Hi Tanu and pulseaudio-discuss,
I'd like to participate as a student in GSoC 2012 working on PulseAudio. Among
suggested ideas I've chosen Configurable maximum volume for sinks and sources,
below is a very draft of my proposal.
I'd be very grateful for any comments, suggestions, pointed-out omissions and
general questions that may arise. I've also thought about extending the
proposal a bit to add code to deal with stereo microphones where one channel
is inverted (supposedly common problem) -- but I've heard on #pulseadio this
is already being worked on. What do you think?
Introduction
=======
Recent PulseAudio versions gained an ability to amplify signal of inputs and
outputs in software. While this is often needed for low-amplitude signals,
software amplification can cause negative effects such as clipping and should be
avoided if possible. Sometimes, even without amplification, outputs may be able
to produce sound that is unacceptably loud. In both situations user should be
able to set volume limit to prevent her ears and/or equipment from discomfort
or even damage. Many portable audio players are able to do this, this project
is about supporting volume limits in PulseAudio.
Project goals
=======
What will be implemented in PulseAudio in this project:
* support for enforcing volume limits in [TODO: ALSA-based?] Sources and
Sinks; the limits can be
changed on-the-fly
* support for persisting volume limits across PulseAudio restarts
* user-friendly GUI to configure volume limits by extending KDE's KMix or
PulseAudio's pavucontrol. [TODO: I would love to make both, but it's hard to
estimate how much time it will take]
Implementation
=========
[this section is based on my rather incomplete understanding of the code;
please correct me where I'm wrong]
PulseAudio's architecture is based on modules that can provide Sources (audio
inputs), Sinks (audio outputs), networking protocols and more. Sinks are
represented with a C struct pa_source defined in pulsecore/sink.h; Sources use
analogous structure pa_sink. Modules then implement Sources and Sinks by
setting various callbacks in the structs to their functions and filling fields
with their data.
[TODO: this project should focus on ALSA, right?] Relevant for this project is
the module-alsa-card (which shares Sink and Source code with module-alsa-sink,
module-alsa-source) that implements Sinks & Sources atop of the most prevalent
Linux sound system, ALSA. For the first iteration, the module(s) will be made
to accept volume_limit parameter that will be enforced in
{sink,source}_set_volume_cb() (TODO: this seems to only work for hardware
volumes?)
For complete integration, pa_{sink,source} structs along with related
functions will have to be extended to cope with volume limits. To expose these
limits, the pulseaudio native protocol, cli protocol (therefore pacmd tool)
and the pactl tool will be extended, along with the PulseAudio client library,
libpulse. Finally, KDE's KMix will be tweaked to allow displaying and setting
the volume limits. (preferably in the same widget where the volume is set)
Timeline
=====
[subject to change as many implementation details are not yet ironed out]
* Community bonding period: ironing out the design; where and how volume
limits should be implemented.
* 1. & 2. week: volume limits passed through module params enforced for ALSA
Sinks & Sources
* 3. & 4. week: pa_{sink,source} extended to cope with volume limits
* 5. week: cli protocol extended so that volume limits can be shown and set
in pacmd
* 6. week: pacmd tool extended, work on preserving volume limits across
restarts
* 7. week: native protocol extended
--- mid-term evaluation
* 1. & 2. week: extending KMix to show & set volume limits
* 3. & 4. week: [TODO] extending pavucontrol? unit-tests?
* 5. week: resolving any remaining issues..
About me
======
I'm a 24-year-old student of mathematical informatics from Prague, Czech
Republic. I've been passionate about FLOSS since high school and recently I've
started contributing to a couple of projects, most notably Amarok [1]; I even
have one small patch in the Linux kernel [2]. I know C, C++, Python, Java, a
bit of French (pun intended) and some other less relevant languages. Thanks to
Amarok I have experience in GUI programming in Qt & KDE libs, I'd like to
learn GTK[mm].
I've chosen PulseAudio because it appears to me as a sound-system-done-right,
in order to learn how to make clean modular apps in C and to work on a project
founded by the famous GNU/Linux innovator, Lennart. To get started I've
fixed bug 38728 [3].
Regards,
Mat?j
[1] https://www.ohloh.net/accounts/strohel
[2] goo.gl/WNsnB
[3] goo.gl/rCVGa
