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



For various transmission types, how much group delay variation can be
tolerated in
the receiver before it causes problems recovering the original
signal? I realize that it
all depends ... I'm just interested in getting a rough idea.

For example, at what point is:

1) SSB voice reception noticeably affected?

2) the ability to receive images from weather satellites affected?

3) various amateur radio modes such as PSK31, RTTY, and MFSK16
affected?

4) HD Radio specs 600 ns as the max for the transmitter, what's the
max for the receiver?

I'm curious since the group delay variation of SAW filters available
for IF filtering seems
to vary widely.

I have been playing with homebrew crystal filters (following W7ZOI and
Bill Carver/K6OLG) for CW, as well as audio filters, and can tell you
that on CW the difference between a super-sharp-in-frequency
Chesbyshev filter (typical in ham equipment for a long time now) and a
more constant-delay (e.g. Gaussian to 6dB or 12dB, or equiripple
linear phase) filter is like night and day.

My impressions are done "to my ears", not to a spectrum analyzer.

The super sharp in frequency Chesbyshev filters have horrible horrible
ringing especially on say 40M or 80M in the summer with the QRN. I can
hardly listen for a few minutes without getting disgusted. BUT... they
do have a real advantage during say a contest when there's competing
signals every few hundred Hz.

At the other extreme the constant-delay filters sound remarkably clear
and transparent. They do not have such a sharp stopband, but my ear
makes up for that most of the time. Ringing from summertime band noise
is not nearly so tiring.

Most of the filter simulation programs (AADE, SPICE, NatSemi's cool
new WebBench filter tools, etc.) let you look at not just frequency
response but also phase response and (maybe most importantly for
summertime QRN) impulse response. Having run the simulations and
listened with my ears to my experiments this summer, I cannot
emphasize how much more enjoyable it is to use a Gaussian-to-6dB or
equiripple linear phase filter in CW.

Often when the bands are not crowded but there is QRN, I far far
prefer a simple two-pole crystal filter designed for CW in the first
place (e.g. my Heath HW-16) to any fancy-pants 8-pole or 12-pole
modern filters in my new rigs.

I notice you ask about a lot of digital modes but not CW. My ears have
been listening to CW for 30-some years now and I can do a lot of
processing in my brain. But what my brain cannot remove is horrible
filter ringing. I don't know how those other digital modes stack up...
maybe computers are better at removing horrible ringing than my brain.

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.

I suspect the people who design RF communications systems using modern
modulation schemes know the answers to John's questions. There's
probably another newsgroup where you'd get more answers. Or--run some
simulations. 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.

Cheers,
Tom