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RE: P3D information



> Digital phones are great when they work.  However, due to the 
> extremely low
> sampling rates that are used in order to get the necessary 
> narrow bandwidth,
> voice quality is sometimes quite poor.  Now this is partially 

I use GSM here, and find the voice quality is quite adequate.  I wouldn't
rate the voice quality of a weak SSB signal that brilliant either, by the
time you factor in noise and the inevitible small frequency error that
creeps in! :-)  It is true that marginal digital audio does require the ear
to be attuned differently than a marginal FM or SSB signal, especially since
digital can be chopped up in some really wierd ways, but in some
environments, the digital signal can still be better (though not as good as
analog FM).  One environment is one that I'm often in, where there is a
narrow acoustic dynamic range available, so the audio level variations of
SSB become more of a problem that digital artefacts.  I also deal with some
grossly distorted and chopped up VoIP audio from those Internet chat
programs, so I'm probably equally used to poor digital and analog signals.

Just making a point that the digital audio on phones is not necessarily as
bad as people make out, and sometimes it maybe a case of what one's used to.

> due to the
> fact that most PCS providers think they can build out a 1.9 
> GHz network with
> the same cell site spacing that an 800 MHz network has.  Or 
> at least the

My experience as a user is soley based on 900 MHz GSM.  I haven't yet used
CDMA or 1.8 GHz, so can't really comment on either.

> marketing people think this way.  The other issue is that 
> background noise,
> and so forth is not digitized well.

True, but I tend not to notice noise these days, unless it becomes
intrusive.

> CDMA in particular is horrible from my experience.  I had 
> always thought
> this was going to be the best and winner over any of the 
> other modes.  In

On paper, you'd think it would be better.  What have you noticed with CDMA?

> fact, at Motorola, we had scrapped our domestic TDMA 
> development because of
> the difficulties Ericsson had when rolling out their TDMA 
> systems.  We went
> with CDMA  and GSM only (with the exception of our PDC system 
> for Japan).
> Now, after having used CDMA, I think TDMA is a better choice. 
>  Personally, I
> use TDMA with Nextel's system and find it's voice quality to 
> be better than

Well, as I said, GSM voice quality is more than acceptable to me.  However,
a TDMA based system has some limitations over here in rural areas, where
users expect wide range coverage.  Timing issues come into play, as you're
no doubt aware, though people in the industry have informed me that there
are ways to extend a cell's coverage at the expense of capacity, which is
usually a reasonable tradeoff in the bush.

> anything else out there (we have no GSM really here in Chicago yet).
> Nextel's TDMA system using Motorola's iDEN technology is 
> really the best I
> have heard.  The voice quality is nearly as good as analog.  
> It beats CDMA
> hands down.

So, what does CDMA sound like?

> The other problem with digital networks as someone else 
> pointed out is that
> you either get S9 reception or S0.  There is no in between.  
> This is good in
> that you get better signal reception with low received signal levels.
> However, when you hit the point where you can't decode the 
> bits any more it
> just falls off like a brick.  With an analog system you can 
> still make sense
> out of the static.

This is true, and there's arguments both ways, I suppose.  For really weak
signal work, where you're in the noise floor, I can see this being a
problem, but for everything else, I don't

> My biggest concern is that all of the "3G" cellular systems 
> are based on
> CDMA technology which I think is the wrong way to go based on 
> the quality I
> have seen from CDMA so far.  We could end up with yucky 
> cellular networks
> very soon.

Well, time will tell.  Is the problem one of the underlying technology?  Or
simply the use of voice CODECs which overly distort the voice?

> Now, to bring satellites into play here as that is what this 
> is about....
> 
> Given the fact that satellites are sometimes weak signal 
> devices, how well
> will digital forms of modulation work?  I know P3D will be 
> far better than
> AO-10, but there are times on AO-10 when I've worked stations 
> that I don't
> think you could work digitally due to low signal levels.  
> Yes, I know that
> things like PSK-31 can copy signals below the noise floor, 
> but that data
> rate certainly isn't high enough to support voice.  Of course 
> if P3D is as
> good as it is supposed to be, perhaps this is all moot and 
> we'll have plenty
> of signal level to play with.

Time will tell, and there's only one way to find out.... wait till the thing
is in the sky and operational. :-)  Perhaps we amateurs are the ones who
will end up looking into some of the issues of digital systems that
commercial operations overlook due to marketing, project timeframes or other
pressures that they face.  And I suspect we are probably in the unique
position of being able to compare analog and digital side by side for an
extended period (years, at least).
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