And I doubt any ham rigs have these added features.
Chuck
WB2EDV
----- Original Message -----
From: Cort Buffington
To: Repeater-Builder@yahoogroups.com
Sent: Saturday, October 03, 2009 8:45 PM
Subject: Re: [Repeater-Builder] Narrow(er) band FM
Eric,
My goal was to not employ any additional "trickery" to make the things sound
better -- to test it with as many things being equal as possible. Once we get a
feel if there's a real-world "range" difference, I'll throw in the extras and
try again. Thanks for pointing this out -- I wasn't really clear that I wasn't
going to enable features designed to improve the super-narrow setting.
73 DE N0MJS
On Oct 3, 2009, at 7:40 PM, Eric Lemmon wrote:
Motorola recommends turning on the "companding" feature in CPS when using
narrow-band channels. To see how this works, go to the Advanced tab for the
personality you want to test. The normal settings on this page are
"Disabled" for Compression Type and Expansion Type, and "De-emphasis &
Pre-emphasis" for the Emphasis Selection. To turn on companding, select
"Full Compression" for Compression Type, and notice that Expansion Type
automatically changed to "Full Expansion." Leave Emphasis Selection alone,
it is overridden. Save the codeplug to your radios.
Do this on both radios, and run your test again. You should see a
significant improvement in clarity and a lower noise level.
73, Eric Lemmon WB6FLY
-----Original Message-----
From: Repeater-Builder@yahoogroups.com
[mailto:repeater-buil...@yahoogroups.com] On Behalf Of Cort Buffington
Sent: Saturday, October 03, 2009 5:14 PM
To: Repeater-Builder@yahoogroups.com
Subject: Re: [Repeater-Builder] Narrow(er) band FM
I said I'd report back....
XYL and I were out with the EX500s today. I copied the channel we normally
use for simplex and changed nothing but made it a narrower channel.
Results. Noise squelch seems sloppier (normally I use DPL or PL, so that
really isn't too big of a deal), audio fidelity is reduced. We weren't far
enough apart to really test range. I think the audio quality was still
pretty good, but when do do "ok, go back to channel 3 now..." It's quite
clear the narrow sounds quite noticeably better than the narrower.
73 DE N0MJS
P.S. I also wonder about the frequency accuracy of radios going to the
super-narrow band. I've looked at a lot of ham rigs on my service monitor.
They are usually worse than the commercial radios in this area. Isn't that
going to have a more pronounced effect?
On Oct 3, 2009, at 4:22 PM, John Sehring wrote:
I think it's worth repeating (no pun intended!):
0. In a narrower band FM system, with only the carrier present, you
may well get a bit more ultimate quieting sensitivity (but not necessarily
better SINAD) as the receiver's IF bandpass (selectivity) is narrower,
letting less noise thru. However, the question is: how much of that slightly
increased sensivity is actually useable?
1. Reducing FM deviation to less than about 5 kHz results in less
power in the sidebands, which sidebands convey the intelligence (the carrier
is just there to enable the usual demodulation (detection) process). As the
detector needs the sideband energy, even granting (1) above, you'll have
less recovered audio available. The signal's spectrum then begins to
resemble that of an equivalently-modulated AM signal; the major difference
is that with an FM signal, the carrier is 90 degrees out of phase with the
sidebands, whereas with AM, carrier and sidebands are in phase.
2. Reducing FM deviation (and narrowing IF bandpass) allows more
distortion in receivers at low (fringe) signal levels, so it's less able to
deal with things like multipath propagation, AM noise, FM noise (yes, there
is such a thing), and co-channel interference. Signal to noise ratio is thus
reduced.
3. Squelch action becomes sloppier because the demodulated audio
spectrum which is used for noise-operated squelch is quite a bit less when
using narrower band FM. Rule of thumb for the squelch detector's bandpass:
it extends from A) just above the voice audio band, say, 4 kHz, to B) about
one-half the IF bandwidth. The latter is distinctly less, so the squelch
sensing bandpass is less making squelch action less responsive.
If you use an audio spectrum analyzer to look at a demodulated FM
signal, you can see the spectral differences between 75 (FM broadcast), 25
(NTSC TV sound), 15, 5, and 2.5 kHz deviated signals, esp. as the signal
strengths are reduced to zero.
--John
--
Cort Buffington
H: +1-785-838-3034
M: +1-785-865-7206
