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Optical Networking & SFPs


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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.  

 

My basis for this is that in previous experiments with coax SPDIF signals to my Chord DAVE, that particular DAC seemed to have a strong susceptibility to parasitic signals in the 2-3GHz band.  Copious ferrites on the dual coax connectors to the DAC had a surprising benefit to SQ.  IIRC Rob Watts (designer of the DAC) attributed the impact to noise floor modulation in the DAC (I would have look up some old notes to confirm).  It is easy to replicate this by having a WiFi basestation configured for 2.4Ghz and bringing it close to this DAC, or having  music server on WiFi that is close to the DAC.  

 

All above is hypothesis or experiential.

 

 

ATT Fiber -> EdgeRouter X SFP -> Taiko Audio Extreme -> Vinnie Rossi L2i-SE w/ Level 2 DAC -> Voxativ 9.87 speakers w/ 4D drivers

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Paul, I have a couple different components that are SFP capable:

 

The  StarTech PEX1000SFP2 PCIe optical Network card in my Taiko Audio Extreme

(2x) Ubiquiti EdgeRouter X SFP's

Sonore opticalModule

 

I recently sold my Uptone Audio EtherREGEN, but I was experimenting with the SFP port on the ER in my Taiko Audio SGM Extreme review (part 4 for those subjective findings)

 

I currently have some TP Link multi-mode SFPs connecting my two EdgeRouter X SFPs.  ERX1 is my router/gateway and connected to my ATT fiber ONT.  It has a audio subnet that I connect by copper to my opticalModule. I run fiber from the opticalModule to my Extreme.  I've run both the Planet Tech SFPs and Finisar SFPs on my audio net.

 

I have the rest of my home network (including WiFi) on ERX2, galvanically isolated from ERX1.  ERX2 is configured as a simple switch, but I do have a couple VLANs for my WiFI (guest network, IoT network, etc)

 

With the Planet Tech SFPs, I preferred copper NIC on the Extreme (fiber to the opticalModule+PlanetTech to copper to the Extreme).  With the Finisars, I'm back to preferring the optical NIC, esp. now that I have my audio network isolated from my home network, but it isn't a clear preference.  

 

FWIW, subjectively, reduced network traffic to the my music server NIC does have an impact.  This is contributing to my hypothesis of laser switching-induced electrical noise being the mechanism in play.

 

All of the above is with streaming TIDAL content to Roon to HQP to DAC.

 

The network part of my system is very much in flux right now, because of what I've been hearing.  There are a couple of permutations I'd like to try to see if it helps develop a hypothesis for a what a core mechanism could be.

 

Here is my wish list for things to try if I had proper equipment in quarantine with me:

 

* Probe power rails on the SFP modules and look at spectrum of noise generated on same by network traffic.  If that spectrum is the same, my hypothesis can filed in the "oh well" cabinet.  If different, then the question shifts to whether that difference is audible (and if so, how).

 

* Build an extender and wire to SFP cage outside of NIC/FMC/Routers/etc.  Rig so that I can swap between powering SFP externally or via the cage leads. If there is variability with different class power supplies, that would generally support the laser-switching-noise hypothesis. If not, back to the "oh well" cabinet we go.

 

* Get access to OEM data sheets for SFP modules and find the lowest power consuming unit available...give it a listen, get another subjective data point

 

@Superdad without asking you to divulge anything proprietary, did you or John look into any of these things when you were designing and prototyping the ER? (and if that is poking at things that you consider proprietary, my apologies in advance)

 

 

ATT Fiber -> EdgeRouter X SFP -> Taiko Audio Extreme -> Vinnie Rossi L2i-SE w/ Level 2 DAC -> Voxativ 9.87 speakers w/ 4D drivers

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6 minutes ago, Superdad said:

P.S. By the way, @plissken's first name is Mark not Paul. (And over at ASR he is Jinjuku; various other names elsewhere...)

 

Mark, my apologies!  I have no idea how "Paul" settled into my brain, but certainly no disrespect was intended.  Thank you Alex for the gentle correction (as well as the info that you shared)

ATT Fiber -> EdgeRouter X SFP -> Taiko Audio Extreme -> Vinnie Rossi L2i-SE w/ Level 2 DAC -> Voxativ 9.87 speakers w/ 4D drivers

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1 hour ago, cat6man said:

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 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)

ATT Fiber -> EdgeRouter X SFP -> Taiko Audio Extreme -> Vinnie Rossi L2i-SE w/ Level 2 DAC -> Voxativ 9.87 speakers w/ 4D drivers

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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)

 

ATT Fiber -> EdgeRouter X SFP -> Taiko Audio Extreme -> Vinnie Rossi L2i-SE w/ Level 2 DAC -> Voxativ 9.87 speakers w/ 4D drivers

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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). 

 

 

ATT Fiber -> EdgeRouter X SFP -> Taiko Audio Extreme -> Vinnie Rossi L2i-SE w/ Level 2 DAC -> Voxativ 9.87 speakers w/ 4D drivers

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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.

 

 

ATT Fiber -> EdgeRouter X SFP -> Taiko Audio Extreme -> Vinnie Rossi L2i-SE w/ Level 2 DAC -> Voxativ 9.87 speakers w/ 4D drivers

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