SPDIF Cable, longer is better! (Interesting articel)

[Flecko]

That's my point - how do we know if the characteristic impedance of the device is correct? I's not simply a case of putting in a 75ohm R, as you know. You might just have substituted one set of reflections for another

I think you can say, that if the signal is terminated by 75ohm+-1% resistor, the impedance that the cable sees is 75ohm+-1%. And the cable itself has again 75ohm+-1% if it is very good. And here you will have reflections. You never get lost of them but doing it like this, is way better than just use a rca connector with some random impedance and normal wires that also have just random impedance. I think I got your point when you say a attenuator is alway better than non (isn't it?). This should be in most situations a good thing. But on the other hand, the attenuator itself induces refelctions again. And IF your system would be really a great design, it could get worse.

I believe that most of the issues with clocks start with the clock PS - most don't seem to get this right. 

Jep    Clean power seems to be a golden rule in audio.

[jkeny]

I think you can say, that if the signal is terminated by 75ohm+-1% resistor, the impedance that the cable sees is 75ohm+-1%. And the cable itself has again 75ohm+-1% if it is very good.

Depends on the receiver input circuit! The signal traces that come after that also need to be impedance controlled. A more linear stage is also better, anyway!

And here you will have reflections. You never get lost of them but doing it like this, is way better than just use a rca connector with some random impedance and normal wires that also have just random impedance.

Agreed

I think I got your point when you say a attenuator is alway better than non (isn't it?). This should be in most situations a good thing. But on the other hand, the attenuator itself induces refelctions again. And IF your system would be really a great design, it could get worse.

Can you tell me where/how the attenuator induces reflections?

I believe that most of the issues with clocks start with the clock PS - most don't seem to get this right. 

Jep    Clean power seems to be a golden rule in audio.


[/quote]Even more critical in digital audio & even more crucial in clock PS - just read what Jocko (Pat) et al have to say about this.

[Flecko]

The signal traces that come after that also need to be impedance controlled.

Assuming that the signal path is very short it becomes less significant but yes.

Can you tell me where/how the attenuator induces reflections?

They have themself an impedance that dissents from 75ohm, thus causing reflections. Also there are other ways how they can manipulate the signal, I would assume, like how constant they reduce the signal according to the frequencies you send through. There are different qualites of attenuator made for different frequencie ranges. There are also attenuators for 30$ or even 120$ made for higher frequencies and better quality.

[jkeny]

Can you tell me where/how the attenuator induces reflections?

They have themself an impedance that dissents from 75ohm, thus causing reflections. Also there are other ways how they can manipulate the signal, I would assume, like how constant they reduce the signal according to the frequencies you send through. There are different qualites of attenuator made for different frequencie ranges. There are also attenuators for 30$ or even 120$ made for higher frequencies and better quality.

have a look at the minicircuits PDF on fixed attenuators to answer/correct your above questions/statements which is what I recommend - $12 http://www.minicircuits.com/pages/pdfs/an70001.pdf

[Flecko]

Quote
Can you tell me where/how the attenuator induces reflections?
They have themself an impedance that dissents from 75ohm, thus causing reflections. Also there are other ways how they can manipulate the signal, I would assume, like how constant they reduce the signal according to the frequencies you send through. There are different qualites of attenuator made for different frequencie ranges. There are also attenuators for 30$ or even 120$ made for higher frequencies and better quality.
have a look at the minicircuits PDF on fixed attenuators to answer/correct your above questions/statements which is what I recommend - $12 http://www.minicircuits.com/pages/pdfs/an70001.pdf

I see no contradiction to what I wrote. Can you name one?

[jkeny]

You seemed to imply that these precision made attenuators (which have data sheets & graphs of their VSWR) introduced some impedance mismatch but I believe that this is a red-herring unless you can show a measured digital audio system where introducing these RF attenuators would have a detrimental effect!

Anyway, point is these are $12 & alleviate/solve a number of difficult issues. Best bang for the buck that I know of!

[Flecko]

You seemed to imply that these precision made attenuators (which have data sheets & graphs of their VSWR) introduced some impedance mismatch but I believe that this is a red-herring unless you can show a measured digital audio system where introducing these RF attenuators would have a detrimental effect!

Anyway, point is these are $12 & alleviate/solve a number of difficult issues. Best bang for the buck that I know of!

Even if they have exactly 75Ohm, you will have reflections, because your cable will not have exactly 75ohm! Impedance changes->Reflection
I have worked with an analyzer once and we had attenuators that were in the range of 50$ (I was surprized that they were so expensive). And as sensitive as audio is, it might be an improvement to use "better" ones. But maybe not. I think you have chosen good ones, no doubt.

[jkeny]

You seemed to imply that these precision made attenuators (which have data sheets & graphs of their VSWR) introduced some impedance mismatch but I believe that this is a red-herring unless you can show a measured digital audio system where introducing these RF attenuators would have a detrimental effect!

