clock.  Jitter on the input does not directly translate to jitter on the output, though input clock jitter can affect your circuit's ability to create a low jitter output clock.  A well designed output clock circuit will average out most of the received jitter by using a phase locked loop whose loop bandwidth is many, many clock cycles.

Something else to remember... If reflections from an improperly terminated cable actually come back during the rise time, they will serve to hasten the rise of the signal.  Not only that, but they will hasten that rise by exactly the same amount of time every time.  (The cable is not changing lengths from bit to bit, nor is the speed of propagation on the cable changing.)  So even if there is an effect on the rise time, it's the same for each and every bit transmitted.  That simply cannot cause an increase in jitter.  Remember that jitter is a variation in bit timing.  Since the effect is always the same, there is no increase in the variation in bit interval that is due to cable reflections.

And again, my main point in my previous post is that THERE ARE NO REFLECTIONS if your cable impedance is the same as the input impedance of the electronics that are connected at the destination end of the cable.  It simply does not matter how long the cable is unless you have an impedance mismatch.

Brian, I understand what you are saying about the buffer solution in dacs.  It sounds like a logical solution.  If dacs with buffers were available to handle this issue, they would be in the caliber of dacs in the reference systems being used to make the comparisons.  A lot of of money, time and effort goes into these systems to make sure they are properly set up and have the best gear available to them. 

SparkE, your assessment of  improperly terminated cables isn't very realistic.  Testing six different digital cables from Atlas and Harmonix (ranging from $93 - $1700) that are all assembled improperly would be statistically challenging to say the least.  Atlas and Harmonix know how to terminate cables.  I'll take that statement to the bank.

The article stated that the rise-time is not consistent from one data block to the next.  It did not go into detail on the issue, but the issue was briefly addressed.  The inconsistency was blamed on the less-than-perfect impedance matching.  If their reasoning doesn't hold water, what other issues could be causing the sound quality to be better on the 1.5 meter cables?

Chris

[JasonR]

... If their reasoning doesn't hold water, what other issues could be causing the sound quality to be better on the 1.5 meter cables?

Psychology for one.

If the reasoning doesn't hold water, the next step is to look at the testing methodology for any holes, and the ones described are far too significant to conclude that one cable really does sound any different (forget better) than another.  At least two problems are that the sample size is too small to reach meaningful conclusions, and one person in the room was aware of which setup was which.

Maybe coaxial digital cables over some length really do sound different, or even better, but it doesn't seem to be supported by the evidence thusfar.

- Jason

[rodeen]

... If their reasoning doesn't hold water, what other issues could be causing the sound quality to be better on the 1.5 meter cables?

Psychology for one.

Wouldn't you capture the bits and diff them?  If the data coming across the wire is the same, wouldn't it sound the same?  I have to imaging if we are dealing in the digital domain that there is some way to measure the differences and explain them. 

[Brian Skalinder]

Wouldn't you capture the bits and diff them?  If the data coming across the wire is the same, wouldn't it sound the same?  I have to imaging if we are dealing in the digital domain that there is some way to measure the differences and explain them.

It's not so much that the bits are different, but the jitter that causes arrival time differences of the bits at the DAC.

[Roving Sign]

Ok - backing up a bit - a lot of this addresses sending a stream to a DAC.

Does any/all of this mean anything when it comes to cables between ADs and Recorders?

If a bit comes out of the AD and is recorded...isnt is still the same bits - no matter when it arrived.

[SparkE!]

Wouldn't you capture the bits and diff them?  If the data coming across the wire is the same, wouldn't it sound the same?  I have to imaging if we are dealing in the digital domain that there is some way to measure the differences and explain them.

It's not so much that the bits are different, but the jitter that causes arrival time differences of the bits at the DAC.

Brian, the speed of propagation on the cable is constant and the reflections are the same size every time.  Nothing about the reflections will change from bit to bit.   As such, there is no increase in jitter, even if there are measurable reflections.

The size of the reflection is given by Vr = Vi * G, where G is the reflection coefficient, Vr is the reflected voltage and Vi is the incident voltage at the mismatched end of the cable.  G may be computed as (Zcable - Zinput) / (Zcable + Zinput) where Zcable is the intrinsic impedance of the cable and Zinput is the effective input impedance at end of the cable that is mismatched.  Notice that if Zcable = Zinput, G is 0 and there is no reflected voltage.  Nothing in any of these parameters changes from bit to bit, so the reflection is the same size for every bit.

The reflections may speed the rise time up a bit, but the rise time of the signals on the installed cable will be exactly the same time for each and every bit transmitted.  Mismatch reflections simply do not and cannot affect signal jitter unless the cable significantly limits the signal bandwidth (which is not the case for S/PDIF signals on commonly available S/PDIF cables).

