[Elecraft] K3 Questions

[Stuart Rohre]

In the basics of digitizing the signal you have to clock the conversion at 
least twice the highest frequency you want to reproduce in the voice (or 
audio) signal.

Now, you can sample a number of ways.  You can use one sample per clock 
cycle, or you could do some of the phase shift techniques in encoding, 
taking multiple samples per cycle. (Quadrature sampling for example).

Thus, if you narrow the band in which the digital signal is transmitting, 
you normally give up frequency response and intelligibility.  Note the AOR 
open protocol system developed by the G -ham transmits into your audio input 
of a SSB radio, thus the bandwidth is no more than the SSB bandwidth, or at 
least that is what I got from an AOR ad.

Another big issue with digital coding is duty cycle.  It might not be within 
the finals power envelope to support digital modes other than CW or TTY; as 
TTY for example, continuously transmits a signal, and its information is 
varied by the shifting of the frequency, while the amplitude stays at a 
constant level.  TTY has a high duty cycle compared to on off CW.

In most rigs where you want to run PSK or TTY you back off the power to half 
of SSB.  For PSK, it often is enough to do that, and still get the message 
for the variants of PSK that are error corrected.

I think hams often want everything in one box, but don't want to pay the 
price that would require.  There is no free lunch in RF engineering or other 
types.

Until there is a world wide standard agreement on a digital modulation for 
voice for ham radio use, there is not much sense in marketing a unique 
system in the radio.  In fact, the advantage of ham radio, is that it is 
mostly analog, and the signal can still be decoded, (heard) under conditions 
of fading, which is not true for digital methods that set a minimal SNR 
threshold to work.  Thus in disaster, we can get a message thru when wide 
bandwidth, power hungry digital encoding fails.

Most digital systems such as trunking radio, have single points of failure 
which are catastrophic.  Ham communications are multi point to multi point, 
or redundant. IF one path fails, there is another station to station path 
that can work.  The cost of sophisticated digital encoding such as Pactor 
III means that not many hams are going to have a spare packet modem box if 
the first one fails.  However, you usually have several microphones that can 
work on a given radio at lower cost, if the first one fails.  It is also 
easier to fix, as the failure modes are mostly mechanical, (connections) 
rather than a combination of electronic circuits and mechanical connections.

-Stuart
K5KVH