Setting up an APRS IGate with OpenWebRx, Direwolf and CDR - Linux

G'Day Guys,

In my previous post "Setting up APRS IGate - Windows via SoundCard" I outlined how to straight forward it was to setup and APRS IGate under Windows was using a Soundcard to decode VHF APRS signals.

Well for me I thought this is cool, but hang on a second!!!  I'm currently making available for internet users access to our local 2m VHF spectrum using OpenWebRx and SDRPlay RSP2Pro.  You can tune in to 145.175 MHz and hear the APRS digital signals. So from my experience of OpenWebRx its a great tool which does the following:

• Using rtl_sdr or rx_sdr tools will start up a session to your SDR what whatever bandwidth you'd like to offer via OpenWebRX.  In my case I've configured the "rx_sdr" to sample and provide a bandwidth of 5MHz which allows me to offer the 2m VHF from 143MHz through to 148 MHz. This feed is then made available via "127.0.0.1:4951"

• The OpenWebRx application creates a single set of process that then samples the SDR feed and present a fantastic waterfall and means for multiple users to select a frequency and mode to listen via a web interface.

• When a user finally connects up and mind you there is a limit to the number of concurrent users that can access the portal and individual select any frequency and mode within the allocated frequency range.   When the user connects up OpenWebRx starts a separate set of CSDR processes pipe-lined together to extract and transform the desired frequency and mode and audio outputted to the users browser.

So with that little bit of background on how OpenWebRx works, then I thought hey why can't I start my own CDR pipeline for APRS tuned to 145.175MHz NFM and pipe the audio through to a APRS decoding tool ?

It didn't take me long to come across "Direwolf " via GitHub.  

Dire Wolf is a software "soundcard" AX.25 packet modem/TNC and APRS encoder/decoder. It can be used stand-alone to observe APRS traffic, as a tracker, digipeater, APRStt gateway, or Internet Gateway (IGate). For more information, look at the bottom 1/4 of this page and in https://github.com/wb2osz/direwolf/blob/dev/doc/README.md  

Follow the instructions on the "GitHub" Readme to download, compile and install "direwolf".

Now before I continue I will also assume you have setup OpenWebRx or you have configure a "rtl_sdr" or "rx_sdr" process which is available via network or to be pipped into the CSDR tool.  Take a look at my post "Getting Your RTLSDR Online Using OpenWebRx" :)

Ok!!! what the dickens is this CSDR?  head on over to GitHub  project CSDR by "Simonyi Károly College for Advanced Studies"

csdr is a command line tool to carry out DSP tasks for Software Defined Radio.

It can be used to build simple signal processing flow graphs, right from the command line.

The included libcsdr library contains the DSP functions that csdr makes use of. It was designed to use auto-vectorization available in gcc, and also has some functions optimized with inline assembly for ARM NEON to achieve some speedup by taking advantage of SIMD command sets available in today's CPUs.

Feel free to use it in your projects.
Most of the code is available under the permissive BSD license, with some optional parts under GPL. For additional details, see licensing.

csdr has already been used to build:

• AM, FM, SSB, CW and BPSK31 demodulators and waterfall display in OpenWebRX,
• AM, FM, SSB modulators in qtcsdr that can also be used standalone with rpitx,
• a demodulator for FSK transmissions sent with the CC1111 wireless MCU, and also a standalone RTTY demodulator.

 On the project page there is a document that outlines of the CSDR functions and their parameters and included examples.  The examples are using "rtl_sdr" to start a SDR stream and pipe through the CSDR tools.

GOTCHA #1

The first gotcha that almost drove me bonkers was that the "rtl_sdr" and "rx_sdr" tool command line arguments are fairly similar. Since my OpenWebRX was configured to use rx_sdr I made a stupid very STUPID assumption / did not pay attention to the fact the CSDR examples used "rtl_sdr" and the very first CSDR function was to conbert to U8_f which caused the sample rate to be 4 times what I needed it since the OpenWebRx passed CF32 option !!  BIG DOH!!!  So basically I just had to exclude the "csdr convert_u8_f" command and leave it as C32F for the next CSDR command.

