Wed 11 Nov 2009
I upgraded my laptop to Ubuntu Karmic recently, which brought with it the ability to use my Bluetooth A2DP headphones natively. Getting them running is now as simple as using the Bluetooth icon in the panel to pair the laptop with the headphones, and then selecting them in the Sound Preferences applet, on the Output tab.
As soon as the headphones are connected, they show up as a new audio device. Selecting it instantly (and seamlessly) migrates my sounds and music from the laptop sound device onto the headphones. The Play/Pause, Next Track and Previous Track buttons all generate media key keypresses – so Rhythmbox and Totem behave like they’re supposed to. It’s lovely.
If that we’re all, it would already be pretty sweet in my opinion, but wait – there’s more!
A few days after starting to use my bluetooth headphones, my wife and I took a trip to Barcelona (from Dublin, where we live for the next few weeks… more on that later). When we got to the airport, the first thing we learned was that our flight had been delayed by 3 hours. Since I occasionally hack on multimedia related things, I typically have a few DVDs
with me for testing. In this case, I had Vicky Christina Barcelona on hand, and we hadn’t watched it yet – a perfect choice for 2 people on their way to Barcelona.
Problem! There are four sets of ears wanting to listen to the DVD, and only 2 audio channels produced. I could choose to send the sound to either the in built sound device, and listen on the earbuds my wife had, or I could send it to my bluetooth headphones, but not both.
Pulseaudio to the rescue! With a bit of command-line fu (no GUI for this, but that’s totally do-able), I created a virtual audio device, using Pulseaudio’s “combine” module. Like the name suggests, it combines multiple other audio devices into a single one. It can do more complex combinations (such as sending some channels hither and others thither), but I just needed a straight mirroring of the devices. In a terminal, I ran:
pactl load-module module-combine sink_name=shared_play adjust_time=3 slaves=”alsa_output.pci-0000_00_1b.0.analog-stereo,bluez_sink.00_15_0F_72_70_E1″
Hey presto! Now there’s a third audio device available in the Sound Preferences to send the sound to, and it comes out both the wired ear buds and my bluetooth headphones (with a very slight sync offset, but close enough for my purposes).
Also, for those interested – the names of the 2 audio devices in my pactl command line came from the output of ‘pactl list’.
This kind of seamless migration of running audio streams really isn’t possible to do without something like Pulseaudio that can manage stream routing on the fly. I’m well aware that Pulseaudio integration into the distributions has been a bumpy ride for lots of people, but I think the end goal justifies the painful process of fixing all the sound drivers. I hope you do too!
Lennart points out that the extra paprefs application has a “Add virtual output device for simultaneous output on all local sound cards” check-box that does the same thing as loading the combine module manual, but also handles hot-plugging of devices as they appear and disappear.