I would like to suggest implementation of ladspa 1421, matrix_ms_st_1421.
It is used for a certain microphone configuration (Mid-Side) that allows you to vary the width of the stereo field from mono to unnaturally wide.
There would be one or two simple controls. A “Width” slider and possibly a “Wet/Dry” slider.
The Width slider would be variable from 0 to 2. 0 is full mono. It gets more stereo width as you approach 1. It gets unnaturally wider than stereo as you go beyond 1 towards 2.
The Wet/Dry slider defaults 1, but is unnecessary if I am understanding it correctly.
Width would work with keyframes, so that you can start with wide stereo, and narrow it as the video zooms in on a subject. In that regard, you would almost zoom in the audio as well.
You can do this with your installation depending on which OS and how it was done. Basically, go into the Shotcut program folder followed by share/shotcut/qml/filters. Copy the folder of the audio_delay filter to a name like audio_ms_stereo. Then, edit the QML files accordingly. It should be fairly obvious how to do it without extensive knowledge of the language.
However, I am concerned that this filter works as expected with the way the ladspa is integrated into MLT, which is generic. The docs say, “MLT will create the number of plugins that are required to support the number of audio channels.” MLT works primarily with mono filters. I think it will create an instance for left and right that output respectively to left and right only. But you can try it.
I checked the MLT code, and it appears that it does support stereo plugins. It creates multiple instances as needed to make filter channels match the project/output channel count. For example, with melt -debug output I see for the audio delay filter (ladspa.1192): [filter ladspa.1192] Plugin Initialized. Channels: 1 Copies: 2
And for Matrix: MS to Stereo [filter ladspa.1421] Plugin Initialized. Channels: 2 Copies: 1
I agree that a Wet/Dry parameter is not logical to include in this filter. That is a generic parameter available to all LADSPA plugins in MLT.