I'm late to the RasPi party, got myself a board beginning of this year to finally follow the crowd. It makes a nice little media box, but with bias on video, it is missing a digital audio output for the music lovers. People have to add clumsy USB sound hardware or get the sound from HDMI, but the RasPi has preparation for native I²S, at least with rev. B. So I've developed an as-small-as-possible addon board for TOSLINK digital (optical) audio output. I did this just for the fun of adding something I felt missing, for friends and the like, no commercial plans. It is ready since a while, but now I found the time to make some pictures and write about it.
While waiting for the PCBs and parts, I learned about the great work of other people. Namely Florian Meier's work on a kernel Alsa driver to support this path. Later I found the announcement of the Hifiberry Digi, for which its creator Daniel has selected the same Wolfson chip than I did (well, not too many choices in that field). So, my hardware is compatible to his, can use the same driver, which is by now already included in the standard kernel. The software part of this project collapsed to almost zero effort, I was prepared for more.
After this history, let's get to the hardware. The neat thing is that it doesn't add extra height and fits in the cases I've tried, makes e.g. a nice compact audio client from the RasPi. To some extent it is still possible to use the expansion port. The tallest component is the transmitter, it reaches the same height as the USB jack of the RasPi.
See for yourself, this is the bare board:
This is how it attaches to the RasPi, firmly soldered into the normally empty P5 header:
This is the "sandwich", the board is soldered with an 8-pin header into P5, the I²S interface of the rev. B boards. It also grabs 2 pins (I²C) from the normal extension port P1:
(My PCB could be more thin, but this wasn't available for a standard prototype run.)
There's another style of mounting, for "developers", plugged and removable. The optical jack instead goes on the underside, giving it even lower profile. (However, I think this is not rigid enough to put in a case.)
Inside the popular Teko case:
Inside the Multicomp case:
The white cable is the optical connection, I guess you get the idea.
The board is minimalistic, but still has something to offer for the audiophiles. It has a 10 ppm precision crystal, which is governing the audio timing. The RasPi audio is slave to this clock. There is extra filtering for the PLL clock supply. Less related, attaching audio to the I²S is as tight as you can get to the CPU core (compared to an extra trip over USB to some external hardware with out-of-control clocking implementation). The Wolfson chip can do 24 bits resolution and higher sampling rates, like the I²S of the Broadcom chip. But I haven't done much testing yet.
How to proceed besides showing off? I hope it's too nice to hand-build only a few, I have some parts left for selected people, if there's really a demand we'll have to think about something.
So long,
Jörg
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