A while ago some colleagues and I had a vivid discussion whether group delay can be considered frequency dependent delay.
What got the discussion started was an article, in German industry magazine Production Partner, stating that group delay specifically isn't frequency dependent delay but delay of the bulk energy in a group of frequencies. Click here to read the article (German).
Group delay is the first derivative of the phase response (figure 1). It describers the steepness of the slopes in the phase trace. The unit is time expressed in seconds.
The definition according to Wikipedia is:
"Group delay is the time delay of the amplitude envelopes of the various sinusoidal components of a signal through a device under test, and is a function of frequency for each component."
I attempted at condensing my thoughts into the PDF at the bottom of this article. Be warned though, this is a controversial topic and as far as I can tell no consensus has been achieved.
Anxious to hear what the hive-mind thinks.
Click here to read (German) the valid concerns of fellow instructor Bodo Felush.