Objective-Fi Optical Networking & SFPs

[Audiophile Neuroscience]

51 minutes ago, bluesman said:

A p value of 0.1 means a 90% probability, not 99%.

My typo. should have been p value =0.01, 99%

 

Of interest, the first blind test methodology based on AB XXXXXXXX failed ( false negative test result) whereas the second blind test procedure, ABX ABX etc succeeded to a p-value of 0.01 being highly significant.

[cat6man]

19 hours ago, ray-dude said:

 

This is the fun stuff!!  If PhD's are allowed in, count me in!

 

(FYI, I put my hypothesis out there in part 1 of my Extreme review...reference voltage, ground plane, and reference timing are the father/son/holy ghost of digital audio, I think, and everything always seems to come back to those fundamentals)

 

Hell yes, this is the fun stuff (well, that and listening to music).

 

Open-minded folks with a inquiring/scientific bent (PhD or not) are most welcome Ray.

I wonder how much Emile&co measure versus listen when tweaking the Extreme?  If they have found a measurable quantity that is correlated with what we consider 'better' sound, it would be a lot easier to optimize the package or at least get it in the ballpark.  For example, looking at power supply/ground plane spikes and tuning to minimize frequency of events?  Measuring latency variation in delivering samples, mimimizing fragmentation?  I personally would love to experiment with a server where the latencies could be manipulated in a deterministic way (but don't have the programming chops to do so myself).

 

 

 

Too bad I don't have access to a world class lab and test equipment anymore

 

 

 

 

[cat6man]

15 hours ago, fas42 said:

 

Go for it ... 🙂.

 

The overall answer for "what is going on" is that electrical noise from a variety of sources internal and external to the rig impacts the analogue areas of the replay chain; just enough to be audible - this was true 3 decades ago, and is just as true, still, right now. Doesn't matter that the music player is "right over there, way, way from the sensitive stuff!!" ... nasty stuff gets around with the greatest of ease, and your challenge, should you choose to accept it 😝, is to track down and nail every last one of these pathways for the SQ to be degraded by interference mechanisms.

 

The precise, technical explanation for what's going on in a particular setup would be handy to know - but ultimately far less important than knocking the relevant interference pathways on the head ... 😉.

 

i accept!

 

and while the precise technical explanation may be hard to elucidate, why isn't the result measurable at the DAC output?

whatever the exact mechanism may be........

 

is it our inability to measure transient and/or peak distortions at a low enough level?

[Audiophile Neuroscience]

On 6/27/2020 at 5:14 AM, cat6man said:

 

I'm a big fan of WU! (and still have my coffee mug)

 

This discussion reminds me of situations that happened repeatedly at work over the years.

We'd measure something in the lab and some arrogant PhD would say it was impossible because he had determined that such and such was optimum, blah blah blah.

 

I always insisted that the theoreticians in my department/lab get their hands dirty and verified that their algorithms behaved as expected when implemented.

In my personal experience, I obtained some of my most interesting insights (and patents) from the lab (i.e. real world) experiments not lining up with the oversimplified theories.........like "Why the hell is that happening?".............answer:  the experiment is trying to tell you something dummy!

 

Back to audio:

It really bugs the heck out of me that putting a better power supply on a digital music server can have such a profound impact on my musical enjoyment................I love the enjoyment part but am frustrated that I don't understand the mechanism (actually worse, I don't have a clue!) by which this happens. 

 

I have no specific theory or hypothesis to fit my experience but I think a framework is in sight.  My engineering intuition (or hubris?) suggests that it is related to behavior on the tail of distributions (a phenomenon I've seen repeatedly in my professional work).

A possible framework, though unproven, might look as follows:  (YMMV, ignore the rest if you detest speculation)

 

Whatever mechanism is at work is likely not measurable in an average sense but is likely, IMHO, to be a low probability event.  Let's hypothesize 1% of the time this "something happens".  For red book, this would be ~440 times/sec.  I can certainly imagine that this might be discernible if it affected 440 samples/sec but would yet be unmeasurable on average due to being masked by the other 43,000+ samples.  Occasional long latencies between samples?  Clock jitter spikes?  Short infrequent power supply glitches?  I've no idea but something is going on.

