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

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.
https://launchpad.net/~diwic