[WARC] Analog vs. Digital

[Leslie Hittner]

Pardon my typing errors!

-Les

----- Original Message ----- 
From: "Leslie Hittner" <lhittner at hbci.com>
To: "Winona Amateur Radio Club general discussion"
<warc at lists.w0ne.org>
Sent: Sunday, December 02, 2007 12:08
Subject: [WARC] Analog vs. Digital


> 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
>
>