Re: about group delay
your can refer to pg.89 of Bruce Carlson (Communication Systems -4th Edition) for a good discussion on the topic - the book can be downloaded from the downloads sections.
In time domain, we know that "The output is undistorted if it differs from the input only by a multiplying CONSTANT and a finite time delay".
Clearly, whenever a signal passes through a system, it must incur some delay. This constant time delay, in frequency domain (you must've studied with Fourier Transform) becomes a LINEAR PHASE SHIFT (linearly related to frequency)....
Let us take an example where a signal suffers from a CONSTANT PHASE SHIFT, i.e. let us assume that all its frequency components incur a phase shift of 70°. Now in time domain, if the frequency component with a frequency of 'x' kHz has a delay of 3 microseconds (corresponds to 70 degrees), then the frequency component with a frequency of '3x' kHz will have a delay of 1 microseconds (corresponding to 70 degrees, i.e. (70/360)*Timeperiod_of_3x_kHz)....Thus due to CONSTANT PHASE SHIFT, different frequency components of the same signal suffer from DIFFERENT TIME DELAYS....which definitely causes distortion, b/c each frequency component has a different tim period...
On the other hand, if the signal suffers from a LINEAR PHASE SHIFT, phase shift proportional to frequency, then the 'x' kHz signal will suffer from the same time delay as the '3x' kHz signal component.
This got a bit complicated....but still I hope it helps...