[Brian Skalinder]

Brian, the speed of propagation on the cable is constant and the reflections are the same size every time.  Nothing about the reflections will change from bit to bit.   As such, there is no increase in jitter, even if there are measurable reflections.

The size of the reflection is given by Vr = Vi * G, where G is the reflection coefficient, Vr is the reflected voltage and Vi is the incident voltage at the mismatched end of the cable.  G may be computed as (Zcable - Zinput) / (Zcable + Zinput) where Zcable is the intrinsic impedance of the cable and Zinput is the effective input impedance at end of the cable that is mismatched.  Notice that if Zcable = Zinput, G is 0 and there is no reflected voltage.  Nothing in any of these parameters changes from bit to bit, so the reflection is the same size for every bit.

The reflections may speed the rise time up a bit, but the rise time of the signals on the installed cable will be exactly the same time for each and every bit transmitted.  Mismatch reflections simply do not and cannot affect signal jitter unless the cable significantly limits the signal bandwidth (which is not the case for S/PDIF signals on commonly available S/PDIF cables).

I think I actually followed that.    Thanks for the correction, and explanation, SparkE!

[SparkE!]

Guys, I'm really not trying to be a jerk about this.  It just bothers me when I see gross misinformation being peddled as the truth.

That said, I could still be wrong in my opinion on the topic, but so far no one has given a plausible explanation as to why short data cables are problematic.  In fact in every other application that I've worked on, there was a concerted effort to limit the length of data cables in order to avoid problems associated with reflections.  Here's why:

The size of the reflection is determined by the impedance mismatch and the size of the incident signal.  For all practical purposes, the reflection happens instantaneously when an coming voltage transition arrives.  If there are mismatches at both ends of the cable, the reflection will bounce back and forth and reduce in amplitude at each reflection.  The amount of time that the reflections can affect things is determined by how long it takes for pulses to traverse the cable from one end to the other and back.  (Remember that the reflections are instantaneous, but each reflection reduces the amplitude of the reflected signal.)  So, the shorter the cable, the less time that the reflections can actually affect the signal that is detected at the destination end of the cable.  In that respect, shorter is good.  I have never seen an application that required longer cable for better performance unless the additional cable was being used to reduce the signal level of an rf signal that's so large that it will overdrive the front end of the device that will receive it.  (For example, GPS receivers don't like too much signal at their input because it can cause intermodulation distortion in their front end amplifiers.  So, when hunting for antennas for your GPS receiver, you need to make sure that you don't overdrive the front end by choosing an antenna that has too much gain.  On the other hand, if you want to drive a longer cable, you may actually want to use a higher gain antenna to compensate for the additional cable losses that are associated with the increased cable length.)

[cshepherd]

What a conversation!  I don't think anybody's being a jerk here.  I knew this topic would stir it up.  If longer digital cables really do sound better than shorter lengths, that's worthy news around this forum. 

SparkE has made a solid case against their explanation.  I believe the reviewer's results though.  I don't see anything faulty with their revieweing process.  As part of the reviewing process at UHF, the person controlling the review comments last to avoid influence.  The fact that they didn't review 400 different cables doesn't change what they heard with four...and what our distributor heard with the two Harmonix digital cables.

I'm inclined to believe the debate lies with the explanation and not with the comparison.  I'm going to email Steve Nugent and ask for his explanation on this subject.  I'm hoping he'll respond.  If he does, I'll post it here.

 

Chris

[SparkE!]

I have no problem believing that 3 people can agree on a single conclusion.  Heck, I can toss 3 coins in the air, they'll all land the same side up about 1/4 of the time on the average.  It's not at all hard for me to believe something like that happened when you are talking a single trial with 3 people.  On the other hand, if I was the one deciding which cables to stock in my store, I certainly wouldn't base my stocking choices on the opinion of 3 people unless they could demonstrate to me that they can actually tell the difference by running the test blind over multiple trials.  I seriously doubt that they could identify which digital cable was in use in random trials any better than this 1980 penny in my pocket can.  I'll flip you to see who goes first!

[ghellquist]

It is very interesting that people trying to make you buy $1000 cables cannot be bothered to measure jitter. There are only two error modes for this kind of digital cables, lost bits or time jitter. Only these errors could influence the sound of a digital interconnect, if the rest is the same.

Lost bits are very unusual and is a sign of a really inferior design. If you need to, this can be easily measured.

Time jitter, or short jitter, is inherent in every transmission of a clock signal. The audible effects of jitter can easily be measured if you know how to do it and has acess to some fairly inexpensive measurement instruments. None, none, none, none of these producers of ultra expensive stuff can be bothered to measure it.

My guess is that there is no difference, only marketing. And as long as people buy it, they make their money and laugh all the way to the bank. Guess the world is full of suckers waiting to get it.

Gunnar