Group Delay Visualized 

The more we learn, the more we realize how little we know. Great video!

This video is awesome! Starting with a delay line, going for a regular filter and ending with a constant phase (= frequency dependent delay) filter is an excellent explanatory journey. For a week now I have been breaking my head over the question if phase actually introduces a delay in the output. e.g. let's take the constant 90° phase shift filter and place 1000 of it in series. Does the 1000x90° correspond to a measurable (or audible) delay at the output? or does the phase wrap around at the 180° mark and there is no delay at all? According to your video it is the latter. I am sure if I keep thinking about this video enough, i can finally wrap my head around it.

Wouldn't delay in amplitude envelope mean that audio gets delayed (if we use real audio for testing)? No one listens to pure sine waves rt? And that's why audiophiles concerned with GD.