I had recently been looking into purchasing an Apple TV for the purpose of streaming audio via AirPlay. I have little interest in the video side of things, so I started searching for a dedicated audio solution, hoping to find a high fidelity airplay receiver. Given that AirPlay data is transmitted using a lossless codec, the potential for super-high fidelity is unlimited.

After a little searching I found that a Raspberry Pi (RPi) could be converted into an airplay receiver. The only problem is the somewhat-less-than-perfect analogue audio output built into the RPi. The digital to analogue converter (DAC) in the RPi works using the pulse-width modulation (PWM) principle to keep costs down. Most high fidelity audio DACs work using the pulse-code modulation (PCM) principle, generally resulting in a more faithful representation of the digitally-encoded analogue signal.

A PWM signal is comprised of a rapid series of electrical pulses. The pulse can either be zero voltage, or maximum voltage. The width (or duty cycle) of the very rapid pulses is modulated such that the the desired output voltage is represented by the average voltage of the pulse train. This is then lowpass filtered to give a steady analogue voltage (the cutoff frequency of the filter can be well above the audible band). The pulse rate in the RPi is fixed at 100 MHz. The bit depth of this pulse stream is 1 bit (the pulse voltage can only be zero or maximum). The fixed pulse rate gives an upper limit on the amount of information that can be transmitted by the audio output built into the RPi. The following formula can be used to estimate the bit depth at the standard CD sampling rate of 44.1 kHz.

This gives us a theoretical maximum of around 11 bits / sample at 44.1 kHz. This is absolutely fine for basic speech intelligibility when running a VIOP application, or for simple sound effects. However, for any serious music listening, at least 16-bit audio is required to get full resolution dynamic range out of CDs. This means that an external USB audio interface is required, but thankfully, there are some great USB external interfaces available.

The Hardware

I happened to have a Creative X-Fi HD laying around that I was able to temporarily repurpose. This is certainly not the best external DAC that money can buy, but it sounds very good for its price, and it is more than adequate for the initial testing phase of this project.

The assembled test rig is shown in the screenshot below. The power is supplied by the USB hub. The xFi current draw is fairly low, meaning it can be powered from the RPi using a Y-cable if desired. The networking is wired for the time being, but the setup could easily be changed to use wifi (I have found the ALFA Networks high power USB wifi devices to work very well compared to the small Edimax devices, but this is a whole other blog post). This setup is not very visually appealing, but it is functional, and this is just the pilot phase. I have been testing this setup by streaming music from my mac using iTunes, through the test equipment, and then onwards to some Rogers LS7t speakers. I have been listening for hours on end over the past few days without noticing a single audio drop or glitch.

The sound quality is great. As far as I can tell, the digital data stream to the USB interface is uninterrupted and error free, meaning sound quality is exactly as if the Mac was directly connected to the the X-Fi Sound interface. Many people on the official Raspberry Pi forums have reported having difficulty getting stable audio when using an external USB interface. The rest of this blog article describes the software steps involved in getting pristine AirPlay audio through an external USB sound interface connected to the RPi.

Preparation

Before we begin, make sure that you have the Debian Wheezy operating system installed, and that you have secure shell (SSH) access to your RPi (the instructions for doing so are beyond the scope of this tutorial, but the steps required can easily be found by searching the web). Make sure that you can log onto the RPi over the network before proceeding.

The very first step is to upgrade the packages on the RPi. As root, do the following …

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root@raspberrypi:~# aptitude update
root@raspberrypi:~# aptitude upgrade

On my system, the uname command gives the following output (so long as the output you see has the same date stamp, or is newer, then things should be OK) …

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pi@raspberrypi ~ $ uname -a
Linux raspberrypi 3.6.11+ #348 PREEMPT Tue Jan 1 16:33:22 GMT 2013 armv6l GNU/Linux

The next step is to upgrade the RPi’s firmware. As root, use the following command …

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root@raspberrypi:~# rpi-update

For me, this failed on the first run, but worked OK on the second attempt. For detailed information regarding the rpi-update command, see the associated github repository.

Test AirPlay using the onboard sound

This section is a summary of the instructions found here. The default audio output should be set to the onboard stereo jack. For this, a single command is required as root …

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root@raspberrypi:~# amixer cset numid=3 1

Before download and compilation of the shairport software (this is the software that makes the RPi mimic an authentic AirPlay device), some prerequisites need to be installed. As root, install the following packages …

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root@raspberrypi:~# aptitude install git libao-dev libssl-dev libcrypt-openssl-rsa-perl libio-socket-inet6-perl libwww-perl avahi-utils

I also needed to get the Net::SDP perl library when I tried this. This was done using the following command as root …

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root@raspberrypi:~# cpan install Net::SDP

With all the prerequisites successfully installed, the next step is to download shairport sources and compile them as root …

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root@raspberrypi:~# git clone https://github.com/albertz/shairport.git shairport
root@raspberrypi:~# cd shairport
root@raspberrypi:~/shairport# make

Shairport can now be tested by launching it in the foreground …

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root@raspberrypi:~/shairport# ./shairport.pl -a AirPi

… and then connecting to it using an AirPlay compatible device. The streamed data should then be audible from the 3.5 mm jack with speakers or headphones attached.

