Category: Raspberry Pi and microcontrollers

I decided that the sound output from the Pi’s built-in headphone jack wasn’t sufficient after all and so went searching for better options (a DAC—digital-to-analog converter).

The Raspberry Pi foundation created a specification called “HAT” (Hardware Attached on Top) a few years ago which specifies a standard way for devices to automatically identify and configure a device and drivers that’s attached to the Pi via its GPIO (General Purpose Input/Output) pins. There’s a number of DACs now that conform to this standard, and the one I settled on is the JustBoom DAC HAT. It’s a UK company but you can buy them locally from Logicware (with $5 overnight shipping no less).

The setup is incredibly simple: connect the plastic mounting plugs and attach the DAC to the Pi, then start it up and edit /boot/config.txt to comment out the default audio setting:

#dtparam=audio=on

Then add three new lines in:

dtparam=audio=off
dtoverlay=i2s-mmap
dtoverlay=justboom-dac

Then reboot. (If you’re running Raspbian Stretch or newer, the i2s-mmap line should not be added).

To say that I’m impressed would be an understatement! I didn’t realise just how crappy the audio from the Pi’s built-in headphone jack was until I’d hooked up the new DAC and blasted some music out. I’m not an audiophile and it’s hard to articulate, but I’d compare it most closely to listening to really low-quality MP3s on cheap earbuds versus high-quality MP3s on a proper set of headphones.

If you’re going to be hooking your Pi into a good stereo system, I can’t recommend JustBoom’s DAC HAT enough!

I bought a Raspberry Pi almost exactly a year ago, intending on eventually replacing my Ninja Block and its sometimes-unreliable wireless sensors with hardwired ones (apart from the batteries needing occasional changing, there’s something that interferes with the signal on occasion and I just stop receiving updates from the sensor outside for several hours at a time, and then suddenly it starts working again). To do that, I need to physically run a cable from outside under the pergola to inside where the Raspberry Pi will live and I don’t really want to go drilling holes through the house willy-nilly. I want to eventually get the electrician in to do some recabling so I’m going to get him to do that as well, but until then the Pi was just sitting there collecting dust. I figured I should find something useful to do it with, but having a Linode meant that any sort of generic “Have a Linux box handy to run some sort of server on” itch was already well-scratched.

I did a bit of Googling, and discovered Shairport Sync! It lets you use the Raspberry Pi as an AirPlay receiver to stream music to from iTunes or iOS devices, a la an Apple TV or AirPort Express. We already have an Apple TV but it’s plugged into the HDMI port on the Xbox One which means that to simply stream audio to the stereo we have to have the Xbox One, TV, and Apple TV all turned on (the Apple TV is plugged into the Xbox’s HDMI input so we can say “Xbox, on” and the Xbox turns itself on as well as the TV and amplifier, then “Xbox, watch TV” and it goes to the Apple TV; it works very nicely but is a bit of overkill when all you want to do is listen to music in the lounge room).

Installing Shairport Sync was quite straightforward, I pretty much just followed the instructions in the readme there then connected a 3.5mm to RCA cable from the headphone jack on the Raspberry Pi to the RCA input on the stereo. It’s mentioned in the readme, but this issue contains details on how to use a newer audio driver for the Pi that significantly improves the audio output quality.

The only stumbling block I ran into was the audio output being extremely quiet. Configuring audio in Linux is still an awful mess, but after a whole lot of googling I discovered the “aslamixer” tool (thanks to this blog post), which gives a “graphical” interface for setting the sound volume, and it turned out the output volume was only at 40%! I cranked it up to 100% and while it’s still a bit quieter than what the Apple TV outputs, it doesn’t need a large bump on the volume dial to fix—there’s apparently no amplifier or anything on the Raspberry Pi, it’s straight line-level output. The quality isn’t quite as good as going via the Apple TV, but it gets the job done! I might eventually get a USB DAC or amplifier but this works fine for the time being.

On macOS it’s possible to set the system audio output to an AirPlay device, so you can be watching a video but outputting the audio to AirPlay, and the system keeps the video and audio properly in sync. It works extremely well, but the problem we found with having the Apple TV hooked up to the Xbox One’s HDMI input is that there’s a small amount of lag from the connection. When the audio and video are both coming from the Apple TV there’s no problem, but watching video on a laptop while outputting the sound to the Apple TV meant that the audio was just slightly out of sync from the video. Having the Raspberry Pi as the AirPlay receiver solves that problem too!

UPDATE: Two further additions to this post. Firstly, and most importantly, make sure you have a 5-volt, 2.5-amp power supply for the Raspberry Pi. I’ve been running it off a spare iPhone charger which is 5V but only 1A, and the Pi will randomly reboot under load because it can’t draw enough power from the power supply.

Secondly, the volume changes done with the “alsamixer” tool are not saved between reboots. Once you’ve set the volume to your preferred level, you need to run “sudo alsactl store” to persist it (this was actually mentioned in the blog post I linked to above, but I managed to miss it).