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[aprssig] APRS Messenger now has APRS over PSK-63

Stephen H. Smith wa8lmf2 at aol.com
Sun Feb 28 21:52:54 UTC 2010


Some further thoughts and observations about simultaneous AX25 and PSK63:


On 2/28/2010 9:41 AM, Frank or Barbara Rossi wrote:
> Trying to get APRS Messenger and UI-View using the AGW Packet engine 
> APRS dial frequencies to line up.
> Currently using the 2100 Hz version of APRS Messenger with the dial 
> set at 10.1476 on USB.
> Several stations and I have exchanged messages, works great.
> I figure if I could get the dial frequencies to line up, I could 
> I-Gate AX25 APRS and PSK-63 APRS at the same time,
> same radio and same computer.
> The AGW Packet engine will only decode tones 2100/2300.
> With dial set at 10.1476 on USB AX25 tones are to low to decode 
> without changing dial frequency, and then PSK-63 APRS is off frequency.
>
> Should I be using a different version of APRS Messenger to get the 
> tones to line up ?
> Also What would the dial frequency and USB/LSB be ?
>

The FREEWARE version of AGWpe only offfers the 2100/2300 tones.     The 
PAY-FOR AGWpe "Pro" version offers the choice of standard 1600/1800 KAM 
tones or the original 2100/2300 tones.


------------------------------------------------------------------------




1)     SSB transmission is essentially a frequency translator that 
shifts audio frequencies to RF while retaining their relative spacing.



2)     The actual RF frequencies of the two tones (a.k.a "mark" and 
"space") of normal FSK AX25 APRS on 30 meters are 10.149.200 MHz and 
10.149.400 MHz.    Chris is trying to place the PSK63 carrier exactly 
half-way between the upper packet tone at 10.149.400 and the edge of the 
30-meter ham band at 10.150.00.  In other words the actual PSK63 center 
frequency is 10.149.700; i.e. 300 Hz above the higher of the two packet 
FSK frequencies.

Note that no matter how you fiddle and diddle around with audio tone 
freqs and dial settings,
      THE ACTUAL *RADIO FREQUENCIES*  MUST ALWAYS BE THE SAME.



3)      At the audio baseband, the PSK63 tone (2100 Hz)  is 300 Hz ABOVE 
the higher of the two standard KAM-style FSK packet audio  tones of  
1600 & 1800 Hz.     By adding these three audio tones to the 
(suppressed) RF carrier (i.e. "dial") frequency of 10.147.600 Mhz USB, 
you get the three RF frequencies mentioned in 2) above:



      10.147.600 + 1.600 =  10.140.200      Packet MARK
      10.147.600 + 1.800 =  10.149.400      Packet SPACE

      10.147.600 + 2.100 =  10.149.700      PSK63



4)      If,   at the audio baseband,   you want to use the freeware 
AGWpe tones of 2100 and 2300, the PSK63 tone now has to be 300Hz above 
the higher of these, or 2600 Hz.  (I.e. new version of APRS Messenger 
required.)  To make these land on the same *RF* frequencies as above, 
you will have to now set the suppressed carrier (i.e. "dial") frequency 
of the radio to 10.147.100 MHz.


      10.147.100 + 2.100 =  10.149.200   Packet MARK
      10.147.100 + 2.300 =  10.149.400   Packet SPACE

      10.147.100 + 2.600 =  10.149.700   PSK63

The problem is that virtually NO SSB rig's have audio passbands/filter 
systems that will pass an audio tone as high as 2600.

[  Unless of course you are one of the hard-core guys at the low end of 
75 meters at night playing with "HiFi" SSB with broadcast audio consoles 
and studio condenser mics, running heavily modified rigs employing 
special wide-band filters on SSB, .)  ]

If the 2600 Hz tone does somehow just barely get through the SSB filter, 
the tone will land right on the filter's upper skirt where it will be 
subject to severe phase shift and group-delay distortion. (Which is 
exactly what you DON'T want in a phase-shift modulation scheme!)



5)     Operating on *LSB* (rather than *USB*)  *INVERTS* the tone 
relationship; i.e. the higher the audio tone, the LOWER the resulting RF 
frequency.

If one places the PSK63 tone 300 Hz below the lower AGWpe packet tone of 
2100, it will be safely within  the SSB audio passband.   (A new version 
of APRS messenger running at 1800 Hz would be required.)   Setting the 
radio to a suppressed carrier (i.e. "dial") frequency of 10.151.500 MHz 
LSB will invert the tone relationships and yield the following:


      10.151.500 - 1.800 = 10.149.700   PSK63

      10.151.500 - 2.100 = 10.149.400   Packet SPACE
      10.151.500 - 2.300 = 10.149.200   Packet MARK

Note that while the dial indicates you are transmitting outside the ham 
band, the suppressed carrier frequency of 10.151.500 IS NOT TRANSMITTED 
(assuming your rig is properly aligned with good carrier supression).  
As long as you generate audio tone(s) higher than 1.500 Khz, the 
resulting modulation products will be inside the ham band, assuming you 
don't overdrive the transmitter modulator and produce intermodulation 
distortion.

HOWEVER, most current ham gear will not transmit when the indicated 
carrier frequency is moved outside the ham bands.  You will have to hack 
the radio for the so-called general-coverage transmit capability.  This 
is usually a trivial exercise, done by clipping a single diode on the 
radio's controller board or slashing one board trace with an X-Acto 
knife and then resetting the radio's CPU.   See

<http://mods.dk> for info on the general-coverage mod for virtually any 
HF/SSB transceiver.



6)     Finally, you are probably going to need TWO sound systems on the 
computer;  one to run AGWpe and one to run APRS Messenger PSK63.  There 
are a few scenarios where you can get a single sound system to serve 
more than one application at a time, but this gets really tweaky and 
requires WinXP SP3 or later.   (I have personally run both mmSSTV analog 
SSTV and EasyPal digital SSTV with a single sound card simultanously but 
as they say "Your mileage may vary"!)

------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
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Home Page: http://wa8lmf.net

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