With the recent frequency change of 30m APRS over VARA traffic to 10.1482 MHz I have figured out a way to be able to decode robust packet? as well as AX.25 and VARA APRS traffic with one radio.
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Usually robust packet APRS uses a transceiver tuned to a dial frequency of 10.1473 MHz with the robust packet audio centred at 1500 Hz.
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The new APRS over VARA frequency now means that the robust packet traffic can be heard in the same receiver but the audio is centred 900 Hz lower at 600 Hz.
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Here's how I worked out a way to mix the frequency up to 1500 Hz using HDSDR SDR software. This may be useful for other applications.
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The receiver audio from the transceiver appears on a USB sound card connected to the transceiver. This audio is applied to the VARA HF modem, UZ7HO sound card modem for 300 bd AX.25 packet and HDSDR.
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In HDSDR select the USB sound card connected to the transceiver as the input sound card.
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As it's a mono audio signal you will see two sidebands on the main display centred at 0 Hz on the HDSDR display.
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In HDSDR set Options/Misc Options/set LO <-> Tune Offset to -900 Hz.
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In HDSDR select Options/Misc Options/Tune Fixed to 'LO <-> Tune Offset'.
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Select the output sondcard in HDSDR. I use the LoopBeAudio virtual audio cable as it has near zero latency. The popular VB-Audio virtual audio cable has nearly has a latency of nearly 0.5 second so it is unsuitable for data operations.
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In WinRPR select LoopBeAudio as the input sound card.
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The original audio centred at 600 Hz now appears at 1500 Hz and WinRPR will now decode robust packet data.
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You can go into HDSDR and tighten up the filter on the robust packet data.
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To do all this you will need a fast PC to handle the three decode programs and the SDR program but it will work reliably. Mine has run for 4 hours so far and I'll leave it running for the next few days.
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I'm using a Tentec Corsair II with a synthesiser I originally developed for a SDR transceiver that we never put into production.
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This has a 1.8 kHz crystal filter originally used for SSB DXing and contests which allows all three APRS data streams through.
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If you want to check on APRS.FI I'm running GM4HYG-1 for 300 bd AX25 and robust packet and GM4HYG-2 for APRS over VARA.
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Regards,
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Chris GM4HYG
