[pulseaudio-discuss] New equalizer module (module-eqpro-sink), some questions

Tue Nov 6 20:18:32 UTC 2018

On Mon, 2018-11-05 at 00:14 +0500, Alexander E. Patrakov wrote:
> Andrea A <Andrea69x at hotmail.it>:
> > I'm writing a new equalizer module for PA,
> > https://github.com/andrea993/audioeqpro/blob/master/pulsemodule/module-eqpro-sink.c
> > I've almost done but I need some information from developer about how to proceed.
> 
> Thanks for attempting a contribution. I have attempted to answer your
> questions regarding the integration, please read below. However, see
> the end of this email for the biggest reason why I am against
> accepting this module or any future form of it (but my "no" holds very
> little weight, so feel free to ignore it).
> 
> However, in order for the module to be accepted (barring the big
> objection at the bottom of this email), we need to review the DSP
> part, and not just the integration part. It would help if you provide,
> in the form of comments in the source code, some references where the
> math comes from. And use more descriptive variable names, such as K ->
> extra_gain. Also, I think it would make sense to use a struct of 10
> well-named floats instead of eqp->c.
> 
> > First of all, I see that current equalizer module manages
> > "autoloaded" have I to manage it? What it does exactly? Old
> > equalizer check "autoloaded" state in a callback "may_move_to",
> > what is it? Have I to implement this callback and manage
> > "autoloaded" like the current equalizer module?
> 
> It is set by module_filter_apply. The intended effect is to prevent
> moving the output of the equalizer to a different sink - i.e. if it
> was autoloaded for "Built-in Audio Analog Stereo" then you cannot move
> it to "HDMI Digital Stereo" using pavucontrol. See
> module-virtual-surround-sink.c for known-good usage. Although, I don't
> know any user of module_filter_apply.
> 
> Regarding the may_move_to callback, it is called when a user tries to
> move the equalizer output to a different sink. Please at least prevent
> creating a loop, i.e. moving the output to the equalizer itself.

There's no need to worry about loops, pa_sink_input_may_move_to()
already checks that (except loops built using module-loopback aren't
checked, but Andrea probably isn't going to solve that problem anyway,
or if he is, it's better to solve that in pa_sink_input_move_to()
rather than in individual modules).

> > After the "autoloaded" management I can send the equalizer as a
> > patch, however I've some questions about how to add my equalizer
> > GUI to the PA branch. Should the GUI remains an external program or
> > the GUI will be inserted to the mainline sources? In the second
> > scenario how the GUI should be inserted? Which is the correct
> > directory in the sources tree and what about the GUI makefile and
> > the GUI installation directory?
> 
> PulseAudio currently does not depend on any GUI toolkit (well, except
> the old equalizer GUI). Personally, I am fine with this GUI (or maybe
> two GUIs - one for GNOME and MATE and XFCE, one for KDE) being in
> external repositories.

GUIs should go to external repositories. qpaeq is an exception, and
probably not that well justified exception. One reason qpaeq made its
way to the main pulseaudio repo was that it's just a simple python
script that doesn't need much support from the build system.

> > The equalizer needs the messages patches from George Chini
> > https://patchwork.freedesktop.org/series/41390/
> > Have I to write this information as a patch comment or other?
> 
> Patch comment.

I'm not sure what "patch comment" means, but the information doesn't
belong in the commit message. If the module is submitted as a merge
request in GitLab, the information can be written to the merge request
description or added as a separate comment in the discussion section.
If the module is submitted via email, the information can be added
below the "---" line in the patch (this stuff is explained at
https://www.freedesktop.org/wiki/Software/PulseAudio/HowToUseGitSendEmail/
).

> > I would like to add some configuration files to my module, for example to load and store equalizer preset, is there a PA specific file format (and directory to store file) to do this?
> 
> There are database files in ~/.config/pulse. There are multiple
> backends supported, see the --with-database=... configure argument.
> The abstraction layer is in src/pulsecore/database.h. Not sure if this
> is suitable for your needs.

