[pulseaudio-discuss] RFC: New volume functionality for PulseAudio

[Tanu Kaskinen]

On Tue, 2014-06-24 at 15:37 +0200, David Henningsson wrote:
> 
> On 2014-06-24 15:01, Tanu Kaskinen wrote:
> > On Tue, 2014-06-24 at 13:19 +0200, David Henningsson wrote:
> >> I remember thinking that pa_ext_volume_api_bvolume should include a "int
> >> mute" field. Then every mute specific stuff, including mute controls,
> >> can be skipped.
> >
> > I don't like that. Any struct that contains both volume and mute will
> > also need "bool has_volume" and "bool has_mute" fields (or a type enum
> > for either/or choice).
> 
> Wouldn't be typical for a control to have both volume and mute? E g, a 
> sink has both volume and mute, right? Instead of having two objects and 
> leave to the client to group them together, it would make more sense to 
> keep them as one object and let clients turn it into two if necessary.
> 
> Or, why wouldn't a volume also have a mute?

If we route things in domains where pulseaudio doesn't have access to
the audio data, we're at the mercy of whatever controls the domain
provides. For example, when routing audio from cellular modem to
speakers without ever seeing the audio data in pulseaudio, there's no
guarantee that the hardware has both volume element and a mute switch
for the audio path. It can have one or the other. I haven't worked with
such systems, though, so I don't know what the situation is with real
hardware out there.

It can also be that e.g. a stream should have both volume and mute, but
those control entirely different things. For example, someone once
requested a feature where the stream volume would actually control the
sink volume. I don't think that was a silly request: I might want to use
such feature myself for selected streams: for example, I might want the
volume control in Totem to control the device volume to get some of the
benefits of flat volumes (easy access to full hw volume range in
full-screen mode) without enabling flat volumes for all streams. If
stream volume actually controls the device volume, it may still be
desirable to have per-stream mute, so different controls are required
for volume and mute.

I'll quote Alexander[1] for another use case for volume and mute
separation: "Sooner or later, someone will implement optional and
runtime-switchable automatic gain control (AGC). A natural question is:
does the whole notion of the main input volume make sense at all if AGC
is enabled?" Main input volume doesn't make sense in this case, but main
input mute does.

[1] http://article.gmane.org/gmane.comp.audio.pulseaudio.general/19694/

> We've already added extra information in order not to lose the balance 
> when the volume is -inf, we could just as well add a mute too.
> 
> > I don't think pa_bvolume is the right struct to
> > do that. This would make more sense to me:
> >
> > struct pa_control_info {
> >      uint32_t index;
> >      char *name;
> >      char *description;
> >      pa_proplist *proplist;
> >      pa_control_type_t type;
> >      void *data;
> > };
> >
> > type is either PA_CONTROL_TYPE_VOLUME or PA_CONTROL_TYPE_MUTE. data
> > points to one of these structs depending on the control type:
> >
> > struct pa_volume_control_data {
> >      pa_bvolume volume;
> >      int convertible_to_dB;
> 
> Btw, could you explain what this field (convertible_to_dB) means?

Sinks and sources have the DECIBEL_VOLUME flag. That's exposed here, so
that clients know when they can convert pa_volume_t to decibels.

-- 
Tanu