Allow sends to multiple buses from a single AudioStreamPlayer node

[ellenhp]

Draft state for now, Implements godotengine/godot-proposals#1836

Right now it uses linear units, and the loudness across all sends is not zero-sum. Those things could both change depending on feedback.

• Production edit: This closes Implement the ability to send audio sources to multiple buses godot-proposals#1836.

[ellenhp]

One comment on this is that we should probably put a little warning on the node somehow if it doesn't have any sends, and perhaps even make the minimum number of sends 1 instead of 0. If a user attempts to remove the last send, one is added for the Master bus. While testing I ran into a situation where I wasn't hearing audio playback and it wasn't clear that the player was configured with no sends.

[fire]

I have a preference for using one unit across every parameter for loudness. DECIBELS.

@ellenhp Mentioned

crossfades with linear units are easier, and a huge reason this exists is crossfades, but we have linear2db.

So I wanted to poll the group.

[ellenhp]

Another tricky thing is that people probably won't understand that the decibels in the sends will be relative to the overall volume, not an "absolute" volume.

[fire]

Won't the linear format will have the same confusion?

[ellenhp]

I don't think so. I do generally agree that we should probably be consistent with units, but people tend to understand linear units better than decibels in general so I doubt that people would have the same confusion.

I will switch this to decibels I think then maybe we (I?) can write a tutorial about audio that covers how to do a loudness-preserving crossfade between buses.

[TokageItLab]

I think that AnimationTree's implicit AudioStreamPlayer volume change should be eliminated and the user should be able to arbitrarily change the volume with ValueTrack. (Ref: #55218)

The only sense in which the current implicit blending exists is to bypass the linear blending of the AnimationTree and blend the volume on a log scale. With this fix, we should be able to consider making the implicit volume change in AnimationTree eliminated.

[ellenhp]

I don't think that's related to the sends of an AudioStreamPlayer though. The sends just determine where audio is sent, not how loud the stream is. Plus, like fire said it probably makes sense to remove all the linear units everywhere for audio. I was very confused by your comment in that PR which is why I hadn't responded, but I just went and left a comment. Let's continue discussion there for animation topics.