"charlesb" wrote

Isn't 1200 baud the limit on 10 meters, and 300 baud lower down?

This is a common misconception about baud and bits. You are correct
about the baud rate limitations, although the baud rate is what is
known legaly as the symbol rate.

Suppose I design a modem that operates at 50 baud (20 ms symbol rate),
obviously I can use this below 10 meters. Now, let's say I modulate
that rate with 36 carriers of DQPSK. That is, there are 36 carriers
of four phases (two bits per carrier), or 72 bits in 20 ms (50 baud).
There's fifty 20 ms periods in a second, so we have (72 bits x 50), or
3600 bps.

Now obviously we have learned something in the last 20 years, and that
is, that HF is a bad medium, and you can either retry your transmissions
until the band quits, or you can insert some sort of error correction into
the transmission, to prevent retransmission. So let's say we limit the
information to 2400 bps and insert 1200 bps of error correction (FEC),
for a total of 3600 bps.

Suppose each of the 36 carriers is a multiple of 62.5 Hz, then we can
say that 62.5 x 36 is 2250 Hz. But we don't really want to go all the
way down to 0 Hz, so let's say we start at 312 Hz (62.5 x 5) and this
will push the right side out to (62.5 x (5 + 35)) or 2500 Hz.

In summary, 36 carriers takes a lot of bandwidth, but no more than
a standard analog sideband. The fact that you can fit 2400 bps
voice, data, and image into the channel might be worth the complexity.

P.S. I just described the G4GUO modem :-)