I have recently purchased the HiFiBerry Digi add-on board (with S/PDIF and Toslink outputs). The hardest part of the set up was soldering the P5 connector on the Pi. It's not hard but my soldering skills are not advanced. Still, it works so I must have managed it OK!
I had previously been using the Pi to transport digital audio (from a powered USB-connected hard drive) over async USB and was beset with the same audio glitches reported by many due to the Pi's well-documented issues with the shared ethernet/USB controller. Lots of tweaking of the Raspbian OS and a powered hub brought many improvements, but there was still the occasional glitch, most particularly on higher bitrate tracks (24/88.1, 24/96, etc.) So, I bought a Cubieboard (A10), installed Cubian, MPD, lighttpd (for album art) and got it set up exactly the same as my configuration on the Pi. It worked flawlessly over async USB, although there seemed to be a harder "edge" to the music from the Cubie than was the case with the Pi. I know, I know, it's just 0s and 1s isn't it? They ought to sound exactly the same, but whether it was placebo effect or not, I definitely prefer the output transported from the Pi than the Cubie. Go figure.
I recently bought a SqueezeBox Duet and pressed the Pi back into service as a SqueezeBox (LMS) Server via the excellent SqueezePlug. However, I hadn't realised that the Duet only supports 24/48 max resolution with anything above that being downsampled via SoX. It turns out the Pi just didn't have enough grunt to do the on-the-fly downsampling so I got no output for anything above 24/48. So, that appeared to be a waste of £120! Still, the Duet remote is nice! I had the choice of converting all my higher bitrate music down to 24/48 or going back to the Cubie.
However, I've always had a soft-spot for the Pi and when I saw the HiFiBerry Digi for €30 I thought I'd give it a go. After installing the Digi, I decided to see whether the Pi was capable of running both LMS and SqueezeLite player. I'm happy to say that it runs both perfectly. SqueezeLite uses ALSA to output to the external sound card and I've got to say, it sounds fantastic transporting via S/PDIF into my LampizatOr Level 3 tube DAC. I quite like the SqueezeBox infrastructure and at least I can still use the Duet's remote controller for accessing my music, etc. The only time the Pi seems to get hammered is when I browse my music via the music directory when it will shoot up to 90% CPU utilisation in TOP. However, it never seems to miss a beat. During normal operation, LMS consumes around 5% CPU and SqueezeLite maybe between 3% and 8% depending on the bitrate of the track, so it's not exactly stressing the Pi out. Anything up to 24/96 plays perfectly (and I disabled the default SoX downsampling in SqueezePlug so that it sends a bitperfect stream to the DAC). However, anything above 96 just results in static. I thought this might be a limitation in the HiFiBerry Digi but if I boot to my MPD installation, anything up to 24/192 plays perfectly over the S/PDIF connection so it's clearly an issue with SqueezePlug. Since I only have a few sample files that are over 24/96, it's not really an issue.
Either way, whether as a SqueezeBox Server/Player or with MPD, the Pi is performing unbelievably well as a transport over S/PDIF using the HiFiBerry Digi. I am currently working on a project to put it in its own dedicated "HiFi" (PiFi?) case with external S/PDIF and Toslink connection and am working with a friend to build a separate linear power supply.
Anyway, apologies for the long first post. I've included a couple of pics of the Pi with the HiFiBerry Digi board:
