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post #196 of 1712
Quote:
Originally Posted by jkeny View Post
It is reported by Radio frequency Engineers that SPDIF cables needs to be 2 meters or longer on a properly terminated 75 ohm line, shorter than this leads to all sorts of cable reflection problems. These reflections will be heard! This could account for some of the perceived differences between cables.

Edit: this is not a recommendation for buying expensive cables - just simple engineering & no magic ingredients!!

Edit Again: I should have also said that making the cable as short as possible will also work with non-ideal SPDIF (RCA connections etc.) - 12" is possible short enough
That is not so, and no self respecting radio frequency engineer or any other electrical engineer will come up with such false claim. In fact, the shorter the cable, the better you are. I am not suggesting to use 3 inches cables, but a 3 foot is better then 10 foot, and at over 30 feet you are certainly asking for trouble.

You said the reason for keeping the length at least 2 feet had to do with reflections. Reflections have to do with MORE LENGTH, not with less length! Reflection becomes an issue when the cable becomes LONG, making the signal propagation delay longer (the signal travel time from the “driver” end of the cable to the destination “end”). What does longer time mean? Longer with respect to the digital signal rise (and fall) time.

A typical cable delay is around 1.5 nano second (nsec) per foot. The velocity is slower then the speed of light, in the range of 1/3 to 2/3 of the speed of light, and it depends almost entirely on one factor - the cable inner material isolation (the dielectric).

The rise time for the digital signal is between 5nsec and 30 nsec. 30nsec is slow but still within the specifications. 5-15 nsec is nice, and the reason that faster is not allowed has to do with setting a limit on the electromagnetic radiation (transmission of interference).

At say 10 feet, the cable delay is around 15 nsec, and a 5nsec rise time is 3 times faster then the delay, so one DOES NEED to terminate the cable and do so properly.

But at say 8 inches length, the delay is around 1nsec and even a fast 5nsec rise is 5 times slower then the cable delay, and the signal will have virtually no reflections at all. The shorter the cable, the better it is from reflections stand point as well as from many other standpoints.

I am not suggesting 8 inch cables. I am not suggesting not terminating. In fact, as a rule the termination is built into the receiver side. The issue here is cable length, and the notion that there is a minimal cable length one should keep is just plain wrong.

Regards
Dan Lavry
Lavry Engineering
post #197 of 1712
Dan,
I was paraphrasing Fred & Jocko from DiyHifi.org who are respected RF engineers - I'm not an RF guy so I'll just quote verbatim:
Quote:
If you look at both the prop delay to the load and the delay for returning reflection from the load end for a 1 meter digital cable you will find sometihng interesting. The relected signal shows up during the logic transistion of the SPDIF waveform. This is kind of a no brainer to try a 1.5 to two meter digital interconnect and listen for the difference. Anybody want to guess along these lines as to why why digital cables are usually audibly directional.........
Edit: And another:
Quote:
just pointing out why digital cables sound better at 1.5 meter and up. I worked this out with SPICE transmission line models, various published cable propagation delays, and different logic family rise times. This was over ten years ago when I was designing digital cables. Jocko was doing something simular but with real life TDR measurements.
post #198 of 1712
Quote:
Originally Posted by Dan Lavry View Post
That is not so, and no self respecting radio frequency engineer or any other electrical engineer will come up with such false claim..........

............ The issue here is cable length, and the notion that there is a minimal cable length one should keep is just plain wrong.
I'm not sure if this has been referenced already spdif

But a quote from it is interesting
Quote:
When the first reflection comes back to the DAC, if the transition already in process at the receiver has not completed, the reflection voltage will superimpose itself on the transition voltage, causing the transition to shift in time. The DAC will sample the transition in this time-shifted state and there you have jitter.
So, I believe the idea is EITHER use a long cable to avoid this situation OR indeed make the Transport S/PDIF driver faster!
post #199 of 1712

"But a quote from it is interesting
Quote:
When the first reflection comes back to the DAC, if the transition already in process at the receiver has not completed, the reflection voltage will superimpose itself on the transition voltage, causing the transition to shift in time. The DAC will sample the transition in this time-shifted state and there you have jitter."



