Stereo bus, mono source, mono destination

[dmitch57]

 

Short version: What happens when a stereo bus connects mono source to a mono destination?

Long version: A mono aux mixer channel is configured for its output to go to a bus. (All busses are implicitly stereo these days.) A mono audio track is configured to take its input from that bus. 

Seems simple. But:

Why is there a pan knob for the aux channel? Why does that pan knob actually effect the level of the signal going to the audio track? 

What's really confusing is the behavior of the pan. I can think of two internal/hidden configurations of the stereo bus, each of which would have a different behavior when panning the mono source:

• The mono aux source drives both sides of the stereo bus, and the two sides of the stereo bus are summed to drive the stereo audio track. In this case the behavior of the pan knob on the aux channel should be symmetric - you'd see the same signal level at the audio track whether the aux pan was full left or full right.

Or:

• The mono aux source drives one side of the stereo bus, and that side of the bus drives the mono audio channel. In this case the aux channel's pan should cut the signal of the bus to zero when panned to one side, and send full strength when panned to the other side. 

Neither of these are what I see:

Channel "mono" is a mono audio track. Channel "gen" is a mono aux channel with a signal generator sending a sine wave to the "Aux 1" bus - not via a send, but by its output. 

Pan track "gen" to center: "gen" shows -12 dB, "mono" shows -12 dB
Pan track "gen" to right : "gen" shows -9 dB,  "mono" shows -15 dB
Pan track "gen" to left  : "gen" shows -9 dB,  "mono" shows -9 dB

This makes no sense to me. I can't figure out a way that a stereo bus, connected to a mono source and a mono destination, could result in these levels. My pan law is set to -3dB Compensated, but I see similar unexpected and asymmetric results with all of the pan laws.

Any words of wisdom out there? 

Thanks.

 

[David Nahmani]

2 hours ago, dmitch57 said:

This makes no sense to me.

Same here. Something looks really fishy here. Looks like a bug to me. 

[JakobP]

Fwiw, same here in 10.5.1...

[dmitch57]

Reported to Apple as a bug on 4/16/2023. The workaround, I guess, is to make sure a signal going to a mono track via a stereo bus is always panned to the center of the bus.

[David Nahmani]

Great, thanks for reporting it. I'm moving this to the Bugs section.