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.
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.
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.