 

[start lecture]

Case in point/analogy:  In wireless communications, using 2 spatially separated antennas give improved immunity to fading and is called space diversity reception, the idea being that if the fading is independent on each antenna the probability that they both are bad (i.e. faded) is greatly reduced.  That's the theory.....and we saw exactly that with a fading simulator in the lab, but not so when testing around a real cell site with space diversity antennas.  WTF?  It turned out that the two antennas have almost independent fading but there was a residual 1-2% correlation in the fading statistics.  On average that would make no difference to the measurements, however the wireless receiver  behaved differently.  Now a 1% correlated signal could be simply modeled as 99% of the time uncorrelated and 1% of the time completely correlated (i.e. both fading at the same time).  The radio receiver was happy as a clam 99% of the time but very unhappy the other 1% of the time, and whenever that 1% of the time occurred the receiver would have to change its operating point and required SNR and the entire receiver loop would have to re-converge.  When we programed the fading simulator to have 1% correlation between the two receiver inputs, we obtained the exact same results as in the outside world.  The 1% correlation did not make any change to the average fading statistics but it had a huge impact on the receiver due to dynamic effects.

[end lecture]

 

So let me put down a friendly challenge.  In the interest of being 'objective' and not 'subjective' in this sub-forum, how about a sub-sub-group interested in getting to the bottom of this technically and not just acting like my old 'arrogant PhD' colleagues who already knew all the answers (but had oversimplified the situation and therefore hadn't formulated the problem accurately)?

 

This will be my last shot at trying to see if anyone else is interested in approaching this in a similar manner to my current way of thinking.  I'm not interested in debating  philosophy of testing and will not reply to such.  Anyone want to try to figure out what is going on here?

 

 

I expect to be excluded from such a group on "philosophical grounds". No problem but despite my philosophical bent, and probably because of it, I do applaud the open minded experimental approach to knowledge as you suggest. I can only offer then, some philosophy stolen from some of my science heroes.

 

If it disagrees with experiment it is wrong....... It's difficult to be absolutely sure about anything (scientifically speaking)..... among the easiest to fool are ourselves (for or against a proposition), and a direct quote from The meaning of it all: Thoughts of a citizen scientist - "Looking back at the worst times, it always seems that there were times in which there were people who believed with absolute faith and absolute dogmatism in something. And they were so serious in this matter that they insisted that the rest of the world agree with them. And then they would do things that were directly inconsistent with their own beliefs in order to maintain that what they said was true.”

 

Now, if I could put it to music...ah somebody beat me to it 😁

"So I hope, through life you never will decline

In spite of philistine
Defiance
To do what all good scientists do

Experiment"

 

Good Luck

[fas42]

6 hours ago, cat6man said:

 

i accept!

 

and while the precise technical explanation may be hard to elucidate, why isn't the result measurable at the DAC output?

whatever the exact mechanism may be........

 

is it our inability to measure transient and/or peak distortions at a low enough level?

 

What matters is what our ears hear - the tiniest, tiniest variation in electrical activity is guaranteed to make the output of a DAC change to some degree; measuring something happening there is highly likely useless in terms of explaining what our minds are perceiving. And the next complication is that integrated playback systems no longer have a DAC to measure - a digital speaker has that part of itself buried inside. The only useful method is to capture what the speaker is producing, and compare before and after versions of its output.

 

Until very recently we had no decent mechanism to do that comparison ... luckily our able member Paul, @pkane2001, has created some software clever enough to sync the two microphone recordings, and hopefully show something useful. I did an exercise some years ago, examining a YouTube video of a percussion group playing, and a supposedly high end rig reproducing an earlier recording of the same piece - sounded very different, and it was trivially obvious why in the waveforms: the treble of the speaker output was miles from being right ... there are no mysteries, if you look at the right things, 😉.

[jabbr]

On 6/18/2020 at 4:34 PM, ray-dude said:

Thank's for the pointer to this topic Ken.  My hypothesis is that switching to the laser is causing a correlated pulsing on the reference voltage and/or ground planes that is having an indirect impact on the DAC.

reasonable theory however,

 

1) the lines to/from the SFP(+) are differential, so constant current.

2) compliance testing for 10Gbe+ disproves significant ground bounce

 

so available measurements fo not support thus theory, nor have I seen measurements of legacy 1Gbe SFPs which demonstrate this.

