[aprssig] APRS Messenger now has APRS over PSK-63
Stephen H. Smith
wa8lmf2 at aol.com
Sun Feb 28 16:52:54 EST 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]
Skype: WA8LMF
Home Page: http://wa8lmf.net
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