[aprssig] weak signal ISS packet

Dave B g8kbvdave at googlemail.com
Tue Jan 13 03:43:35 CST 2015


Dont' ask me exactly "How" to do it, but with modern soundcard modems, should 
it not be possible to cater for all eventualities, "on the fly"?

Analyze what comes in, to decide if the RX in use is using de-emphasis, or not, 
and apply correction as  needed.

Have a configuration setting, to select if TX Pre-emphasis is in use, and what sort 
of modulation (Frequency or Phase modulation) is in use.

Then for transmission, pre-distort the TX waveform accordingly in software, and 
on RX, apply correction if needed, in software.

Like I said, I wish I knew how to do that sort of stuf myself.   Similar things are 
done these days at celular sites, to use "economic" less than linear economic to 
PA stages, pre-distoring the TX signal to counter the distortion in the TX. 

If the likes of UZ7HO's soundmodem and DireWolf, can have multiple "decoders" 
running, it should in essance be possible to cater for all eventuallities on RX, 
even on a single "FM" analogue channel.    Doable now on most capable PC's I 
suspect, doable soon with systems based arround such modules as the 
BeagleBone boards, and maybe (if the FPU can be used for open source stuff) on 
other ARM based boards.

Lastly...  With the advent of various digital voice modes, why the heck are we 
not moving Packet/APRS forward to use faster but robust modulation schemes 
and methods?

Yes, there is a S+N:N penalty, but if I can reliably work a DMR repeater (GB7NS) 
some 63 miles away via a non-LOS path (IO92MA to IO91WG to be precise) with 
only 1W out on 70cms to a mediocre white stick at some 25' off the deck, why 
are we not trying to advance the packet network forward to such transmission 
formats?   On receive, my DMR radio (an old DP3600) at most only ever shows 
"one bar" of signal, but decodes to very good audio just fine.

The same rig, antenna, path etc using GB3NS (analogue FM, co sited with 
GP7NS) though workable, the audio both ways is "very" scratchy.   That 
comparison was a real eye opener for me, in regards to "Digital vs Analogue" 
capabilities over the same path.   Yes, there is a "cliff edge effect", but that "cliff 
edge" seems to be a lot further away than many assume.

I won't even start on about HF packet, that's stuck in even darker places than 
1200bd VHF packet/APRS.

OK, my first Spleen venting of 2015.  Appologies, but I know I'm not the only 
one who holds these views.

73, rock, coat, hat(tin) ready.

Dave G0WBX.


> Yes, again, "best packet"  all depends on the settings of the
> transmitting station (and matching it on receive).
> 
> When the ISS used a kenwood, then another kenwood would always work
> the best. Because both are built at the factory for optimum
> performance (to each other) and they use direct modulation and
> demodulation to the same levels. My kenwoods can decode another
> kenwood (direct) with only 3 bars.    BUT similarly, I can see many
> signals that appear to have full scale signals and then barely deocde
> because so many signals on the air were never adjusted properly.  AND
> there are two standards.
> 
> Do the sending and receiving radios use pre- and de-emphasis (speaker
> and Mic connections)? Or do they operate flat with direct modulation
> and demodulation.  Is the receiver an FM demodulator or Phase
> demodulator?
> 
> BUT on ISS, I believe they are currently using another radio, not a
> kenwood. SO now you need a receiving system best matched to this
> radios transmitted waveform.
> 
> Maybe someone on the AMSAT-BB has captured the existing ISS waveform
> and we can see what kind of receiver is best.  But then again, the
> captured waveform MUST indicate exactly what kind of receiver (with or
> without de-emphasis) was used in capturing the waveform!
> 
> Two manufactured APRS radios work BEST between them because they were
> factory adjusted for BALANCED tone levels and NO preemphasis or
> de-emphasis. But other stations may just use the Mic connections and 
> speaker connections and get over 6 dB of performance loss if they are
> not accounted for.
> 
> The use of speaker/mic connections(with pre- and de-emphasis) and
> direct modulation and discriminator demodulation can each have optimum
> performance. But there are always problems when crossing between the
> two.  BUT, it IS possible to adjust on the transmit side for a
> compromise between the two and then receivers of each type can decode
> pretty good.
> 
> Though maybe 1% of hams go to this trouble.  Packet radio is NOT
> Plug-N-play.  The transmitter must be adjusted for optimum waveform!
> 
> I should be able to tell everyone what the RIGHT way to do it is, but
> I never trust my memory anymore, and don´t have time today to go do
> all the research again...  But I'm going to recall from memory, that I
> think the WORST situation of most TNC's is if the LOW tone is higher
> than the HIGH tone, that decoding always suffers.  And suffers FAR
> WORSE than the other way around.  So that is why the compromise is to
> make the high tone about 3 dB higher (30% on an OO scope)  so that it
> doesn´t get completely lost when it goes through a speaker connected
> receiver and gets de-emphasized.
> 
> OR, If  I remember wrong and it is the other way around!
> 
> Good luck.
> 
> Bob
> 
> -----Original Message-----
> 
> I was hoping for some ideas to better my station and am very pleased
> you have responded. I want to better utilize my station and increase
> my throughput with the ISS in particular. I have available 10 element
> crossed Yagi antennas and Yaesu az el rotor and several radios with
> TNC's built in including Yaesu FTM-350 and FTM-400 and Kenwood TM-D700
> and TS-2000. MFJ and Kantronics TNC's as well as products from Byonics
> and Argent Data are found on my bench. I am currently running a
> Kenwood TM-D700 to a M2 Eggbeater as a 24/7 ISS station with UISS
> software and modules to upload to the internet.
>          I want to increase my capabilities with respect to the ISS
>          and
> APRS. I have found that the Kenwood TM-D700 requires that you get
> about almost a half scale reading to effect good packet reception. I
> don't want to use the fully automated  station (FT-847) that I have
> when I'm not home because in the past a failure caused my rotor to jam
> and I had to lower my tower to remove and repair the az-el rotor. I
> suspect that the best way might be to use a sound card software and
> computer to give the best decoding. Thanks for the response Steve
> KB1CHU
> 
> 




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