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Ubuntu Touch Audio stack for 13.10 release
Up until now, we've been using Android's AudioFlinger for playing back
and recording audio. Starting with tomorrow's image, that is no longer
true. Instead we're talking directly from PulseAudio to ALSA, or the
Android audio HAL when necessary.
In short, here's how PulseAudio now works:
* For normal playback and recording, PulseAudio talks directly to
alsa-lib, just as on the desktop.
* For detecting whether a headphone/headset is plugged in or not,
PulseAudio now has code for reading that from the Android kernel,
through the "switch" interface .
* For normal mixer setup, we use ALSA UCM  mixer files.
* For setting up voice calls, we talk to the Android Audio HAL through
a PulseAudio module.
This provides somewhat of a compromise between features and porting
effort: By using the ALSA library whenever we can, we can access
PulseAudio's timer scheduling and dynamic latency features. Having the
straightest path possible for playing back music should help efficiency
(and in extension, battery life). At least in theory - we haven't
actually done measurements.
Using the Audio HAL for everything mixer related would have been
optimal, but it turns out that the audio HAL is too smart: it refuses to
set up the mixer, unless PCM data is also sent to it, which is what we
wanted to avoid. So then we had to set up the mixer manually too.
However, we still could not avoid using the Audio HAL altogether: when
starting and stopping voice calls, the Audio HAL talks to the modem and
other components in the kernel to route the voice call between the modem
and the sound card. Hence we ended up with this compromise approach.
== Current status ==
At the time of this writing, this is working best on Nexus 4. The Galaxy
Nexus works for the most part, except for bug 1217072. I intend to add
Nexus 7 support shortly. If anyone wants to help testing Nexus 10, let
== For porters: if you need to do the same ==
Unfortunately, this means some additional work for porters, because you
need to write UCM mixer files. What's worse, UCM is lacking good
documentation. For that reason, I hesitated somewhat before deciding to
actually use UCM at all, but it's the closest we have to a standard for
setting up mixers on embedded devices right now.
But to give you a two-minute crash course in UCM and how it's used in
Ubuntu Touch - start by having a look in
/usr/share/alsa/ucm/apq8064-tabla-snd-card/ directory (shipped with
alsa-lib). You'll need to create a similar directory for your device.
You'll find the right directory name if you look in /proc/asound/cards.
Second, look at apq8064-tabla-snd-card.conf. Rename and copy into your
own UCM directory. If you're making a tablet image (that can't make
voice calls), you can remove the VoiceCall part (and the corresponding
Third, look at the HiFi file. This is where all fun happens. Notice the
device names, which are hardcoded into telepathy-ofono and need to
match: "Speaker", "Earpiece" and "Headphone" for playback, plus
"Handset" and "Headset" for recording.
Fourth, if you need voice calls, also look at the VoiceCall file. Btw,
the verb names "HiFi" and "VoiceCall" also need to match.) This is
largely empty, because the mixer setup is handled by the Audio HAL, but
there is a twist here that took a while to get right: For PulseAudio's
UCM to work, it needs to open a PCM device. However, at the time where
UCM tests this, the voice call is not yet set up. So, you might need to
set up the mixer just a little, so that the PCM can open. (On desktops,
PCM can always open, regardless of mixer state. This is not always true
on embedded devices, that are using ASoC.) It's a bonus if you can find
a PCM that actually plays back audio, because then you can get
notification sounds while on the phone.
And this concludes the two minute crash course - happy porting!
(Side note: This has also been posted on my blog, but the blog link 
currently leads you to a blank page. I've reported the bug to the
relevant team in Canonical, but at the time of this posting, they have
not yet fixed it.)
David Henningsson, Canonical Ltd.