Hi,
> Since the ML401 board that uses the LM4550B codec exhibits significant > noise on the outputs as well (at least on my board), I suspect the codec > chip may have something to do with that. I have just requested samples > of WM9707, we'll see if it improves things (it should be an easy PCB > rework btw). > While you try to switch a different codec, we could take considerations on: 1) What real differences between ML401 board and our rc2? Including layout/routing, etc. We currently don't have precisely measurement system about acoustic mechanism to compare them though. For example, those AC97 wires came from fpga. Probably they possibly don't suffer from digital circuit radiation already when GUI running. 2) Did our part pick up right exactly can eliminate/cut off noises induced from digital circuits effectively? for example, pick different L12 and L13 FB to try to give better noise reductions as well. 3) I tested a very extremely lower voltage 3.829V, this extreme test conflicts with the recommended operating supply ratings for LM4550B, then I can surely hear very loud noise.(That's because of bias inside codec starts to work malfunctioned.) See. results of Sch. C : http://en.qi-hardware.com/wiki/Protection_of_Reversed_Polarity_on_DC_plug-in#Experimental_Results Since this cannot directly approve that the noise caused by this, we still know our codec indeed designed to work at a lower supply boundary. Meanwhile it's good that you requested samples for further experiments then. > BTW, Adam, do you have other AC97 codec chips you could recommend? > > I currently have no idea on this. but select another part, you may >From thinkings above, the WM9707 has wider AVDD supply range(3.3 ~ 5.0V) which is better than LM4550B (4.2 ~ 5.5V). This of course we exactly utilize U23 TPS76301's 4.3V to provide a nearly central boundary of range. This might you already thought about it. With regards to other specifications on audio performance I think most of them are quite the same. A SPDIF out surely is a good extra-feature but it needs another transformer circuit, we don't need it or just pull connection out. -adam
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