soundmodem TX on HF again

soundmodem TX on HF again

[John Goerzen]

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?

Thanks,

-- John

Re: soundmodem TX on HF again

[Larry Levesque]

Ask the sun God for some solar activity. :) 

On Sat, Dec 04, 2010 at 10:07:47AM -0600, 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?
> 
> Thanks,
> 
> -- John
> --

-- 
Larry Levesque
KA1VGM

Re: soundmodem TX on HF again

[Dave Platt]

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.

Re: soundmodem TX on HF again

[Ray Wells]

On 05/12/10 05:05, Dave Platt wrote:
> 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.
>
> --
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>
>    
Just a few jottings.

I've been running 0k3 packet for about 14 years (BBS for 13), using both 
TNC's and a Baycom USCC>4 card. I've NOT used Soundmodem however, I 
expect my finding will be just a relevant, being that they are 'mode' 
specific, not modem specific.

One TNC parameter that doesn't get much mention (except in old 
documents) is txtail, which holds the tx up to ensure the last packet is 
FULLY sent. I spent ages trying to figure out why I couldn't connect 
reliably to remote stations. txtail is the -l parameter for KISS TNC's 
and I use a value of 40 for HF. If I set txtail to zero I can be assured 
of unreliable (if any) connections to remote stations. Some (old) 
documentation suggests txtail is deprecated but it's still available in 
kernel ax25.

I DO NOT know if txtail is an available parameter in Soundmodem, having 
never used it.

Also, don't overlook slow PTT. One old HF rig I had here needed TXD of 
450mS.

Clipped audio from excessive PCM level is more specific to cheap 
domestic sound cards than it is to laptops or any specific brand. My 
professional card (for serious audio work) cannot even be forced into 
that condition.

There is NO substitute for an oscilloscope (a software CRO is ok) when 
looking for audio related problems. That picture is worth a thousand 
head scratches.

RF feedback can be a serious issue on HF, hence the advice in (5) above 
to isolate and filter. If you can listen to your signal, it will sound 
quite different to other signals if RF is getting into the tx audio.

Reducing audio drive also reduces RF level for SSB and this can be a 
useful aid when looking at RF feedback as a potential problem.

Not related to the current problem but, HF packet suffers less retry-out 
events if FRACK is set above 10, I use around 13 (seconds). I found it 
greatly improved circuit reliability.

Ray vk2tv

Re: soundmodem TX on HF again

[John Goerzen]

On 12/04/2010 12:05 PM, Dave Platt wrote:
> (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.

That I managed to do today.  KQ0I looked at my signal from Des Moines 
(I'm in Kansas).  We were having strong signals on 40m at the time and 
he believes I've now got things set up right, and any remaining problems 
are due to poor band conditions.

- John

Re: soundmodem TX on HF again

[John Goerzen]

Thank you very much, Ray and Dave.  I am gathering the wisdom I'm 
collecting from here and elsewhere at

http://wiki.complete.org/PacketRadio#hfsettings

(it's incomplete but will get more)