[audioengr]

Background:
I recently got the 75 ohm termination plug-in for my GHz scope.  Very expensive BTW.  This new tool allowed me to see what I could never see before: exactly what is happening at the S/PDIF termination and the jitter there.  This revealed that my current circuit, which is typical for transformer-coupled galvanically isolated S/PDIF, was holding-back the performance of both the Synchro-Mesh and the Off-Ramp 5.  Bummer   Not that the jitter was high, but it should be a LOT lower that what I am measuring.

Investigation:
In order to determine the cause of this, which is incidently very difficult to observe, I embarked on a multi-week prototyping of different pulse transformers and associated circuits with several Synchro-Meshes.  I got help from the transformer designer too.  Yesterday I had a breakthrough.  I realized that I don't need to change to a custom transformer as I had anticipated.  Some circuits changes and tuning will be sufficient.

Conclusions:

1) I discovered that the impedance of the S/PDIF source is critical to getting low jitter from S/PDIF coax.  Even 1 ohm off makes a big difference.  This is why most manufacturers cannot get it even close and why so many users believe that S/PDIF coax is a broken standard.  Most other products that I modded in the past did not even have 75 ohm output impedance, much less exactly 75 ohms.
2) Pulse transformers can be inconsistent in characteristics.  They are wound with tape etc, so the spacings vary from one to the next.  These differences result in ratio and impedance differences.
3) I believe based on anecdotal evidence that break-in of the device changes the impedance, so tuning it when the device is not broken-in might require a re-tuning later to optimize for jitter.

I am in the final stages of testing of the mod now.  I still need to verify the repeatability of the mod and the effect of break-in.  I will be posting some before and after jitter plots soon.

The impact for me and my customers is an inconvenience for both, but I believe that this one will be worth it.  I expect this mod to cost about $200.00 for either product.  Other companies would put it in the next product, but I believe this is so important that I have to get it out ASAP.  This is another reason to stick with Empirical Audio.

Exciting development.  More to come.

Steve N.

[andrewd01]

Great news Steve! Looking forward to reading more.

can the mod be applied to OR4?

[acg]

Aww Steve,  I am still running in my OR5.

Is this the mod to which that you have eluded previously, or are there others?

Anthony

[liddown]

I would assume that this would not apply to those of use using OR5 with I2S....?

[path73]

Steve, does this mod replace the need to connect some BNC-RCA adapter on the unused output port of SM?
/path

[path73]

Hard to imagine that Steve's really good stuff keeps on getting even better 

[audioengr]

Great news Steve! Looking forward to reading more.

can the mod be applied to OR4?

Yes, at least the later baseboard.

[audioengr]

Aww Steve,  I am still running in my OR5.

Is this the mod to which that you have eluded previously, or are there others?

Anthony

No, that was a tweak that users can do using a stub, which was a BNC to RCA or similar connector.

This mostly negates the need for tweaks, although the tweak may imrpove things for some cables.

Steve N.

[audioengr]

I would assume that this would not apply to those of use using OR5 with I2S....?

Correct.  This is only for S/PDIF coax outputs.

Steve N.

[audioengr]

Steve, does this mod replace the need to connect some BNC-RCA adapter on the unused output port of SM?
/path

Yes.

[JPS]

Hi Steve,

How long will be needed to have the mod applied?

Regards,

Jean-Pierre

[audioengr]

Hi Steve,

How long will be needed to have the mod applied?

Regards,

Jean-Pierre

I can do it in an hour.  I should be able to fit mods in every 2-3 days.  If I get 30 at once, its obviously going to delay things...

I have a customer with 2 SMs, so maybe he will be the first to compare an unmodded to a modded in his own system.  I'll ask.

Steve N.

[werd]

No, that was a tweak that users can do using a stub, which was a BNC to RCA or similar connector.

This mostly negates the need for tweaks, although the tweak may imrpove things for some cables.

Steve N.

Hi

Not sure what this means can you elaborate or point to a thread... Thanks much.

[audioengr]

Hi

Not sure what this means can you elaborate or point to a thread... Thanks much.

See:

http://www.audiocircle.com/index.php?topic=106328.120

[audioengr]

As promised, here are the before and after mods jitter plots of Synchro-Mesh (both use standard wall-wart):

Before Mods - the scale is 106psec/div, so most of the jitter is in the 150 psec range


After Mods  - the scale is 57 psec/div so most of the jitter is in the 60-70 psec range


Here is the Sonos Jitter plot:


As you can see, the worst case jitter is 10nsec.

Here is a zoom to one of the Sonos peaks


the peaks each have around 150-170psec of jitter and then there are other smaller peaks as well, but less samples.

Steve N.

[Watthead]

I'm not familiar with these graphs, but can we conclude that the mod version produces about half less jitter?

Simon

[joeling39]

Oh dear, I have 2 SM on the way & this happens 

[audioengr]

I'm not familiar with these graphs, but can we conclude that the mod version produces about half less jitter?

Simon

That is correct 50% reduction at least.

Steve N.

[audioengr]

Oh dear, I have 2 SM on the way & this happens 

Sorry, just bad timing.   I didn't know at the time.   If you wanted to return one and pay shipping costs both ways, I'll do the mod to it for free if you can compare the modded and unmodded and report back here.

Thanks,
Steve N.

[DaveBSC]

Very impressive work, Steve. For what it's worth, Stereophile measured about 70psec from the Simaudio Moon 650D, an $8K CD player/DAC, so that's good company to be in. Most of the average USB > S/Pdif converters are in the 300+ range.