Return currently selected video source object
src = getselectedsource(obj)
src = getselectedsource(obj) searches all the video source objects associated with the video input object obj and returns the video source object, src, that has the Selected property value set to 'on'.
To select a source for acquisition, use the SelectedSourceName property of the video input object.
obj must be a 1-by-1 video input object.