RadioBanter - View Single Post

July 6th 05, 03:17 PM

wrote:
Dee Flint wrote:

Please show me and everyone else how we can run more than 300
baud on HF
without exceeding reasonable band widths. There are a whole
lot of things,
not just video, that would be nice to do.

The answer (in large part) is to use different modulation and encoding
schemes. Such as QPSK instead of BFSK. Multiple carriers spaced just
far enough apart to avoid interference. Simultaneous AM and PM.

Of course such modes may not have the HF performance we're used to
from, say, PSK31 or SSB. You don't get something for nothing; Shannon's
Theorem shows that the increased data rate in a given bandwidth comes
at the price of needing more S/N to get acceptable results.

That old s/n bugaboo. PSK31 does very well at low power levels in great
part because the PA of present day transmitters work well at lower power
levels. Crank 'em up to full power, and things aren't always so pretty.
You can visually see the results on the waterfall, just as you can see
when the drive level is too high.

Now is that an insurmountable problem? No. But it means that we would
probably have to design new transmitters that can put out a PSK-friendly
output at high power levels.

Another method is of course to increase the ERP. So we can put up a
directional antenna.

But now we're certainly a long way from simple, and many present day
rigs and users wouldn't be able to participate in the fun.

I wonder if any experiments have been performed by Amateurs along these
lines to find a practical limit to how many different phase angles can
be accommodated. Sounds like fun.

As K0HB points out, the 300 baud limit only applies in the CW/data
subbands. If you can stuff "TV" into a reasonable bandwidth, it can be
sent in the voice/image subbands.

As a simple example, imagine 25 PSK31 carriers spaced cheek-by-jowl in
a typical SSB bandwidth, running QPSK. If one gets you 100 baud, 25
will get you 2500 baud. Go to 8PSK and you get 5000 baud. Then add
compression on top of that...

PSK31 is 31 baud, so the numbers have to be shifted a bit. The baud
rate was chosen because it is around the level that a good typist can
type at. But it can be changed PSK100 baud easily, just sacrifice a bit
of bandwidth.

How can we do it? Bandwidth is directly related to baud rate.

Only if "all else is equal". The trick is to make the tradeoff
somewhere else. The familiar "56K" modem trades off S/N rather than
bandwidth.

It's not a complex subject at all.

You've probably heard the old engineering adage:

"You can have it fast, good or cheap. Choose any two"

Same with light bulbs:

Bright, long lasting, or cheap.

All Shannon's Theorem does is equate fast to data rate, good to S/N,
and cheap to bandwidth.