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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
