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As a red circle in the picture, What does 1700 number means? When I change it to another value seems like it can not decode APRS sound. So what is it(1700 in textbox after ChA label)?

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I'm not familiar with the SoundModem program, but APRS uses an AFSK modulation which switches between two tones at 1200 Hz and 2200 Hz. 1700 Hz is exactly halfway between those.

Therefore I would guess that it's setting the demodulator's threshold for discriminating between the two tones. You would change it if you are decoding packets that differ from the APRS standard.

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