MSG # TR  SIZE TO     FROM   @BBS   DATE    TITLE
24847 PN  2477 KA9SCF WB9MJN   ---  921009 56 KB...
Forwarding path: WB9YAE N9HSI 



Hi, To do 56 kb in the legal spectrum permitted by the FCC on 220 , and
440, a special kinda modulation is required. It s called MSK for medium
shift keying, or sometimes FFSK for fast FSK. This is what a WA4DSY modem
is all about. To achieve the narrow enuf spectrum, which fits in a 100 Kc
channel, and is about 60 KC wide at -10 dB, a Inphase, and Quadurature
modulators are used, in the 30 Mhz range. The outputs of these modulators
is combineed, to generate the modulation. Each modulator is driven by
a digital function generator. Sound allot like a phasin SSB modulator?
Yes, it is, except the waveform applied to the modulators is derived from
a PROM lookup table, instead of voice. 

The WA4DSY modem kit costs 250 dollars. That puts out a signal around 30
Mhz. Then the signal has to be translated to 220 or 440. That usually takes
a 300 to 400 dollar transverter. Thats the radio side of things. To do 
56 KB packet, u need a controller that can go that fast. TNC2 s can t,
even tho they were used in the early testing of the WA4DSY modem, and there
is a KISS56 prom image that comes along with the modem. DMA cards available
from Gracillis, or the Ottawa Ontario Canada packet club are probably the
most appropriate for the user station. These are around 200 to 250. The
PacketTEN was developed initially for our Cellnet project. It has 3 ports
that can do 56 KB. Actually allot faster, but the three fast ports can 
handle 56 KB all simultaneously, full duplex, no problem. It s around 
1000 dollars. 

So, yes, the stuff is expensive. Probably the minimal station , providing
u have an AT already, is 800 dollars. And u have to build up and package
the modem, urself.

At this point in time, there is no 56 KB user support access in the area.
There are no plans for it either. There are a few people expermimenting
with simplex 56 KB stations, but not in a very active manner. The one
link, the CELLNET prototype would be jammed if users tried to access it,
since its full duplex. That s part of the CELLNET idea. Being full duplex,
it can t be used for access, at least not legally. Since transmitting on-
top of one of the active channels, that run all the time while in use, is
an act of malicious interferance. 

73, Don.


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