I might have stumbled upon a very cost-effective way to get pretty good sound quality out of a Pi. Long story short is that I use a Mojo2 DAC and tied hooking up my RockPi 4b (I know, not a Raspberry Pi 4 but pretty much the same except with more CPI grunt) using a decent Audioquest USB cable and USB isolator. The initial result, with the Roon volume setting defaulted to “device” was dreadful: lots of hiss. Changing the volume setting to “DSP” pretty much sorted the problem but it did make me think.
I recalled a post by Chord Electronics’ Rob Watts (the DAC designer) explaining how the Mojo deals with SPDIF and USB input. With regard to SPDIF he said that: “The FPGA uses a digital phase lock loop (DPLL) and a tiny buffer. This re-clocks the data and eliminates the incoming jitter from the source. This system took 6 years to perfect, and means that the sound quality defects from source jitter is eliminated. How do I know that? Measurements - 2 uS of jitter has no affect whatsoever on measurements (and I can resolve noise floor at -180dB with my APX555) and sound quality tests against RAM buffer systems revealed no significant difference. You can (almost) use a piece of damp string and the source jitter will be eliminated.”
Regarding noise, he also said: “Note that optical connection does not have any of these problems, and is my preferred connection.” Of course, out of the box, Pi’s don’t have Toslink so unless you have a HiFiBerry Digi board or the like, there’s no way Jose … but then again, maybe not.
I happened to have a little box I bought years ago with HDMI input and both an HDMI and Toslink output, which I used to pipe video sound from a media server into a small Bose system I had back in the day. So I tried it with my RockPi and, badabing-badaboom, it works perfectly: no noise whatsoever.
I had a look on Amazon to see if the Portta Digital Audio Converter can still be found and sure enough, the latest version sells for €18.99 here in France which seems unbelievably good value.
Bottom line, for my particular application, with the Chord DAC eliminating any jitter and the Toslink eliminating any noise, I seem to have a pretty decent Roon endpoint.
Can you tell me if you see any holes in this? Has anyone else tried this HDMI/Toslink splitter fix?
I have done the same thing using my Mojo2 and an HDMI audio extractor. It works fine, but I normally use WIFI from my Dell to RPi4 and USB to Mojo2. This is my normal setup when away from home. I don’t use USB from the Dell to Mojo2 because it is noisy. Something to do with the Dell USB.
EDIT: Thanks Crystal for your ever thoughtful and useful post. Very helpfull.
EDIT: The Rpi and Mojo2 are not the issue. It’s the Dell USB. It’s a non-issue for me because I use Ethernet at home and WIFI away from home.
Makes me wonder why you would pay nearly £500 for a dac that doesn’t work over USB. I’ve had many dacs all of which have never suffered any noise issues over USB from any source. Sounds like defect by design to me. It’s not hard to have proper USB input handling, even my DragonFly Black doesn’t have these issues and that was cheap as chips.
I have the Mojo, and it works perfectly well using USB. Indeed, both Mojo and Mojo2 jitter measuremens for USB are excellent. The Mojo2 improves on the original.
Nope as I dont get any hiss from any pis in the 7 years of using them to lots of different dacs. I had a Mojo one it was supposed to be used as a mobile solution but picked up GPRS signals from the phone which you could hear. Sorry but the USB input isnt good enough on them if its picking up noise. I have used lots of PC’s too and not once has any added what you describe when using them directly plugged in.
Noisy USB can be a problem. I guess it has something to do with it not having been explicitly designed for audio applications. Hi-Fi manufacturers go to a lot of trouble to galvanically isolate their outputs, e.g, the Pi-based Holo Audio Red which sells for €999!
Using a Raspberry Pi as Roon Bridge is not problematic. Indeed, it is the DAC, not the transport, that should filter out noise sources. Once again, the Mojo2 does an impressive job here.
I’ve eliminated the nasty hiss on the USB connection by putting the Pi on a separate power supply than the Mojo2 and using a shorter USB cable together with the isolator. So, maybe it was a power supply problem or maybe the 0.75m Audioquest USB cable was picking up interference somewhere along its route.