 

That said I would be surprised if a generic SFP or NIC failed to meet the standards ascribed to by major manufacturers. For me the differential cost of going with a well known manufacturer is low enough that I do that and don’t worry about this.

[The Computer Audiophile]

2 minutes ago, jabbr said:

reasonable theory however,

 

1) the lines to/from the SFP(+) are differential, so constant current.

2) compliance testing for 10Gbe+ disproves significant ground bounce

 

so available measurements fo not support thus theory, nor have I seen measurements of legacy 1Gbe SFPs which demonstrate this.

 

That said I would be surprised if a generic SFP or NIC failed to meet the standards ascribed to by major manufacturers. For me the differential cost of going with a well known manufacturer is low enough that I do that and don’t worry about this.

Are there links to any of that, that I can read up on?

[jabbr]

It’s interesting that in this so called “objective” thread we are hearing many speculations and philosophical arguments — remember that real world equipment has to pass compliance testing so perhaps we should consider the specifications. How much ground bounce does a 10 Gbe eye pattern allow? 100 Gbe? He he not much 😉 I know that certain manufacturers throw this out as a speculation but seriously given the known compliance testing, let’s see the data! 
 

From my perspective this equipment is all very cost effective, so like my XLR cable for which I go with well known manufacturers such as Neutrik connectors and brand name cable (pick) I also use brand name SFP(+) modules  and I consider Finisar/Avago to be brand names. 

[jabbr]

9 minutes ago, The Computer Audiophile said:

Are there links to any of that, that I can read up on?

Do you mean the Tektronix stressed eye pattern testing primer — I’ve posted the link

 

SFP(+) I/o : https://en.m.wikipedia.org/wiki/Small_form-factor_pluggable_transceiver

 

lines 12/13 are receive +|- and 17/18 are transmit +|-

[ray-dude]

I remember looking at this page when I first heard differences.  On pins 15 and 16, the laser is actually turning on and off (even if fed with a differential signal).  Unless the power is getting dumped into an offsetting load when laser is off, that will create a correlated pulsing load on the power lines, no? My (very) naive assumption was that the power driver circuit biases the laser to the critical voltage, and the differential signal switches it on and off.

 

All that being said, I'm obviously not familiar with how the driver circuits in these modules are typically implemented nor the compliance requirements for noise on ground plane or power plane, so above is an honest question, not a belligerent one (I'm honestly delighted to toss wrong hypotheses on the "Oh Well" pile...life's too short to chase dead ends)

 

[jabbr]

17 hours ago, ray-dude said:

I remember looking at this page when I first heard differences.  On pins 15 and 16, the laser is actually turning on and off (even if fed with a differential signal).  

 

Why do you assume that in all or any implementations the laser actually turns on and off? https://www.optcore.net/transceiver-laser-types/ the more modern lasers are DML/DFB and EML https://community.fs.com/blog/silicon-photonics-and-lasers-in-100g-optical-transceivers.html

 

In any case if there is an irregularity on the power/ground planes, this would be evident in the eye pattern, no?

[kennyb123]

 

5 hours ago, jabbr said:

 

Why do you assume that in all or any implementations the laser actually turns on and off?

 

Whether it's on and off there is electrical activity that brings about whatever the laser is doing.  

 

5 hours ago, jabbr said:

 

https://www.optcore.net/transceiver-laser-types/ the more modern lasers are DML/DFB and EML https://community.fs.com/blog/silicon-photonics-and-lasers-in-100g-optical-transceivers.html

 

In any case if there is an irregularity on the power/ground planes, this would be evident in the eye pattern, no?

 

I'm really trying to understand what lasers are doing if not on and off.  Any chance you could explain what is actually occurring instead of providing links that don't clearly explain this either?

 

The article from FS that you provided mentions "injecting current".  The link mentions this too.  If current is being injected, why is it wrong to assume that this could result in a "pulsing load on the power lines"?

 

[jabbr]

10 hours ago, kennyb123 said:

 

 

Whether it's on and off there is electrical activity that brings about whatever the laser is doing.  
 

 

of course that’s silicon photonics.

Quote

 

 

I'm really trying to understand what lasers are doing if not on and off.  Any chance you could explain what is actually occurring instead of providing links that don't clearly explain this either?

