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

David Henningsson david.henningsson at canonical.com
Tue Jul 1 04:16:52 PDT 2014

On 2014-06-27 17:59, Tanu Kaskinen wrote:
> On Fri, 2014-06-27 at 14:41 +0200, David Henningsson wrote:
>> On 2014-06-25 11:47, Tanu Kaskinen wrote:
>> Well, with that argument we could just as well use pa_cvolume. Because
>> we don't know if the hardware maintains balance when main volume is -inf
>> either.
> It's enough if we remember in pulseaudio the accurate balance that was
> set before the overall volume changed to -inf. The hardware doesn't need
> to (and can't) remember it. (Btw, I prefer term "overall volume" when
> referring to pa_bvolume.volume, because "main volume" is already used to
> mean a different thing.)
> The balance information is potentially lost only if we copy the volume
> from the hardware to our own volume representation, but that's rarely
> necessary. Usually we copy the volume from our own representation to the
> hardware, not the other way around.
>> We decided to have an extra "main" volume not to lose balance
>> information, we can have an extra mute not to lose "main" volume when
>> the stream/sink/etc is muted.
> Losing the overall volume information is not a problem if volume and
> mute are in separate control objects, so I fail to see the point you're
> trying to make here.

It's more common than not to have volume bundled with a mute: almost all 
sinks, sources, and streams have that. Therefore, it feels logically 
more correct to keep them together, rather having every GUI having to 
link them together themselves.

That said, I do acknowledge that there are valid use cases, especially 
for mutes without volumes. But it the answer to that really to separate 
*all* mutes from *all* volumes?

>> Do you also plan to add stream and sink pointers to two pa_control_info
>> structs then, one for volume and one for mute? I e for introspection of
>> sinks/streams/etc.
> The second "odd" assumption was that having pointers from controls to
> sinks/streams/etc would be somehow relevant for sink/stream/etc
> introspection. If you want to introspect a sink, you need a pointer from
> the sink to the control, not the other way around.

Yes, this is what I meant: sinks and sources would have pointers to the 
control. Like this:

struct pa_sink_info {
	uint32_t volume_control_index;
	uint32_t mute_control_index;

Are you planning to add that?

> Now, some kind of mechanism to figure out the "primary" purpose of a
> volume control object is needed, if a UI wants to indicate that the main
> volume currently controls the call volume, or the headphones device, for
> example. I initially thought that it's enough to check what stream,
> device, or audio group is using the same volume control as what is
> assigned as the main volume control, but this is ambiguous if multiple
> entities share the volume control.

Hmm, but are you also planning to change the internals of PulseAudio so 
that if a stream and a sink share the same volume control, only one of 
them will actually apply the volume? Otherwise the volume will have 
double effect.

>>>>> 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.

I think we should call it "int decibel_volume" instead for consistency.

>> So in short, expect convertible_to_dB to always be true, if it's false,
>> we're dealing with "bad" hardware that does not know what their volume
>> steps mean?
> Yes. (By the way, all Bluetooth headsets are "bad" in this sense too.)
>> We could do the full capabilities thing that ALSA does - tell exactly
>> what steps and so on that the hw supports. In which case we can merge
>> volume and mute - a mute control is just a volume control with two
>> steps. But this does not seem to be what you're planning.
> Steps are indeed something that probably should be added to volume
> controls at some point, to allow UIs to show the user what steps are
> available.

Since pa_sink_info already have a n_volume_steps field, I think it would 
make sense to have that here too - shouldn't be too hard to add?

> Another use case for volume steps is to limit the dynamic
> range by using artificial volume steps and setting the lowest non-mute
> step to relatively high value. This is useful if the device has low
> dynamic range and is too quiet on lower volumes. There might be some
> better solution for the dynamic range problem than forcing the use of
> artificial volume steps, but I've heard that Sailfish is using volume
> steps for this purpose (via a custom volume API).
> I'm not sure what design you're thinking here. Earlier you suggested a
> design where a device would have one control that has both volume and
> mute information. Is this paragraph about a design where a device would
> have two volume controls, one of which actually represents mute with a
> two-step volume slider? Another possibility is that you're thinking a
> design where a device has just one merged volume/mute control without
> separate mute field, but that's not workable because then we couldn't
> remember the previous volume when the device is muted.

This was not a design proposal, rather the opposite, an attempt to step 
back a little.

For me, the common case is bundled volume and mute. Hence one bvolume 
with both volume and mute is my primary suggestion.

Then we have all the specials: mutes without volumes, per-channel mutes, 
volume in steps only, and so on. The question is which one(s) of all 
those specials do we want to, and need to, solve.

>> I can understand that mute and volume are the most common types of
>> (volume) controls. But conceptually, we have everything in between too:
>> Per channel mutes (common in hw and ALSA), volume with a limited number
>> of steps, and so on. Considering this, and the boundary between volume
>> and mute is fluid.
>> But maybe that is just to over-generalising things and we should stick
>> to volumes and mutes, like you suggest.
> I'll try to summarize:
> Bundling volume and mute into one control is not feasible if we want to
> have flexible control sharing (see my example of sharing the device
> volume control with Totem's stream,

I don't think the Totem example makes sense. If Totem wants to control 
the device volume instead of the stream volume, then Totem can just do that.

>  while also having per-stream mute
> controls), and it's at least awkward in situations where only mute is
> relevant (e.g. passthrough streams). For these reasons I want to
> separate volume and mute into separate control objects.
> Another idea was to represent mute with a volume control, so each device
> would have two volume controls, one of which would be a two-step volume
> that emulates mute. That should work, but I think that would look
> strange to application developers, and I don't think it would save much
> implementation effort if volume and mute controls are both based on a
> common pa_control class anyway.
> Then there's the possibility that we drop the control sharing feature.
> This makes some issues go away, with the downside that we lose the
> transparency in how different volumes relate to each other, so if we
> want to describe in the client API how stream volumes and mutes are
> grouped, some other mechanism is needed in place of control object
> sharing.
> I briefly mentioned my problem with flat volumes earlier, I'll explain
> it a bit more here. In flat volume mode, the idea is that clients see
> the absolute volume of the stream, i.e. the full hardware volume range
> is available through the stream volume control. If a stream volume is
> shared with an audio group, this becomes a problem, because the audio
> group's volume control can't represent the absolute volume, because
> there can be multiple streams connected to multiple devices - some
> devices may use flat volume and some not, but even if all devices use
> flat volume, their volume scales are different, so absolute volume means
> different things on different devices. Therefore, the only sane choice
> is that the audio group volume represents the relative stream volume.
> But then, if a stream shares the volume control with an audio group, the
> clients see the audio group's volume as the stream volume, so the
> requirement of providing the full hw volume range is not met.
> The solution is to create a volume control object for providing access
> to the stream absolute volume, and propagate changes to that volume
> control internally to the audio group's volume control. But then
> transparency is lost, i.e. clients don't see the connection between the
> stream volume and the audio group volume, which I wanted to be visible.
> Since the volume control sharing doesn't work in a common case like
> this, I don't see it as a big loss if we add the constraint that one
> volume control can be associated with only one entity. It probably makes
> some things simpler.

Yeah - where possible/convenient, let's design the API so we can add 
volume sharing later (if we figure out that's a good idea), but for now 
it seems simpler and easier to understand if one volume control controls 
one volume and nothing else.

David Henningsson, Canonical Ltd.

More information about the pulseaudio-discuss mailing list