Hi Phil, This is a great reminder. Thus far, my data has been collected with the TS-2000x using a "mid" setting for AGC; and with the HDSDR software set to "AGC Med" when using/playing back Funcube Dongle data. I've set both to OFF now, since it's possible
I wonder if anybody has experimented with AGC settings and the HDSDR decoding? Might be worth running a recording through a few times at various settings..hrm. In any event, Phil...THANK YOU for making this code real. I have seen it print data when the signal was visibly "in the dirt" which is impressive and fun to see. 73! Mark N8MH At 02:31 PM 8/16/2011 -0700, Phil Karn wrote: >I forgot to offer some advice when receiving the ARISSat-1 BPSK-1000 >telemetry beacon: turn off your receiver AGC if at all possible. If you >can only choose between fast and slow, pick slow. If this causes a large >variation in audio level, reduce the gain to avoid clipping on the >peaks. A sound card A/D is 16 bits so you have plenty of dynamic range; >don't be afraid to use it. > >Ideally the background noise level should be constant with the signal >going up and down. > >This greatly helps the demodulator and decoder to distinguish signal >from noise. The error correction uses the Viterbi algorithm, and one of >its big features is the ability to distinguish between "strong" and >"weak" bits; a strong '1' or '0' is considered less likely to be in >error than a weak '1' or '0'. The decoder can even accept "I don't know" >for a limited number of bits. > >The decoder can still fix errors in strong bits. But it can fix more of >them in the weak bits and still more in the "I don't knows" (known >technically as "erasures"). > >This is especially important when the signal fades deeply, as it often >does with ARISSat-1. With the AGC off, the audio signal level falls >during a fade and the decoder can recognize it as a burst of erasures or >near-erasures. > >As with many questions in life, "I don't know" or "I think it's X but >I'm not sure" are better answers than being sure of the wrong answer. > >73, Phil >_______________________________________________ >Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. >Not an AMSAT-NA member? Join now to support the amateur satellite program! >Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