Once I had that little gem sorted, then everything worked as expected, but the CSDR notes where not 100% clear to me for some of the CSDR function paramters but I was able to reverse the values and work them out.  So below I have used variable to hold important values and results of calculation that help clearly show how the arguments to these CSDR function are calculated.  It also allows you to quickly and safely alter / play with values.

The first actually passes the stream to "mplayer" to allow you to hear it. The second is example of passing the stream to "Direwolf"

Example 1: MPlayer

       

export SDR_SAMPLE_RATE=5000000
export SDR_GAIN=IFGR=35,RFGR=2
export AUDIO_SAMPLE_RATE=48000
export AUDIO_GAIN=0.32
export FREQ_CENTER=145500000
export FREQ_MON=145175000
export FREQ_MON_BW=12000
export FIR_DISC_FACTOR=`python -c "print float($SDR_SAMPLE_RATE)/float($AUDIO_SAMPLE_RATE)"`
export FIR_DISC_TRANS_BW=`python -c "print float($FREQ_MON_BW)/float($SDR_SAMPLE_RATE)"`
export SHFT_ADD=`python -c "print float($FREQ_CENTER-$FREQ_MON)/$SDR_SAMPLE_RATE"`
 
nc -v 127.0.0.1 4951 | csdr shift_addition_cc $SHFT_ADD | csdr fir_decimate_cc $FIR_DISC_FACTOR $FIR_DISC_TRANS_BW HAMMING | csdr fmdemod_quadri_cf | csdr limit_ff | csdr deemphasis_nfm_ff $AUDIO_SAMPLE_RATE | csdr fastagc_ff | csdr gain_ff $AUDIO_GAIN | csdr convert_f_s16 |  mplayer -cache 1024 -quiet -rawaudio samplesize=2:channels=1:rate=$AUDIO_SAMPLE_RATE -demuxer rawaudio -

       
 

Example 2: Direwolf

       

export SDR_SAMPLE_RATE=5000000
export SDR_GAIN=IFGR=35,RFGR=2
export AUDIO_SAMPLE_RATE=48000
export AUDIO_GAIN=0.32
export FREQ_CENTER=145500000
export FREQ_MON=145175000
export FREQ_MON_BW=12000
export FIR_DISC_FACTOR=`python -c "print float($SDR_SAMPLE_RATE)/float($AUDIO_SAMPLE_RATE)"`
export FIR_DISC_TRANS_BW=`python -c "print float($FREQ_MON_BW)/float($SDR_SAMPLE_RATE)"`
export SHFT_ADD=`python -c "print float($FREQ_CENTER-$FREQ_MON)/$SDR_SAMPLE_RATE"`
export DW_HOME=/home/drifter/sdr/aprs/noniss
export DW_CFG=$DW_HOME/direwolf-local.conf
export DW_LOG=$DW_HOME/logs
 
nc -v 127.0.0.1 4951 | csdr shift_addition_cc $SHFT_ADD | csdr fir_decimate_cc $FIR_DISC_FACTOR $FIR_DISC_TRANS_BW HAMMING | csdr fmdemod_quadri_cf | csdr limit_ff | csdr deemphasis_nfm_ff $AUDIO_SAMPLE_RATE | csdr fastagc_ff | csdr gain_ff $AUDIO_GAIN | csdr convert_f_s16 |  direwolf -c $DW_CFG -l $DW_LOG -a 10 -t 1 -n 1 -r $AUDIO_SAMPLE_RATE -b 16 -d aupim -

       
 

You can substitute the "nc -v 127.0.0.1 4951" with the one of the following if testing outside of OpenWebRx:

For RTL SDR Dongles (just remember to adjust the SDR_SAMPLE_RATE to something more suitable for the RTL SDR ie 2400000):

rtl_sdr -s $SDR_SAMPLE_RATE -f 145000000 -g 44 - | csdr convert_u8_f 

For SDR configured and available via RX_SDR:

8Bit examples:

rx_sdr -F CU8 -s $SDR_SAMPLE_RATE -f $FREQ_CENTER -g $SDR_GAIN - | csdr convert_u8_f 

32Bit Example:

rx_sdr -F CF32 -s $SDR_SAMPLE_RATE -f $FREQ_CENTER -g $SDR_GAIN -

That's All Folks, Hope this helps you out with you APRS and any other digital decoding project you be having in conjunction with your OpenWebRx portal.

73 de VK4TMZ