This quote is the OPPOSITE of what should be stated!

A reflection IS when a signal super imposes itself on a transition. In fact, when the cable is long and the rise time is short, you are likely to have a lot of reflections. The first reflection is a signal traveling back from the load to the source. The second reflection is from the source to the load. The third reflection is back from the load to the source…. And so on.

For a single rise (or fall), the reflections amplitude a sequence where each “next one” gets smaller and smaller exponentially. Also, if the termination is HIGHER IMPEDANCE than the cable, the reflection is positive. If the termination is lower impedance than the cable the reflection is negative.
Call the reflection “Ref”, and it represents the portion of the original signal that will travel back from the cable “edge”. There are 2 factors for figuring the reflection coefficient:

RL = load resistance and Z = cable impedance.

REF = (RL-Z)/(RL+Z). That is the reflection coefficient.

Let’s look at examples for first reflection:

Example 1: 75 Ohm cable and 75 Ohm termination REF=(75-75)/(75+75) = 0/150 = 0 thus no reflection.

Example 2: 75 Ohm cable and 100 Ohm termination REF=(100-75)/(100+75)=0.142 so about 14% of the signal is reflected

Example 3: 75 Ohms cable and 50 Ohm termination REF=(50-75)/(50+75)=-0.2 so 20% of the signal is INVERTED and reflected.

But for a signal to be reflected, it must first happen. Nothing will be reflected if there is no signal to reflect. We do not terminate our DC power supply lines, because the “signal” is DC. It does not move, so the reflection coefficient means nothing.

We also do not terminate audio lines, trying to match the line characteristic impedance with an equal termination resistor. Why is that? Because audio signals are very slow in relationship to the speed of the signal which is at least 1/3 the speed of light. We can be safe to assume that a 100 foot cable conducting analog audio (say 20KHz or even 100KHz) has the SAME voltage at all cable locations at any given time.

We are “lucky” that we do not need to terminate lines when the signal speed is slow with respect to the line delay time. Why? ALL cables and wires have an impedance lower then 200 OHMs. You would need to separate 2 wire by the distance of earth to the moon to end up with 300 OHMs. I mean it literarily!

Most lines and cables are in the 50-130 Ohms impedance. Imagine having to terminate each line with around 100 Ohms. If so, the each time you have a 10V signal, you would need to supply .1A current and dissipate 1 watt!.

Say the rise time “step” is say 1nsec, and it takes the signal 10nsec to get from one end to the other, you can have different voltages at both ends of the cable. The driver puts fast 1V step at the input of the cable, and you will not know about it at the end until 10nsec later… In this case you need to consider reflections and proper termination.

But say the rise time is 10nsec, and the cable is say 8 inches (thus 1nsec delay), the signal at the input side has barely began, (it only got to be 10% of it’s value) and you already see it at the cable end. So you can not reflect the whole transition amplitude, because it has not yet happened. It will no be finished for another 9nsec… Clearly, the shorter the cable and the slower the rise time, the less one needs to be concerned about reflections. Take it to extreme - the slowest signal is DC, and the shortest cable is zero length... You really need no termination for reflections of slow signals and short cables.

Reflection is a good view to adopt for the cases where the cable is long enough, and the signal rise time is fast enough to have the complete transition happen at one cable end BEFORE it ever got to the other end. Another way to view it all is from the point of view of “Standing Waves”, which is more oriented to what radio frequency engineers do (certainly so for antenna designers).

I hope that helps clarify things. I wrote it to be helpful and set things straight. I am not going to twist arms to convince people that Ohms law is right, or that 1+1= 2. I did not express an opinion, I stated technical facts. While I will be glad to clarify and answer questions, there is tons of basic engineering materials covering the subject.

Again, this is not about who you like and who you want to respect. It is about physics of how electrical signals behave in conductors.

Regards
Dan Lavry
Lavry Engineering
post #200 of 1712
Quote:
Originally Posted by Dan Lavry View Post
[I]

This quote is the OPPOSITE of what should be stated!

