FSK extension & framing mode

Hi, I am a new user and just trying find out how some of the less obvious things work in KiwiSDR.

One thing I have not been able to find when searching the forum is how to use he framing mode in the FSK extension. The shift and baud rate of FSK signals are fairly easy to find out, but the framing I find puzzling.

Perhaps someone could shine some light on that.



BTW: I found that PBB (Dutch Navy) also works FSK on 12840.5 USB


  • jksjks
    edited December 2022

    The framing mode is a visual tool I wrote while developing the bit-level decoders for the various specialty modes (e.g. DSC, EFR, CHU etc). You will note that when you select one of the signal presets from the menus not only is the baud and shift set but also the framing and encoding.

    The framing tool is a way of confirming what the framing and encoding actually are by plotting the bits received in rows and columns using various word lengths, phasing and selection of the number of bits that are used for the data decoding (e.g. ITA2, ASCII).

    It's probably best described by example. Unfortunately there are few simple format FSK signals that are strong and easy to receive. The strong ones are all slightly advanced which makes them a little more difficult to explain. Namely:

    The DDH/DDK weather signals use fractional stop bits (1.5) which have to be accounted for in a special way: word=15, 1 start bit, 5 data bits (ITA2), no parity bits, 1.5 stop bits = 7.5 bits, but 7.5*2=15 bits, twice what it would normally be if there were no fractional stop bit; data=5 (ITA2)

    PBB uses a format which isn't supported by the framer currently.

    EFR: word=11, 1 start bit, 8 data bits (ASCII), 1 even parity bit, 1 stop bit; data=8 (ASCII) But as shown below the decoded packet isn't printed until all the data is received (decoding can't occur character-by-character).

    So start by picking a Kiwi in Europe that has strong reception of DDH47 on 147.3 kHz. Set word=15 and data=5 as explained above. Click the sample button. Wait for the display to fill up. Note that the start and data bits are "doubled up" across two rows with the same values in each row. The fractional 1.5 stop bit becomes 3 rows, all of the same value.

    Now use the phasing buttons + and - Note how the rows are shifted up and down. Phase until the start bit pattern (two lines of solid blue bits) are at the top of the display and three lines of solid yellow bits are at the bottom (stop bit). The red lines bracket the 8(16) bits that are taken as ASCII and displayed as characters at the top. Note how the ASCII decoding won't be correct unless the phasing is correct.

    You should be able to repeat this for the EFR (Teleswitch) signals. Use word=11, data=8.

    p.s. I added 12840.5 to the Military/PBB menu. Thanks.

  • @jks

    Many thanks for the clear explanation of the framing mode tool and I will try this out on some other HF fsk transmissions. There are unfortunately few left that are unencrypted nowadays, including probably the frequently heard T-600 and Stanag 4481 signals.

    Best regards,


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