Confused about grouped volume controls

I’m confused by what happens to volume controls when I add an iPhone into a group.
One of my endpoints is an old Mac mini which has been persuaded to run Monterey (thanks to dosdude1). When I group that with my iPhone 13 Pro, the Mac volume in the group shows on a scale labelled with dB. The iPhone shows with a scale of 0 to 100. But when I change the volume for the iPhone, the volume changes on the Mac as well.
If I group the Mac mini with just another Mac (a bit newer and running Monterey and my Roon Core) then the volumes work independently. If I add the iPhone in then the second Mac volume continues to be independently adjustable.

Sometimes I see a message something about Airplay, which I don’t understand.

I’ve read lots of articles about levelling and zone grouping but have failed to find anything that explains this behaviour. Doesn’t mean there isn’t an explanation, just that I’ve failed to find it☹️
Volume levelling is Off.

Any idiots guide explanation gratefully received😊

Are you using dsp volume on the one showing db scale as that’s the only time it shows up.

How are you grouping them as AirPlay zones are using Roon on them for Roons RAAT output?

Hi, I’m using DSP volume on all my endpoints, various macs feeding into either USB DACS or Meridian digital processors. That way I can set the volume for each zone from my iPhone.

Grouping is simply done by selecting an audio zone, clicking on Group and adding the other zones. As far as I know AirPlay is not involved. Everything is hard wired Ethernet - apart from the iPhone which is connected by WiFi to the same domestic network.

And are you using dsp volume on the iPhone as well?

Hmm. After a good nights sleep, I’ve checked the Audio Device settings again.
Kitchen mac min = DSP Volume
iPhone 13 Pro = Device Volume
2nd mac = Device Volume

So to simplify and experiment, I changed the group to just the Kitchen mac and the iPhone 13 Pro.

The experiments in the table below go through all the combinations of volume control type for each endpoint and the effect of changing each endpoint’s volume.
Screenshot 2023-01-24 at 16.29.02

Result: if the iphone volume control is set to Device then it interferes with the Kitchen volume. Any other combination works as expected.

I haven’t tabulated the results but I also tried adding 2 other zones into the group. Only the Kitchen mac has the interaction with the iPhone volume.

I tried building groups starting from different endpoints. In each case the iPhone 13 Pro interferes with the first member of the group and none of the others. If the group is created starting with the iPhone then there are no interference between volume settings for any members of the group.

The obvious common device in these experiments is the iPhone 13Pro. So I built a group using an old iPhone 5S. I got exactly the same results as with the iPhone 13 Pro.

Finally I built a group starting with the iPhone 13 Pro and adding the iPhone 5S.
Screenshot 2023-01-24 at 16.29.21

Summary of findings:
If an iPhone in a group is set to Volume Control = Device then when the iPhone volume is changed, it interfers with the volume of the first member of the group.
If the first member of the group is an iPhone set to volume type = Device then it will misbehave when its volume is changed.

I don’t think that this behaviour is an intended ‘feature’! Sounds more like a bug :space_invader: to me.

Continuing the discussion from Confused about grouped volume controls:

I’ve found an issue where changing the volume of an iPhone using Device Volume causes the volume of the 1st zone in a group to also change in a very confusing way.

The experiments I’ve carried out to investigate the problem are in the linked post.

Could somebody take a look and tell me if this is a bug or a ‘feature’?