If the preset files are expected to be shared between users, then the
database.h stuff isn't good, because different users can have their
pulseaudio configured with different database formats. I like the "ini-
style" configuration file style that pulseaudio uses for .conf files.
There are no helpers for writing those files, though, but that's
probably not a big issue.

You mentioned presets only as an example, do you have other kinds of
configuration in mind? I'd expect the module arguments to provide all
the necessary configuration.

> > Execuse me for the wall of questions and thanks in advance.
> 
> You are welcome.
> 
> Anyway, just a small nitpick: the rewind callback is implemented
> incorrectly. The real problem is - nobody implements it correctly,
> especially because the comment in the template module-virtual-sink.c
> suggest doing such a stupid thing as resetting the filter. And, at
> least for the case of a resampler, users other than me do notice the
> imperfection, see https://bugs.freedesktop.org/show_bug.cgi?id=50113 .
> There are two solutions that I would accept as "proper". 1 - store the
> history of your input and/or state, restore it when asked to rewind. 2
> - do not pretend to support rewinds (but in this case, please limit
> the latency to something like 20-30 ms, so hat PulseAudio reacts
> quickly to the new streams). In the name of simplicity, and because
> the power-saving argument behind the original rewind operation does
> not hold if there is non-trivial processing, I would prefer option 2.

In the name of simplicity, I'd strongly prefer option 2.

> Big warning: I have not tested the module.
> 
> And here at the bottom of the email, let me explain why I think
> keeping this module outside of PulseAudio, in a different form, may be
> a better idea.
> 
> The reason is that, by accepting this module, we are implicitly taking
> the responsibility to support it inside the tree. And, you are the
> best person to support it. So there is an additional (avoidable!)
> latency between the time when you develop improvements and the time
> when users see them: namely, the time for someone else to understand
> and review your code, for PulseAudio team to make a release, and for
> distributions to package it.
> 
> A better alternative, in my opinion, would be to create a LADSPA
> plugin instead. PulseAudio already has module-lasdpa-sink since ages
> (even with D-Bus interface to change parameters at runtime), so your
> filter will be available also to all users of old PulseAudio versions.
> It will be also available to users of pure ALSA-based setup, if they
> still exist. You can publish improvements any time you want, without
> needing any potentially slow review from PulseAudio maintainers (but
> feel free to contact me privately if you do need a review), and your
> module will be quick to compile, because it is separated from the rest
> of PulseAudio. You can then publish a GUI application that loads the
> module into PulseAudio and then controls its filter via D-Bus. And you
> don't need to care about rewinds and may_move_to and all other
> pulseaudio-specific boilerplate. Sounds like a win-win situation.
> Could you please investigate this approach?

I would love to have the equalizer as a LADSPA plugin rather than yet
another separate filter sink. It's not very uncommon that some core
change requires changes in all sinks, so even if the module is perfect
and doesn't require maintenance in form of bug fixes, there are other
kinds of real maintenance costs.

The LADSPA plugin approach isn't without issues, however:

LADSPA doesn't seem to support parametrized plugin instantiation,
meaning that the number of bands needs to be fixed. This can be
mitigated by creating a few separate plugins with different band
counts, but that of course can't scale to support arbitrary band
counts. But maybe a few common cases is good enough?

There is a D-Bus interface for changing the parameters, but it's rather
limited. It may very well be enough for a specialized GUI that knows
exactly what plugin it's dealing with, though. But the D-Bus protocol
has some serious issues, such as having no API stability guarantees. I
think the D-Bus protocol can be regarded kind of deprecated, it's not
likely to ever become the preferred protocol as originally envisioned.
Implementing a control interface using the message API from Georg for
the LADSPA sink would be an awesome contribution in itself, especially
if it's more complete than the D-Bus one. "More complete" means
supporting introspection: applications should be able to enumerate the
LADSPA sinks, find out which plugins they have loaded (including label
and name information) and what controls the plugins have (including the
control name and type).

-- 
Tanu

https://www.patreon.com/tanuk
https://liberapay.com/tanuk