 

To simplify, the silicon modulates the light after it leaves the laser. It might disperse the light, decreasing its intensity, or absorb the light. 
 

Think about this like a gate on a transistor, a small gate current switches a much larger current between the collector and emitter. 

Quote

 

The article from FS that you provided mentions "injecting current".  The link mentions this too.  If current is being injected, why is it wrong to assume that this could result in a "pulsing load on the power lines"?

 

Anything could happen in theory. The question is whether there are fluctuations in the power and ground planes. The stressed eye pattern testing should pick this up, no?  People make all sorts of assumptions about what is going on in an electrical circuit. For example there is constant current switching, so an increase in current draw by one part in a circuit is met with a decrease in a complimentary part. 
 

Bottom line is whether there is noise on the data lines (electrical). That is what is measured. Consider a 60 femtosecond jitter Budget for a clock. You can’t very well have a pulsing power supply and meet that, can you? The stressed eye pattern testing is where the rubber meets the road.

[ray-dude]

The presumption I've been working with is that signal integrity is effectively a given with these devices, which is why I've been paying more attention to possible impact on power/ground planes (other candidate is radiated RF, but that seems a stretch as well)  I most certainly was not expecting to hear differences between SFPs (pretty stunning actually, for all the reasons you cite, as is anything on the network having an impact on SQ)  

 

I should give the caveat that my only experience has been wth 1 gigabit SFPs.  Do you (or others) happen to know the type of laser typically used in these devices?  At a $20-30 price target, I was presuming these low cost modules wouldn't be using more advanced photonics (I'm a geezer...it's been almost 30 years since I've done work on DFB lasers, and back then, we were delighted just to get them to work).  

 

I spent a little bit of time looking for a published reference design for a 1G SFP module, but didn't (yet) find anything current.  What I did find had a simple driver on the LED laser module, with the differential signal switching the laser on and off:

 

https://www.analog.com/media/en/reference-design-documentation/reference-designs/5693022520349015544867851905SFP_RDK_pra.pdf

 

If someone has a pointer to a more modern published reference design for a 1G SFP module (with presumably more advanced photonics than an LED laser), it would be very helpful to understanding what is going on inside of these beasts.

 

If 10G modules have more advanced photonics switching, that could be an argument for making the investment in 10G network gear (I have not had access to this sort of kit to test, although it is definitely on the list). 

 

 

[jabbr]

5 hours ago, ray-dude said:

I should give the caveat that my only experience has been wth 1 gigabit SFPs.  Do you (or others) happen to know the type of laser typically used in these devices?  At a $20-30 price target, I was presuming these low cost modules wouldn't be using more advanced photonics (I'm a geezer...it's been almost 30 years since I've done work on DFB lasers, and back then, we were delighted just to get them to work).  

This has a list of SFP/SFP(+) and QSFP(+) modules with the type of laser in https://www.finisar.com/sites/default/files/resources/finisar_optical_transceiver_product_guide_3_2015_web.pdf

As you can see the SFP multimode modules tend to be VCSEL and the single mode modules tend to be DFB ... as of 2015 

 

[ray-dude]

47 minutes ago, jabbr said:

This has a list of SFP/SFP(+) and QSFP(+) modules with the type of laser in https://www.finisar.com/sites/default/files/resources/finisar_optical_transceiver_product_guide_3_2015_web.pdf

As you can see the SFP multimode modules tend to be VCSEL and the single mode modules tend to be DFB ... as of 2015 

 

 

Fantastic, thank you!  My (currently) preferred FTLF1324P2BTL is a Fabry-Perot laser, which seems to be operated like a laser diode? (direct modulation, limited to lower bandwidth channels)  Alas, I'm not finding more modern reference designs (yet), but the older ones I've found all have Vcc direct to the laser diode or driver in the block diagrams.

 

If the 10G modules have a constant power load to go with their tighter operating tolerances, that certainly makes them even more interesting.

 

 

[kennyb123]

 

4 hours ago, jabbr said:

This has a list of SFP/SFP(+) and QSFP(+) modules with the type of laser in https://www.finisar.com/sites/default/files/resources/finisar_optical_transceiver_product_guide_3_2015_web.pdf

As you can see the SFP multimode modules tend to be VCSEL and the single mode modules tend to be DFB ... as of 2015 

 

Thanks!  
 

