DSC Monitoring

edited June 2022 in Signals Received

I have been monitoring that on 12577 and receive 100's of TX without issue. I also note about 5% of the TX visible on the waterfall either don't decode even though they seem on freq. and other ares -25 Hz or so off freq. Anybody else experience this?


  • After a DSC freq is selected you can set "show errs" from the DSC menu to see which TXs are not being shown because the FEC failed (a character plus its 400 msec delayed copy both had parity errors so the message content can't be trusted). I'm currently still showing a message if there are no FEC failures, but the overall checksum failed. Another error situation is when no end-of-sequence (EOS) could be detected.

    I hear off-frequency TXs all the time. Some far enough removed from the fixed FSK mark/space filters that they won't decode.

  • edited June 2022

    I wonder if the "official" RX stations can decode the off freq at all. If not, the safety is compromised by the poor TX.

    +/- 10 Hz seems to be the common spec amongst the commerical radio OEM

  • I'd consider JRC to be top of the line, here are their specs

  • jksjks
    edited June 2022

    The spec from ITU-R M.493 section 1.3.3 is +/- 10 Hz. The current Kiwi mark/space filters can handle that fine. I bet real Selcall radios use PLL or FFT based FSK decoding that tolerate greater off-frequency signals.

    But this makes me realize another feature I need to add: User-defined offset. Suppose a public Kiwi you're interested in is not using GPS and has a large frequency offset. The owner/admin can do a manual calibration to remove the offset (right-click menu, cal ADC clock) But that's a privileged operation as it applies to the entire Kiwi.

    Suppose you select 6312 kHz from the DSC menu and find you have to adjust the frequency downward 50 Hz to center the FSK markers to get good decoding (e.g. click the small circled-minus icon five times). That's fine, but as soon as you select a different DSC frequency in the menu you have to apply the offset again. What if you could just setup the offset once someplace and not have to worry about it again?

    This is slightly more complicated because the offset is not a constant value in Hz. It's really an ADC clock ppm adjustment. So the 50 Hz offset at 6312 kHz is actually a bit larger at the DSC 16805.4 frequency. But I have the code to figure that out. It's all in the cal ADC clock dialog which shows you the computed ppm value given the offset being applied.

  • This is a really cool enhancement , and I'm having fun seeing the huge variety of ship traffic using the DSC decoder.

    Iv'e spent some time looking for a "Idiots guide to DSC message format interpreting" but haven't found any info on what the interpretation of the final part of the string that is transmitted means (6PE, 1PE, 8PE, etc) . Anyone know?

    I've got my Kiwi parked on 16804 and it's copying shipping in the Persian Gulf, and all over central and south america. I'm really impressed both by how versatile the Kiwi is, and the quality of the decoding algorithm is.

  • jksjks
    edited June 2022



    All the stuff in darker blue (and red) is not part of the DSC protocol, but related to errors seen in decoding.

    As usual with the Kiwi extensions there are much better decoders written as external PC/Mac software that you can feed with Kiwi IQ output. The forum's own @ChrisSmolinski of Black Cat Systems has a DSC decoder with some very impressive features: blackcatsystems.com/software/black_cat_gmdss_decoder.html A comprehensive review from Nils, DK8OK: dk8ok.org/2021/11/22/ahoy-decoding-eight-gmdss-channels-in-a-convoy

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