How to obtain unloaded quality factor using group delay?

Hi everyone
The equation below shows how to obtain unloaded quality factor using group delay(f0 is resonant frequency. IL is insertion loss at f0. td is group delay). But I do not know how this equation is obtained. Can anyone give me some clues? Thanks.
Qu = pi*f0*td*( 10^(IL/20) )/( 10^(IL/20) - 1 )

That equation is derived in Matthaei's book in section 6.15 pag. 343. The equation number is 6.15-4.

I'm also looking for a similar, but more general, derivation. If you find one please let me know.

Reference: Microwave filters, impedance-matching networks, and coupling structures. Matthaei, Young, and Jones.