SmoothAdjust is pretty neat. I found I could stretch the blacks to an extreme degree without banding or blotchiness.
Code:
#avisynth
C1=ImageSource("sgrad-test-2.jpg")
C1=C1.Crop(0, 128, 512, 256).ConvertToYV12()
## SGrad
C2=C1.SGradation2(0.6, 0.6)
## Smooth - moderate
#C3=C1.SmoothCurve(Ycurve="0-0;4-0;8-12;36-66;245-235;255-255", smooth=200, dither=100, show=false)
#C3=C3.SmoothLevels(23, 1.1, 255, 0, 255)
## Smooth - extreme
C4=C1.SmoothCurve(Ycurve="0-0;4-0;8-26;36-66;245-235;255-255", smooth=200, dither=100, show=false)
C4=C4.SmoothLevels(30, 1.35, 255, 0, 255)
S=StackVertical(
\ C4.Subtitle("Smooth"),
\ C2.Subtitle("SGrad"))
## exaggerate the artifacts
return S.Levels(0, 1.9, 255, 0, 255)
Note the "exaggerate " line is just to show off the smoothing.
Agree w/ Mug Funky on the futility of trying to rescue this particular video, especially if the stage lights are constantly changing.