APRS & Soundmodem

I was not satisfied with my prior results with the APRS testing, so I setup soundmodem in an Ubuntu environment on my trusty netbook and set-off to decode packets. As expected, everything went well – soundmodem was decoding local packets with little trouble. To be sure that the sound card itself was not at fault, I used the same USB sound-card as before.

Soundmodem working as intended

Soundmodem working as intended

It appears that my electronics workbench may not be an ideal APRS RX station locale after all – I moved the test setup to the loft of my out-building and was pulling in packets more easily. The next step will be to replicate the setup using a Raspberry Pi in place of the netbook – I just need to configure wireless networking on the Pi before I relegate the hardware to the -20°C temperatures we are experiencing.

Perhaps the received signals were in fact too weak for soundmodem to properly decode before? If this is the case, a good argument for a purpose-built TNC can be made 😉

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3 Responses to APRS & Soundmodem

  1. Per L says:

    I have a project to build a low bitrate packet data link using two raspberry pi, two audio UHF transcievers and soundmodem.
    What would be a good trasciever to use under these conditions:
    – outdoor all-weather operation
    – 1W xmit power (1 km reliable range)
    – possible to switch transmitting on/off from raspberry (to save battery when there is no data to send)

  2. Marcel says:

    Hi James,

    I have been reading your posts on aprs and soundmodem and got the thing to go on raspberry pi. I’m using an el-cheapo CM106 External USB sound card that works very well on RX (haven’t made a cable for TX yet) with my handheld 2m tranceiver. I also played with Direwolf and Xastir but aprx is my favourite so far. I noticed you’re not having much success in receiving/decoding packets, is that still the case? One thing I noticed was that my audio-input level should be around 60-70% to get the maximum decoding success rate. I also used the test CD and once got up to around 990 successfully decoded packets.

    The other tool that I used to measure input levels is with the alsa utility called arecord. With the method described on this page you can get a rolling graphical view of input levels which may help you set the appropriate output level.


    The syntax for this command is:

    arecord -Dhw:0 -c2 -d 0 -fS16_LE /dev/null -vvv

    But you may want to set the device name to a different card.

    The Direwolf software nicely shows the measured volume level on every decoded packet and even shows when it’s too loud.

  3. Jaques says:

    I too am very interested in getting some APRS and BBS action going on a Raspberry Pi 🙂
    Holding thumbs that I’ll be able to send you a BBS mail some day

    ZS6JV de Jaques

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