There is still more to do. This was a prototype board, I found six hardware issues which took a number of "red wires" on the prototype, I have to redo the board to incorporate these. One of the big problems was the startup of the XMOS processor, it turns out you have to have a specific sequence of voltages coming up and reset pulse. Unfortunately these requirements are not listed in the spec sheet, they are in a separate app note. Unfortunately I missed that when I designed the board. It's very easy to design a circuit to implement the requirements so it's not a problem.
One thing I'm changing is the "aux interface". I originally had a hardware interface using the FPGA and I was writing a protocol to send stuff over it. But Once I got working with the XMOS processors I'm throwing that out and using their Xlink interface which is specifically design to talk between chips. The interface and protocols are already built into all their processors. This makes it really easy to implement add on boards to do all kinds of things. One neat aspect of this is the remote processors can booth themselves over the Xlink from the "master" so you don't even have a way to program the remotes, they just program themselves over the link.
So there is some work still that I have to do, but all the basic functionality is in this prototype so Doc and company can start working on the tube output stage and floorplanning the top plate etc.
As to buzz words, the filter is intermediate phase. A linear phase filter has equal amounts of pre and post ringing but fairly short amount of time in each. The minimum phase has no pre ringing, but in order to get the same over all similar functionality it has to have a much longer amount of post ringing. The intermediate phase splits the difference and has a small amount of pre ringing and post ringing that is a little bit longer than linear phase but much less than minimum phase. This is also deliberately a short filter, that means the amount of time it spends ringing is a very short amount of time compared to other filters. The tradeoff is that the ultimate amount of alias reduction is only about 80db, whereas those in modern DAC chips are going to 130 db or more. This is the tradeoff I mentioned, by not aiming for nearly as much ultimate image rejection I get a filter that sounds much better.
There is no dither involved, it simply i not needed in the DAC.
Lets see, other buzz words, it has ground plane isolation with special high quality isolators on the signals. This does a very good job minimizing the ground plane noise from the digital chips from getting into the analog side of things. This gives a marvelous very "black" background. It also significantly cuts down on the outside world affects such as source component interaction and cable affects. It doesn't completely eliminate them, but it does significantly reduce these interactions. For example I have been listening to all three inputs (S/PDIF coax and TOSLINK and USB) and cannot tell hardly any difference between them. There are some very slight differences, but they are very small and hard to determine. There is only a very tiny difference in sound with different source components (different computers, OS etc, at least the ones I have on hand) I've spent a lot of work trying to decrease these sensitivities and I think I've done a pretty good job.
John S.
