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If there’s one particular weakness of the Raspberry PI for audiophile use, it’s definitely its poor interface implementation. Not that it’s really bad but even the PI 2 Revision B has to share Ethernet and USB bandwith due to hardware limitations. Many enthuasiasts have reported annoying pops and clicks when listening to music over USB connections.
Personally I’ve had a few glitches once in a while.
Adding to these problems are compatibility issues with certain USB DACs. Yes, I know most of them should be working out of the box without any drivers or configuration changes since the PI’s operating systems are all Linux-based. But some DACs such as the Chord Hugo or even the fantastic Chord Mojo just simply cannot be talked into co-operating with RuneAudio.
Following Hans Beekhuyzen’s rather enthusiastic YoutTube review of the Hifiberry Digi+ daughter board for the Raspberry PI, I thought why not give that a try?
What is the Hifiberry Digi+?
The Hifiberry Digi+ is an add-on board for your Raspberry PI (works on A/B as well as the newer Revision 2 models, haven’t tested the latest Rev 3 yet). It supposedly provides ultra-clean good old fashioned SP/DIF outputs via coaxial and optical.
What’s really interesting is that they take the underlying I2S signal from the GPIO header, feed that into a Wolfson WM8804 SP/DIF receiver chip that has an external on-board Master Clock and let you tap into a high quality SP/DIF signal. Most DACs on the market support either coaxial or optical – most of them even both standards.
If you opt for the output isolation transformer – which I highly recommend, you can get a completely galvanic isolated output for your DAC. This means no electrical noise will pollute the circuitry of your DAC. All you need then is a standard digital cable. No more USB drivers, no hassles, Nada.
One drawback however is its 192kHz limitation. So if you want to play back DSD or anything above 192kHz, you are going to need USB. As for me, I’m not so sure whether I can actually hear the difference to be honest…
Setting the whole thing up in RuneAudio was a breeze. You simply need to edit /boot/config.txt with a text editor of your choice (I use Nano through my SSH session which is built right into the PI’s Linux distribution RuneAudio but it’s a personal choice).
Your config.txt sections should look similar to the one below – note to remove the pound symbol in front of the line to enable the appropriate kernel module for your daughter board:
# Uncomment one of these lines to enable an audio interface
Finally, save everything and reboot. But wait! When I’m saying reboot, don’t just do a software reboot which is what I did and it just didn’t work. Make sure youu actually physically power down (aka “turn off”) your PI completely. After a cold boot, I was finally able to configure the card in the RuneAudio MPD configuration menu.
The actual steps may of course be different depending on whether you use Volumio, RuneAudio or whatever distribution you favor. The guys from Hifiberry have published a pretty nice guide on their website.
Initial Testing and Sound Quality
As always, it could be my imagination but I think the sound quality is much better than over USB. Also the pops and clicks are completely gone! Not surprising though since the USB bus is now no longer busy serving the XMOS asynchronously while at the same time sucking data via WIFI from the NAS. I also use a DIY 5V power supply for my Raspberry PI to give it clean DC to begin with. This seems to be pretty crucial for best performance because the whole clock generation magic relies on clean power regulation. So I’d not use any sort of wallwart switchmode PSU. To be frank, I don’t think the XMOS implementation in the SMSL M8 is that great. With a “better” USB to SP/DIF converter (an external one), the results could be in favor of USB or at least similar in performance. But for the 35 Euros I paid, I cannot complain at all. And you are going to save a lot of money in expensive USB gadgets that most likely don’t perform any better. If you are not listening to anything above 192kHz, I’d say go with the Hifiberry Digi+!
Where to buy
The transformer version is going to cost you less than $10 more so I’d very highly recommend you purchase the upgraded version.
Updates as of December 20, 2016
The HifiBerry foundation has just released the Digi+ Pro board with a low-noise regulator and dual-domain clock generation (cleans up the jitter depending on the sample rate of the source material). I’d definitely get this particular board over the “standard” Digi+. It cost just a few dollars more. For the price of a latte, you can potentially upgrade your sound quality and giving your DAC an easier time to lock to an even cleaner SP/DIF signal 🙂