K7ITM wrote on Thurs, Jun 26 2008 2:45 pm

On Jun 25, 6:45 am, Tim Shoppa wrote:
On Jun 24, 11:46 pm, wrote:

I don't think I have a "golden ear" or any other audiophile quality.
In fact I'm pretty sure my ears are less good than they were when I
was a kid doing CW.

Tim N3QE

Obviously, both the time and the frequency response are determined by
the positions of the poles and zeros of a linear system (filter), but
be a bit careful about equating group delay and ringing. It's easy to
make an FIR digital filter that has constant group delay, but rings
quite nicely.

'Ringing' and group-delay over a passband are separate things but, in
practice they are are related. In passive-component filters they are
quite related.

'Ringing' phenomena can be investigated analytically in any time-
domain circuit analysis program. I use LTSpice from National Semi-
conductor...totally free for download and works on any PC. SPICE
compatible, the source stimulus can be set as a pulse of several
cycles with the rise-time, fall-time adjustable. If there is
electronic-cause ringing, it will show up at the output.

Every single passive-component filter has time delay. If the time
delay is unequal across the passband, then one will hear the ringing.
Such ringing isn't always a physical-electronic thing IN the filter
but more in the way the human brain perceives sound. True high-
fidelity electronic music systems will have near-equal time delay
over its entire passband. Unfortunately, few, if any, of the first
Hi-Fi systems makers ever published specifications on group-delay
or delay of any kind. [excluding speakers, of course, since those
and their interrelationship with a room are so acoustically
variable that no common standard could be reasonably adopted]

It is very hard to describe sound that is FELT rather than
measured by instruments. As Tim said about CW use, ringing causes
an actual discomfort. With wideband home music systems there is
very little perceived 'ringing' but there exists 'quality' which
can only be graded by comparison with another system as 'A-B'
testing. The one that FEELS like it sounds best would be the winner.

For non-audio use, such as in AM-PM ('QAM') modulation combinations
in modems, inter-symbol distortion with/without an 'eye' display
can grade things...and group-delay effects aren't the only things
to blame there, lots of others in the total path.

Simpler FSK systems such as single-channel teleprinter need
concentrate on group-delay only over a passband about twice the
frequency of its frequency shift between Mark and Space. Group-
delay there shows up more on demodulated pulse edge ditortion.
Ringing there can be seen easily at the edge transitions. A
compromise there is to have group-delay greater beyond the
needed passband limits resulting in rounded transitions;
demodulator output can be shaped afterwards as desired.

NTSC analog video examples do not really apply since the common
ACTUAL bandwidth of most smaller TV sets was so limited (down to
1 MHz in some) that group-delay effects are hidden in the
resulting video passband distortion of details on objects.

Broadcast FM is hard to define in felt-quality since FM's
'quiet' spectrum use is relatively narrow. 'Loud' passages of
music uses more spectrum space, thus group-delay effects over
a passband are more pronounced on loud passages.

... I can imagine creating ideal signals in Scilab (or
Matlab) and feeding them through various filters, and then
demodulating them. It shouldn't be terribly difficult to do that, but
I'm not volunteering.

Machine-in, machine-out systems such as teleprinters and
modems can be compared relatively easily with computer
analysis programs. Regardless of the computer program,
none can substitute for what is FELT in the ear-brain
sensory system with acoustic input. I see a valid test as
only A-B or A-B-C (or more) comparisons using the same
audio or audio-modulation input. That is a LOT more work.
Most of what I've done in that delay subject are locked into
lab notebooks in corporate ownership and it involves many
man-weeks of investigation. The best one can hope for,
in my estimation, are general guides on limits from the
many and varied radio services.

73, Len AF6AY