Anyway, point is these are $12 & alleviate/solve a number of difficult issues. Best bang for the buck that I know of!

Even if they have exactly 75Ohm, you will have reflections, because your cable will not have exactly 75ohm! Impedance changes->Reflection
I have worked with an analyzer once and we had attenuators that were in the range of 50$ (I was surprized that they were so expensive). And as sensitive as audio is, it might be an improvement to use "better" ones. But maybe not. I think you have chosen good ones, no doubt.

Thank you, yes they are good ones.

In answer to your point about them having their own reflections - a quote from the PDF I linked to - the first lines read

Fixed attenuators help minimize impedance mismatches
Proper application of fixed attenuators can help reduce
impedance mismatches in high-frequency circuits and systems.

And another which shows that there is no such thing as an ideal transmission line in the real world:

Under ideal conditions, when a
load is perfectly matched to a source, maximum power available from that source is
transferred to the load. Under these ideal conditions, there are no reflections, and the
reflection coefficient is zero. But when the operating conditions are less than ideal (as in all
real-world applications), not all of the source power is absorbed by the load; the remaining
power is reflected back to the source.

As I said, I think your statement is a red herring, however if you can nominate a system in which the attenuators would be detrimental, I would stand corrected.

[PeterSt]

By now ... somehow this comes to me as catching flies.
So, I thought to change the subject a bit ...

But still the hifacde has 45ps jitter figure according to Philip Gruebele.

... who took that from somewhere else. Besides I won't believe it. And merely, what's the phase noise ?

So, John, what oscillators are in there ? (it may even be a synthetic thing, I just don't know).

Peter


PS: I just wanted to say something to deviate from the subject a bit, because otherwise you may end up fighting. Haha.

[jkeny]

By now ... somehow this comes to me as catching flies.
So, I thought to change the subject a bit ...

But still the hifacde has 45ps jitter figure according to Philip Gruebele.

... who took that from somewhere else. Besides I won't believe it. And merely, what's the phase noise ?

So, John, what oscillators are in there ? (it may even be a synthetic thing, I just don't know).

Peter


PS: I just wanted to say something to deviate from the subject a bit, because otherwise you may end up fighting. Haha.

Don't worry Peter, no fighting from me!

Here's a Phase noise plot of the stock hiface

taken by JosephK & posted on DIYA here http://www.diyaudio.com/forums/digital-source/168901-rf-attenuators-jitter-reducers-30.html#post2352984 He says this about the above plot "The spdif bit rate at 44.1khz is 2.8224 MHz. Half of it is 1.4112 MHz, and that is the strongest frequency line if you look at an SPDIF data stream on a spectrum analyzer.
So probably this is the jitter caused by clocking out the data..

The tall line at the bottom of the spectrum is the ~80kHz power supply noise."

He goes on to say about some of my scope shots of this 80KHz PS noise in the stock Hiface & how they correlate with his plot above "Now, because of the correlation, I am quite sure that removing that power supply noise component from the clock, the recovered jitter would be much different.."

The clocks used are MEC clocks which are good performers - a 22.xxx & a 24.XXXMHz ones to deal with the two families of speeds!

[PeterSt]

Thanks John. But your picture doesn't work and the link doesn't either.

[jkeny]

Doe sit work now?

[PeterSt]

Yes, both work now. Thanks !

[jkeny]

BTW, seeing as we are talking about RF attenuators here are before/after scope shots of the stock Hiface SPDIF output
Without Attenuators:

With Attenuators:

The initial overshoot is because of the SPDIF output transformer & attenuators can't do anything for this! You can see a significant reduction in the reflections, however.

[Flecko]

Quote
Fixed attenuators help minimize impedance mismatches
Proper application of fixed attenuators can help reduce
impedance mismatches in high-frequency circuits and systems.


And another which shows that there is no such thing as an ideal transmission line in the real world:
Quote
Under ideal conditions, when a
load is perfectly matched to a source, maximum power available from that source is
transferred to the load. Under these ideal conditions, there are no reflections, and the
reflection coefficient is zero. But when the operating conditions are less than ideal (as in all
real-world applications), not all of the source power is absorbed by the load; the remaining
power is reflected back to the source.

As I said, I think your statement is a red herring, however if you can nominate a system in which the attenuators would be detrimental, I would stand corrected.

I do not want to say you are not correct. I think too, that a system where an attenuator has a negative impact is not easy to find. Maybe don't exist. But theoreticaly it is possible that the attenuator has a negativ influence because you can have a mismatching impedance between the attenuator and the cabel/dac/whatever. It might be realy small. One could calcuate that but I am to lazy for that now. We do not need an ideal system. Just a cable that has for example 76.ohm and a attenuator that has 75ohm. The impedance suddenly changes and this causes reflections. If the system is so good, that this reflections are higher than the reflection of the system itself, the attenuator will have a negative impact. But this is just theory and may be not the case for 99.99% of all hifi systems.