[WARC] Analog vs. Digital

[Leslie Hittner]

This is not a pitch for D-Star. It is a discussion of digital
vs. analog. There have been comments about how the analog
signals can get weaker and weaker and when they do the noise
increases and the readability decreases until at some very weak
point no communications is possibl. This has been used recently
to argue that the "all or nothing" feature of the digital modes
is bad. The argument has been made that the digital signal is
100% or nothing with nothing (but dead air) in between. A recent
QST letter even proposed that artifical "noise" be inserted in
the repeated signal to indicate that there is a received signal
but it is inadequate.

I believe that these comparisons are often made with SSB voice
signals, where the weaker and weaker scenerio does indeed play
out. Such is not the case with FM signals, however. With FM
voice (indeed, all FM) there is a threshold effect. As long as
the signal remains above the threshold, the signal is virturally
noise-free. As the signal strength decerases (and channel noise
increases) there is a very narrow band of signal strengths where
we heare a vary noisy signal. Then as the signal strength
continues to decrease the noise suddeenly takes over and the
squench closes. Please note: In all the years of FM voice there
has never been a suggestion that we insert artificial noise into
the repeated output to inform the operator that there is indeed
an unreadable signal on the input.

Digital voice is more like FM voice than SSB voice. Like FCM
voice, digital voice systems exhibit a threshold effect although
it is much narrower in terms of the range of signal strengths
over which the decoding is problematic vs. non-existant.
Nevertheless the end result is the same even if the mode is
different - communications is virtually 100% noise free or it is
non-existant with a narow range of "noise (FM) or intermittant
dropouts (Digital).  I think the argument that digital is "all
or nothing and that's bad" is a red herring when making a
comparison with FM voice.

With respect to D-Star, lets keep our discussion to the
strengths and weaknesses of that system (For instance, I would
also expect voice quality to be better on digital than analog. I
see that as a problem with the D-Star vocoder.) and not try to
generalize to the entire relm of digital lest we find ourselve
locked in a "conceptual prison" in the near future.

One important question to ask relative to DIGITAL vs. FM might
be how the digital systems compare over the "useable signal
strength range" of our present FM voice systems. If the digital
dropouts and shutdowns take place at or below the useable signal
levels of an FM system, then the digital system offers an
improvement. That is one objective and measureable
test/comparison I have not seen.

This whole issue came to my mind yesterday as I installed a new
TV set (Yes, we got rid of our old analog sets and got a new
digital-ready set.) I watched channel 8 (analog) with my antenna
pointed north. Got a lot of ghost images. Switched to their
digital channels (There are two digital information channels.)
NO GHOSTS! The signal was perfect. I am going to do some
experimenting with channel 10 and 13 because I can rotate the
antenna to create quite noisy images (channel 8 is too strong)
and want to see at what point the digital encoder shuts down.
I'm going to experiment with channel 31 in La Crosse as well
(They have 4 channels of digital programming - who needs cable?)
I think comparisons on UHF might be more representative, since
all of the digital channels appear to reside on UHF..

-Les, K0BAD

----- Original Message ----- 
From: "James Jefferson Jarvis" <jj at aprsworld.com>
To: <warc at lists.w0ne.org>
Cc: <robert.andrews at us.ibm.com>
Sent: Saturday, November 24, 2007 18:28
Subject: [WARC] attenuator club project?


> Hi Group,
>
> Walt lent me his Fox Hunt Attenuator today. It worked great!
>
> Basically it is an inline device that allows you to switch on
attenuation as
> needed. So when fox hunting it allows you to lower the signal
level so the
> fox isn't S9 in all directions.
>
> Walt's attenuator is from Arrow Antennas. Here is a link to
the manufacturer's
> site:
> http://www.arrowantennas.com/atten.html
>
> I believe the design started from an article published in QST
some years ago.
> It's something that can easily be built. Clare built one years
ago. If
> somebody wanted to help me, we could CNC mill the aluminum
bodies and kit up
> the rest of the parts.
>
> Would there be interest from the group in making this a club
project? Perhaps
> something that we could do at a meeting? I can't imagine it
would take more
> than 45 minutes to build. If the labor to machine the boxes
was free, I think
> the thing could be kitted for $25 or less.
>
> ?
>
> -Jim KB0THN
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