[amsat-bb] Digital Radios

[BobsImsai8800@xxxxxxx]

In affirmation of Wayne's comments, it appears that some people have been 
getting straight digital to digital radio communications mixed up with 
communicating through a network (CDMA, TDMA, etc.).  Through the network, 
there is a master control and all communications goes through it. Hams are 
more used to straight one on one communications.  

However, there are some great uses for the network type of digital radio in 
satellite communications here.  How about many simultaneous conversations on 
a single frequency FM type satellite instead of just one at a time?

Bandwidth is another problem compared to analog, according to Bell Labs, it 
takes 64 KB per second of data input to get the human voice through -- at 
least that is the rate at which it is first digitized in some equipment.  
With the current technology that is for sale and in use commercially, using 
FEC, etc; the bandwidth has been reduced to an FM channel width or less -- 
but it is not quite an SSB bandwidth yet. That would require about a 20 to 1 
reduction in bandwidth.  Widespread use of even narrower bandwidths will be 
commercially available and in widespread use soon, but I don't think they are 
an improvement over SSB yet.

With 15 years of experience in digital 2 way radios, I have seen that the 
commercial digital radio technology that is in use today does not enhance the 
range, it actually reduces it.  What digital radio gives you is a very 
constant (but smaller) circle of good communication coverage compared to 
analog.  Analog radio will work from a very strong signal level to a very 
poor signal level.  Analog communications is there until the signal level is 
very weak and a lot of information is being missed.  With analog, you have 
that extra range where the communications is very poor but understandable. 

Digital radio will work very well until the FEC, etc, fails and the 
communications goes from very good to unreadable with a small additional 
reduction in signal level.  Unlike analog, digital is pretty much there and 
very readable, or not there.  You don't get that extra range with digital 
where the signal is very poor but readable.

TNX,

Bob Seydler
K5GNA

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