[Lowfer] Calling all NAVTEX Experts

[JD]

The nominal 518 is the center frequency.  The concept of "dial frequency" 
and sorta-agreed offsets for SSB bandwidth reception is virtually unknown in 
the commercial world.  Where a signal really exists within the spectrum, the 
amount of shift, and the direction thereof, are the only important things 
outside the amateur community, and what the person and hardware at the 
receiving end does with the signal thereafter, frequency wise, is entirely 
up to them.

It came about in amateur practice directly (and probably solely) because of 
the convenience of using an audio frequency exciter for digital modes, to 
drive existing SSB rigs for transmitting.  Unfortunately, with the 
prevalence of transceivers as both the signal source and receiving hardware, 
we've also come to accept the converse/downside of that approach, which is 
the wasteful degradation of receiver dynamic range that results from using 
full SSB bandwidth where CW bandwidth would be better.  (It's not a question 
of the software excluding signals outside the detection band...it's an issue 
of what the receiver itself does with them before the audio leaves it.  I 
know I'm not the only one to have noticed that, having read what at least 
one member of this group said about it on the RSGB reflector last winter.)

Since 1500 is a terrible pitch for listening to CW, no receivers 
conveniently accomodate that offset for digital modes.  One is therefore 
faced with the relatively poor alternatives of lying to the receive software 
about the dial frequency to make the displayed frequency correct (which I 
find most effective for weak signals, but woe be unto him that maketh 
arithmetic errors, for surely he shall receive naught for his night's 
labors); or cobble together a combination of manually set bandwidth and IF 
offset that may not be terribly compatible with the radio's internal 
gain/loss budget (I do that sometimes, but it's tricky and works slightly 
less well than using CW mode and lying about the dial frequency); or just 
live with the shortcomings of the voice bandwidth sideband approach (and its 
artifically large SNR numbers) which I wimp out and do by default most of 
the time myself, though it's clearly less effective.

Between the time I started writing this and the time I returned to it, I see 
Lloyd has posted the link to Steve's Web site, which is one of the best 
NAVTEX introductions I've ever seen, so I'll cease and desist now. :)

John D