Re: HF DV:


Richard - VE7CVS
 

Actually, 1200 bps over FM has a lot of redundancy that makes it work well in most conditions (if properly configured) - it's just not a very efficient use of bandwidth!

I implemented some of the very first Bell 202 modems for packet radio back in 1979. That 'standard' that you see know comes from the VADCG (Vancouver Amateur Digital Communications Group), we got a stack of Bell 202-standard modems from the Canadian Imperial Bank of Commerce for a whopping $1.75 each - we bought them by the pound!

The idea was to come up with a cheap, easy-to-implement-with-existing-2m-rigs modulation scheme - radios of that day were notoriously poor for digital radio, I modified ICOM IC22-U and IC22-S radios to bring their squelch detect down from several hundred to several tens of milliseconds - and other radios were even worse, some of the early synthesised rigs took forever to stabilise. Some of the squelch mods, and the 9-pin plug wiring standards for ICOM rigs came from me (although I was totally oblivious that I was creating anything resembling a 'standard'...).

Modern radios can do 1200 bps with relative ease. It's still terribly slow, we at the VADCG saw 1200 bps as a stopgap until we could bring out fast, 220 MHz radios. Unfortunately, we never got that far. :(

Another group that I belonged to in Vancouver brought out a very sturdy 56 kbps standard system (Dennis Rosenauer was the main instigator - and stellar RF engineer - for this system - for Dennis, 70 cm 'is DC' :-)


- Richard

On 3/17/17 4:12 PM, Stuart Longland VK4MSL wrote:
On 17/03/17 20:54, Andrew O'Brien wrote:
While I would like this to succeed, the SNR required for digital voice
on most HF bands is such that it renders DV as only useful between
stations with a lot of power and/or really high gain antennas.
Well, this can be true of other systems like FM too. 1200 baud AFSK
over FM is actually pretty terrible as a modem in performance.

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