A reflection IS when a signal super imposes itself on a transition.
I believe this is contrary to the accepted understanding of a reflection - as a portion of the signal that bounces back along the transmission line (multiple times) & one of the main causes is when a signal encounters a variation in impedance on the transmission line. Is this not what you understand by a reflection?

Quote:
In fact, when the cable is long and the rise time is short, you are likely to have a lot of reflections. The first reflection is a signal traveling back from the load to the source. The second reflection is from the source to the load. The third reflection is back from the load to the source…. And so on.
There are multiple return reflections on EVERY line, long or short. Whether there are reflections is not a function of the length of the line - only the time it takes them to travel from one end to another is effected by the length of the line.

Quote:
For a single rise (or fall), the reflections amplitude a sequence where each “next one” gets smaller and smaller exponentially. Also, if the termination is HIGHER IMPEDANCE than the cable, the reflection is positive. If the termination is lower impedance than the cable the reflection is negative.
Call the reflection “Ref”, and it represents the portion of the original signal that will travel back from the cable “edge”. There are 2 factors for figuring the reflection coefficient:

RL = load resistance and Z = cable impedance.

REF = (RL-Z)/(RL+Z). That is the reflection coefficient.

Let’s look at examples for first reflection:

Example 1: 75 Ohm cable and 75 Ohm termination REF=(75-75)/(75+75) = 0/150 = 0 thus no reflection.

Example 2: 75 Ohm cable and 100 Ohm termination REF=(100-75)/(100+75)=0.142 so about 14% of the signal is reflected

Example 3: 75 Ohms cable and 50 Ohm termination REF=(50-75)/(50+75)=-0.2 so 20% of the signal is INVERTED and reflected.
I don't think this adds anything to the statement that the main way reflections are caused is by a mismatch/change in line impedance along the transmission line?

Quote:
But for a signal to be reflected, it must first happen. Nothing will be reflected if there is no signal to reflect. We do not terminate our DC power supply lines, because the “signal” is DC. It does not move, so the reflection coefficient means nothing.

We also do not terminate audio lines, trying to match the line characteristic impedance with an equal termination resistor. Why is that? Because audio signals are very slow in relationship to the speed of the signal which is at least 1/3 the speed of light. We can be safe to assume that a 100 foot cable conducting analog audio (say 20KHz or even 100KHz) has the SAME voltage at all cable locations at any given time.

We are “lucky” that we do not need to terminate lines when the signal speed is slow with respect to the line delay time. Why? ALL cables and wires have an impedance lower then 200 OHMs. You would need to separate 2 wire by the distance of earth to the moon to end up with 300 OHMs. I mean it literarily!

Most lines and cables are in the 50-130 Ohms impedance. Imagine having to terminate each line with around 100 Ohms. If so, the each time you have a 10V signal, you would need to supply .1A current and dissipate 1 watt!.
I'm not sure what this has to do with digital signal cables?

Quote:
Say the rise time “step” is say 1nsec, and it takes the signal 10nsec to get from one end to the other, you can have different voltages at both ends of the cable. The driver puts fast 1V step at the input of the cable, and you will not know about it at the end until 10nsec later… In this case you need to consider reflections and proper termination.
Is it not true that in all cases you need to consider reflections & proper termination?

Quote:
But say the rise time is 10nsec, and the cable is say 8 inches (thus 1nsec delay), the signal at the input side has barely began, (it only got to be 10% of it’s value) and you already see it at the cable end. So you can not reflect the whole transition amplitude, because it has not yet happened. It will no be finished for another 9nsec… Clearly, the shorter the cable and the slower the rise time, the less one needs to be concerned about reflections. Take it to extreme - the slowest signal is DC, and the shortest cable is zero length... You really need no termination for reflections of slow signals and short cables.
This logic sounds flawed to me - you are taking a moment in time snapshot & trying to show something at that moment - now move the time forward to say 9nsec (the signal is at 90% of it's value) & the reflection has 1nsec delay - it returns right at the transition step & interferes with it = jitter = what was said in my quotes above, I believe?