The text at the end is repeated in their data sheets. “Finisar’s transceivers feature a microprocessor and diagnostics interface that provides performance information on the data link. Users can remotely monitor—in real-time—received optical power, transmitted optical power, laser bias current, transceiver input voltage and transceiver temperature of any transceiver in the network.”

 

The only way I’ve found to see this is when I use one of their SFPs in one of UniFi switches.  Not sure why current is 0, but it could be that it requires UniFi products at both ends.  

[JohnSwenson]

I think I might be able to explain a little bit about what is going on here with modulating lasers and the "current injection". Solid state lasers have two primary modes they operate in, coherent and non-coherent. (LED mode). At lower currents they are expensive LEDs. At currents above a certain threshold they are a laser. (the "threshold of course is not a "point", it is a small range, inside that range weird things happen).

 

The modulation takes advantage of this, it is not turning the laser "on and off", zero current and full current. The "off" state is a current a little below the threshold and the "on" state is a little above. Thus the light beam is not turning completely on and off, it is changing from non-coherent to coherent. After the laser is some form of optical system that lets a lot of light through if it is coherent and not much if it is non-coherent. Don't ask me WHAT that optical component is, I have no idea.

 

Of course the actual current value of the threshold varies with temperature, which means you need some sort of servo system tracking the threshold. The simple matter of engineering.

 

So the laser system is being modulated by a fairly small current change. Its not zero, but the change is a lot less than the full current needed to make the laser lase.

 

This temperature dependence is one of the reasons optical protocols are such that the average amount of time in "on" and "off" are fairly constant so the temperature is not flailing around with the data.

 

John S.

[jabbr]

We have focused extensively on the properties of different SFP, SFP(+) and SFP28 modules and lasers etc but that is only part of the overall story.

 

As of introduction of 10Gbe 20 years ago, end to end compliance testing was recognized as essential to prevent, among other things, jitter from propagating down a network. The "stressed eye pattern" test was introduced and is implemented by Tektronix and Keysight measurement systems, among others.

 

Switches and NICs compliant with 10Gbe+ must pass these tests.

 

A 1Gbe switch which was designed to pass this stressed eye pattern test would similarly not transmit jitter across its link. I have not seen a 1Gbe switch vendor provide such testing (its not required), but that said, I have not seen evidence that *any* 1Gbe switch of any design, perform better in this regard than the cheapest 10Gbe switch which is also compliant to this specification.

[R1200CL]

I’m wondering if Digital diagnostics monitoring of SFP can help us in any way to ensure jitter free transmissions ? 

http://www.fiber-optic-tutorial.com/do-you-know-digital-diagnostic-monitoring.html
 

I did a search “digital diagnostics monitoring interface for sfp optical transceivers”

 

I also saw this article in understanding SPF+ transceiver testing. I think I stay away from that topic 😀

 

God explanation of eye pattern.

[R1200CL]

Does anyone knows difference between Finisar  wide and narrow extraction bail ?

This is the 3 last letter in product code. 

What does it mean ?

(@plissken @jabbr)

[plissken]

5 hours ago, R1200CL said:

Does anyone knows difference between Finisar  wide and narrow extraction bail ?

This is the 3 last letter in product code. 

What does it mean ?

(@plissken @jabbr)

 

It's the swing arm you use to pull the module out with. They are color coded for the wavelength they run at.

 

Check page 7/8 for the difference in size: https://optical.communications.ii-vi.com/sites/default/files/downloads/finisar_ftlf1519p1xnl_2_gbs_rohs_compliant_long-wavelength_pluggable_sfp_transceiver_product_specification.pdf

 

[jabbr]

https://www.keysight.com/us/en/assets/7018-06220/data-sheets/5992-3083.pdf

 

This shows you the requirements for the stress tests.

[The Computer Audiophile]

I started a topic on buffers and streaming audio to endpoints that I think will be a good discussion with many of the folks in this thread. I can't say where the thread is going, but I think this is an interesting topic.

 

Here's a link.

 

 

[NoNaim]

Anyone tried the latest FMC by Melco... The ADOT MC Fibre Kit? 

 

Seems like it could be an interesting.... second review here.

 

I was going to upgrade my StarTech Gigabit (ET91000SFP2) with an opticalModule but this maybe an alternative option!