The Raspberry Pi is a fairly powerful $25 single-board computer targeted toward the educational market, though just because it’s for kids doesn’t mean it’s not fun for adults. I’ve been wanting to buy a Pi for a while now, but couldn’t justify purchasing hardware I have no use for – that is, until I saw this blog post detailing how to use the Pi as an Apple Airplay receiver. This is perfect. Let’s get started turning our little computer into a single-purpose appliance.

Here it is, after a few weeks of waiting!

The actual board itself is very small. So small, in fact, that I wondered whether I needed to buy a project box to stuff it in. While looking around online at different expensive plastic boxes to put this in, I realized that I have the perfect enclosure for the Pi already in my audio stack.

The Edirol UA-5 usb audio interface

My USB audio interface is made of thick steel and aluminum, and will be plugged in to the Pi anyway (since the Pi uses onboard pulse-width modulation to approximate an audio signal at low quality). Why not put the Pi inside the UA-5? Let’s see if there is room for a Pi.

Plenty of room inside

After removing the thick aluminum cover with a few screws, I see that there looks to be room directly over the digital signal processing side of the board. This is good, because I don’t want to stick a little computer spewing RF noise all over any analogue circuits. Let’s remove the main board.

The board comes right out, and now we’re left with a thick steel case. Let’s cut a hole in it! We need a hole to pass the USB ports, power, and WiFi radio through. I have a feeling this case efficiently blocks and RF signals emanating from inside. At this point I should say that I rushed in to cutting the case without proper tools (I used a drill and some snips). I should have had at least a file to smooth out the rough edges, but I can go back and fix this later.

A very rough looking finish. Oh well.

After cutting a hole in the side of my beautiful equipment, let’s figure out how to fit the Pi in. I’ll screw the main board back in and then cut a piece of the packaging cardboard to make a standoff that will insulate the two boards from each other.

Reduce, Reuse…

And secure the Pi to it’s new home. I’ve added an 8GB SD card with Raspbian along with an Edimax USB wireless dongle and a microUSB cable for power. This is all secured to the cardboard standoff with zip ties.

It fits well

Before I screw the lid back on I need to power up the Pi and log in via SSH using the onboard ethernet. After editing my /etc/network/interfaces and adding “auto wlan0″ I just add the proper wpa_supplicant.conf, grab the wlan0 MAC address, add a static DHCP reservation for the Pi on my router, and reboot. Now let’s put the lid on.

Not bad!

It looks pretty good! Let’s log in to our Pi and install Shairport. It’s pretty straightforward.

# Change to our home directory, install all the packages we need.
cd ~
apt-get install build-essential libssl-dev libcrypt-openssl-rsa-perl libao-dev libio-socket-inet6-perl libwww-perl avahi-utils pkg-config git emacs
git clone https://github.com/albertz/shairport.git
cd shairport
make
# Install Net::SDP Perl module
sudo -s
perl -MCPAN -e 'install Net::SDP'
# Demote the onboard soundcard and allow USB soundcards to become default
# Add 'options snd_bcm2835=-2'
# Comment out line 'options snd-usb-audio index=-2'
emacs /etc/modprobe.d/alsa-base.conf 
# Copy the example init script
cp shairport.init.sample /etc/init.d/shairport
# Edit the init script to point to Shairport
# NAME=RecordPlayer
# DAEMON="/home/pi/shairport/shairport.pl"
emacs /etc/init.d/shairport
update-rc.d shairport defaults
reboot

That’s it! Let’s hook it back in to our amplifier. I’ve connected the UA-5 to the remaining USB port on the Pi.

The Raspberry UA-5 sitting on the phono preamp and stereo amplifier.

With everything connected and powered on, we can now select “RecordPlayer” from our iOS or Mac OS devices and stream music to the nicest speakers in the house.

There’s a computer in there.

The UA-5 is capable of 96KHz 24-bit playback, but because the Pi is not too powerful, and I suspect that Airplay only transmits 16-bit resolution, I have it set up for 48KHz 16-bit playback.

UPDATE:

I’ve recently encountered an issue with the default Pi ALSA configuration. It seems like the Raspberry Pi developers worked in a direct memory access mode for the onboard sound, and this was giving me some issues while using a USB sound card. Sometimes, but not always, the audio would cut out for about a second every five to ten seconds. With some configuration changes I’ve been able to eliminate this. Also, to be safe I’m now using 41KHz playback. Here’s the /etc/asound.conf:

pcm.!default {
    type plug
    slave.pcm anticracking
}

pcm.anticracking {
    type dmix
    ipc_key 1024
    ipc_key_add_uid yes
    slave {
        pcm "hw:0,0"
        period_time 0
        buffer_time 0
        #period_size 1024
        #buffer_size 8192
        rate 44100
    }
}

Thanks to [maximeh] for the suggestion.