APRS using Soundmodem_9k6 (FCD + SDR#)

I tried to receive the digipeating signal of local APRS_144.640MHz
using Soundmodem_9k6 software. This frequency is defined as APRS_
9600bps data communication in Japan. APRS differs in the frequency
by the area as follows.

  Japan: 144.640MHz (9600bps GMSK), 144.660MHz (1200bps AFSK)
  North America: 144.390MHz
  Europe: 144.800MHz
  Australia: 145.175MHz
  New Zealand: 144.575MHz
  Argentina: 144.930MHz
  Brazil: 145.570MHz

As shown in the figure below, SSID of 9600bps digipeater is -2
in Japan. APRS recommends as follows Setting of the other SSID.

  -0  Your primary station usually fixed and message capable
  -1  generic additional station, digi, mobile, wx, etc
  -2  generic additional station, digi, mobile, wx, etc
  -3  generic additional station, digi, mobile, wx, etc
  -4  generic additional station, digi, mobile, wx, etc
  -5  Other networks (Dstar, Iphones, Androids, Blackberry's etc)
  -6  Special activity, Satellite ops, camping or 6 meters, etc
  -7  walkie talkies, HT's or other human portable
  -8  boats, sailboats, RV's or second main mobile
  -9  Primary Mobile (usually message capable)
  -10 internet, Igates, echolink, winlink, AVRS, APRN, etc
  -11 balloons, aircraft, spacecraft, etc
  -12 APRStt, DTMF, RFID, devices, one-way trackers*, etc
  -13 Weather stations
  -14 Truckers or generally full time drivers
  -15 generic additional station, digi, mobile, wx, etc


More details,


I tried to simultaneously received APRS_9600bps signal by the software
MixW(3.1.1) and Soundmodem_9k6(0.10b), and I compared the decode rate.

  Date: 14 Jun 2013, 22:51-23:06UTC
  Frequency: 144.640MHz FM, 9600bps GMSK
  DCD threshold: 5
  Results: Soundmodem_9k6 ... 16
           MixW ............. 49

It was the result MixW wins overwhelmingly on the decoding experiment.
I think that there is a point of setting unadjusted in Soundmodem_9k6.

In addition, I can see the word "Mt.AKAGI" in the image, so I could hear
the signal from Mt.Akagi in Gunma away 130km from Yokohama.


Default value of "NRMonitorLines" in "g3ruh_sm.ini" is 50 lines.
I was advised to change for higher value from the author of SCM. 
So I tried to playback the saved wav file as NRMonitorLines=1000.
Then the decoded number in SCM_9k6 has increased to 47 at once.
Now, It was almost comparable to the decoded number in MixW.

  Date: 14 Jun 2013, 22:51-23:06UTC
  Frequency: 144.640MHz FM, 9600bps GMSK
  DCD threshold: 5
  Results: SCM_9k6(Ver0.10b) ... 47
           MixW(Ver3.1.1)    ... 49

I have succeeded in decoding of 49 pcs by reducing the input level
and setting LPF2_Width=9500 in SCM_9k6.

  1. Sound Recoder -> Open -> aprs_9k6.wav
  2. Sound Recoder -> Effects -> Decrease Volume (Press 5 times)
  3. Soundmodem_9k6 -> Settings -> Modems -> LPF2_Width=9500

GS said:
> I mean recording level. Look at the screenshot, there you see cut off
> the top of a sine wave. Try to reduce recording level, just the signal
> will be without hard limit. I guess it will improve the dynamics.



Subject: Soundmodem_9k6 LPF
From: Mineo Wakita
Date: 17 Jun 2013, 19:53 +09:00

Hello Andrei,

I tried to decode 9k6 signal of M-Cubed satellite using
a wav file which was received by JA0CAW on 12 Jun 2013.
I could decode it setting LPF1 Width=7000 only in SCM_9k6.

Well, I don't know the difference between LPF1 and LPF2,
I want to ask you a question about it.


Subject: Re: Soundmodem_9k6 LPF
From: Andrei Kopanchuk
Date: Mon, 17 Jun 2013 15:44:04 +0400

Hello Mineo!

This is structure of the RX-part of the modem:

Input signal -> LPF1 -> DC offset corrector -> AGC -> Limiter -> LPF2 -> ATC -> AX.25 Decoder

I guess in some cases, for different values of LPF, the ATC (Automatic Threshold Control)
block may return different values of the bit. That may happens when noise level is high.

Andrei Kopanchuk


The figures below are some experiment of FCD individual difference
receiving APRS in Japan. There are individual differences of -13kHz
(in 144_band) and -39kHz (in 430_band) in my FCD. So I need to set
[+ its value] as Shift value of my FCD.  If you set PPM value
described in Supplement2, it is not necessary to set Shift value.




PPM (Parts Per Million) calculation example:


  Since the initial value of PPM was -21.0
  then my PPM=-21.0-90.0=-111.0



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