On 12/04/2010 08:07 AM, John Goerzen wrote:
Hi folks,
Well I've got soundmodem 300 baud working, sortof, with 900Hz and 1100Hz.
I'm using it on HF. I seem to copy other stations just fine, but they
have difficulty copying me. They can, sometimes, but it is hard going
and lots of retrans on my end.
On the output side, I've tried running loud volume and quieter volume.
K7TMG suggested that I set the power on the rig to 100W, then reduce the
audio volume to it until it is transmitting with only 50W. That maybe
helped marginally.
Any other suggestions on what I could do to improve the TX side of
soundmodem on HF?
Several suggestions:
(1) Use a standard oscilloscope, and take look at the waveforms
of the signals you are generating from your sound card's
line outputs.
I found that on one of my laptops, trying to generate full-
amplitude signals was "flat-topping" the audio waveform...
it was being clipped quite badly. Turning down the sound
card "master" volume reduced the amplitude but did not
eliminate the flat-topping... turning down the individual
"PCM volume fixed the problem entirely.
This was clearly a design flaw in the sound "card" in the
Dell laptop... other sound interfaces did not exhibit
the problem.
(2) Check the voltage levels of the signals going into the
HF rig's input (auxiliary or mic) - you may be overdriving
the input, or there might be a DC offset on the signal for
some reason. You may need a DC-blocking capacitor or
a resistive padder to match the levels required.
(3) Use an RF monitor of some sort (either a monitor scope
such as an old Heathkit HO-10, or an oscilloscope)
to take a look at your RF envelope and make sure it
is clean and not distorted.
(4) Have a friend some distance away take a look at your
signal on a "digital modes" program's waterfall
display... make sure the distance between the tones is
correct, and make sure you aren't "splattering" due
to distortion somewhere along the way.
(5) If you aren't using a transformer-isolated audio
interface between PC and radio, try one... if you
are, try adding a few ferrites around the cable to the
radio. You might be getting transmitter RF coupling
back in the the audio, which would cause distortion and
mess up the waveform.
--
To unsubscribe from this list: send the line "unsubscribe linux-hams" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