Quote:
Reflection is a good view to adopt for the cases where the cable is long enough, and the signal rise time is fast enough to have the complete transition happen at one cable end BEFORE it ever got to the other end.
This is what is being said in recommending longer cables - are you now agreeing with this????
Quote:
Another way to view it all is from the point of view of “Standing Waves”, which is more oriented to what radio frequency engineers do (certainly so for antenna designers).

I hope that helps clarify things. I wrote it to be helpful and set things straight. I am not going to twist arms to convince people that Ohms law is right, or that 1+1= 2. I did not express an opinion, I stated technical facts. While I will be glad to clarify and answer questions, there is tons of basic engineering materials covering the subject.

Again, this is not about who you like and who you want to respect. It is about physics of how electrical signals behave in conductors.

Regards
Dan Lavry
Lavry Engineering
I think you can see by my questions that you have not clarified anything for me, anyway. I think your logic has some flaws & there's some fuzzy thinking in what you wrote.

As I said I'm no Eee or RFee but I have a brain & don't patronise me by suggesting that my opinion is formed by who I like rather than on logic (if you knew Jocko or Fred, you would know that they are hard to like )
post #201 of 1712
Guys

think we should respect the original poster and the subject - this going way way off topic (don't shoot the messenger!)
post #202 of 1712
Thread Starter 
Quote:
Originally Posted by hawkhead View Post
Guys

think we should respect the original poster and the subject - this going way way off topic (don't shoot the messenger!)
Agreed. The original intent of this thread was not to discuss theory of jitter and audibility of cables.

The intent of this topic is to discuss different listening experiences with usb to spdif converters used as a transport. Thanks hawkhead for reminding it, I hope nobody shoots the messenger
post #203 of 1712
Yep, no problem at my end dropping this now, I think we have all stated our positions. All messages delivered & messengers still alive

BTW, slim.a It seems we're in agreement on this one, not that we were ever in disagreement
post #204 of 1712
But, before we return to the 'shoot-out' let me just
post that my pc freezing problem with the mod'd
usb cable was due to the ground wire slipping past
the insulation tape and touncing the cable shield.
Flexing of the cable loosened the tape.
post #205 of 1712
Aha, Ultratone, very good, I'm glad you found the prob & reported it here. I have since changed from a hacked USB cable to a USB socket that has an external supply soldered into it. I was worried about the stability of the hacking & needed something a bit more permanent & worry free.
post #206 of 1712
Quote:
Originally Posted by hawkhead View Post
Guys

think we should respect the original poster and the subject - this going way way off topic (don't shoot the messenger!)
I thought that it is best to point out that a minimum cable length could be misleading to many, so I stated the technical facts about it.

I stated the facts and I will drop it, and you can get back to the subject at hand.

Regrads
Dan Lavry
post #207 of 1712
Thread Starter 
Quote:
Originally Posted by Dan Lavry View Post
I thought that it is best to point out that a minimum cable length could be misleading to many, so I stated the technical facts about it.

I stated the facts and I will drop it, and you can get back to the subject at hand.

Regrads
Dan Lavry
Dan Lavry,

Thanks a lot for taking the time to explain to us your view on the relationship between digital cables lengths and jitter. It gives valuable information for anyone using a usb to spdif converter as a transport with an external dac which obviously requires the use of a digital cable and choosing the adequate length (shorter or longer).

If you don't mind, I am going to link your posts in the main review (as well as Steve Nugent's article) so that readers can be aware of both views and make up their minds and choose the right length of digital cables accordingly.
post #208 of 1712
Hey Slim.a
In the interests of equitable & fair treatment, I would appreciate my posts be similarly linked.
post #209 of 1712
Thread Starter 
Quote:
Originally Posted by jkeny View Post
Hey Slim.a
In the interests of equitable & fair treatment, I would appreciate my posts be similarly linked.
Ok
post #210 of 1712
Quote:
Originally Posted by jkeny View Post
Hey Slim.a
In the interests of equitable & fair treatment, I would appreciate my posts be similarly linked.
Who are you man?
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