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[aprssig] Fwd: Soundmodem recognized by Xastir, not receiving

Jim Alles kb3tbx at gmail.com
Wed Aug 25 17:46:53 UTC 2010

Can anyone suggest a good place to post something along the lines of
the following, I pulled it from a thread I was writing, and I will
need to edit it if anyone thinks it is worthwhile.

Thanks, Jim Alles KB3TBX

This will address the stuff external to the computer, mostly audio.

Another note is to double check the scope diagnostic - make sure the
wave form expands from the center of the screen.  If it 'grows' from
the bottom (or top) of the screen, you have a DC offset on the audio
line - unplug it.  I fried the audio section of an HTX-202 this way.
The Shack sells audio isolation transformers for $17 - cheap

John said:

>Could the signal coming from the scanner be kerchunking it where the signal is
>received but the squelch opens too late for the complete signal to come through?
It is possible, but I would assume you know how to adjust the squelch,
and the radio is working OK.

I run my scanning radio with the squelch wide open.  Try this - go
into soundmodemconfig Diagnosis, and look at the modem screen.  Make
sure all of the packets you can _solidly hear_  on 144.390 (with
another receiver, maybe) show up reliably as mostly readable text (as
**** said - it is not all clear text, some things are encoded), but it
should make sense each time - call signs, 'wide2*', etc.  You can't
just rely on the DCD indicator - the carrier is a single modulated
tone, easy to pick out of the noise.  The data is different, and you
have to have a much better signal to decode the packets.  The antenna
can make all the difference in the world.

Try the effects of your squelch control while comparing.  Soundmodem
seems to be perfectly happy listening to static all day.  (Squelch
wide open is not the solution if it is a  transceiver and then refuses
to transmit on a 'busy' channel).  One possible effect is if your
sound card has an automatic gain control, or some other form of
compression, the static will be seen as louder than the packet data,
and this *may* reduce the volume level for you.

Another audio issue is mic level, line level, and speaker/headphone
level interconnections.  Although they can all _appear_ to use the
same connector, each is about an order of magnitude different voltage
(well, power).  The unknown variable is what level does a compter's
particular input expect - too many variations to advise anyone in a
generic fashion (and laptops are worse).  If you are coming out of a
speaker jack on the receiver, however, I would use an attenuator.
These are readily available at the mall with a 1/8" phone jack on one
end, and RCA connector on the other.  You may need additional
adapters.  Be aware, that most of the jacks on a computer are stereo
(except a dedicated microphone jack), and you should take that into
account as well.

Start by running the volume adjustment on the receiver at about 75%
up, and attenuating/adjusting downstream from there.  This will
minimize the effect of amplified noise/hum from your connections.

My scanning receiver happens to have a tape output, which is line
level, and I have a separate line level input on the expansion sound
card, so I have no attenuation in-line.

I adjusted my input level with rf static at about full scale on the
'meter', and the packet data comes comfortably below that (you want a
little headroom).

I hope this helps.

Jim A., KB3TBX

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