Getting an external USB DAC to work without pops / clicks / noise

Firstly, install some prerequisites as root …

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root@raspberrypi:~# apt-get install libasound2-plugins
root@raspberrypi:~# apt-get install libesd0
root@raspberrypi:~# apt-get install nas

The next step is to edit /boot/cmdline.txt to fix some of the potential causes of pops when using a USB sound interface. Open /boot/cmdline.txt in the nano editor as root …

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root@raspberrypi:~# nano /boot/cmdline.txt

Add the following text to the file. I’m not sure if the position of the text makes a difference, but I appended the text to the front of the existing text. Once the text has been added, quit the nano editor using ctrl+x and save the changes by hitting return when prompted.

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dwc_otg.speed=1 dwc_otg.fiq_fix_enable=1 

Using nano as in the previous step (or your favorite editor), edit /etc/libao.conf so that it contains the following …

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default_driver=alsa
dev=default
use_mmap=no

Edit /etc/modprobe.d/alsa-base.conf, commenting out the snd-usb-audio line and adding the snd_bcm2835 line so part of the file looks like the following …

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#options snd-usb-audio index=-2
options snd_bcm2835 index=-2

For testing purposes, create a hidden file, .asoundrc, in the home directory of a regular user. For example, as pi …

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pi@raspberrypi ~ $ touch .asoundrc

Then edit this file to contain the following configuration data…

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pcm.!default {
    type plug
    slave.pcm "softvol"
}
pcm.dmixer {
       type dmix
       ipc_key 1024
       slave {
           pcm "hw:0"
           period_time 0
           period_size 4096
           buffer_size 131072
           rate 44100
       }
       bindings {
           0 0
           1 1
       }
}
pcm.dsnooper {
       type dsnoop
       ipc_key 1024
       slave {
           pcm "hw:0"
           channels 2
           period_time 0
           period_size 4096
           buffer_size 131072
           rate 1
       }
       bindings {
           0 0
           1 1
       }
}
pcm.softvol {
       type softvol
       slave { pcm "dmixer" }
       control {
           name "Master"
           card 0
       }
}
ctl.!default {
    type hw
    card 0
}
ctl.softvol {
    type hw
    card 0
}
ctl.dmixer {
    type hw
    card 0
}

Restart the RPi before proceeding. Attach the external USB sound interface to some speakers or headphones, manually start up shairport again like in the previous example, then test it by connecting to the shairport server and streaming some music

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root@raspberrypi:~/shairport# ./shairport.pl -a AirPi

If this all works OK, then make a demon process so that shairport always runs on startup or after the RPi is reset.

Running the AirPort server at startup

The first task is to copy the local .asoundrc to /etc/asound.conf so that it can be found by the demon process. This tripped me up for a bit!

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root@raspberrypi:~# cp /home/pi/.asoundrc /etc/asound.conf

As root, install the shairport software, copy the default configuration, make the shairport init script executable, and then update rc.d by issuing the following commands …

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root@raspberrypi:~/shairport# make install
root@raspberrypi:~/shairport# cp shairport.init.sample /etc/init.d/shairport
root@raspberrypi:~/shairport# cd /etc/init.d
root@raspberrypi:/etc/init.d# chmod a+x shairport
root@raspberrypi:/etc/init.d# update-rc.d shairport defaults

Before starting the daemon, we have to add the AP Name in the launch parameters. Edit the file using nano shairport then change the DAEMON_ARGS variable line so it looks like the following …

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DAEMON_ARGS="-w $PIDFILE -a AirPi"

Replace AirPi with whatever you want it to appear as on your network. The demon can be started using the following …

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root@raspberrypi:/etc/init.d# ./shairport start

The AirPlay service will now start whenever the RPi is powered on.

Coming soon

In the next installment of this blog series, I will attempt to interface the Raspberry Pi with a Schiit Modi dedicated USB DAC. I have one on order from the manufacturer in the USA (it is currently in the post). These are supposed to provide incredible sound quality for the price. the problem with the xFi is that it tires to do too much (optical / audio in / headphone amp). the Modi just has a USB input and a stereo line level analogue output. As an AipPlay device for use with the Raspberry Pi, no money (or size) is wasted on superfluous functionality.

In the next installment, I also intend to some controlled tests to compare the setup’s sound quality to an Apple TV. I’ll do this with a number of listeners. I’m excited to find out what happens!

UPDATE: Seeing as this post generates the most traffic on my blog, I’d like to add a shameless plug www.aud1.com to my latest project.