Hi Mark, Phil, That's interesting. I have collected all my passes on the TS2000 with the AGC on and set to the longest setting. This is mainly because I often record the signal level every 0.5 seconds during a pass which requires the AGC to be on and the longest setting irons out any short fades. I will turn the AGC off and look at the statistics over a few passes but given that with the AGC on I get almost all the available frames I don't expect to see much difference. It will be interesting to see.
73 Alan ZL2BX -----Original Message----- From: amsat-bb-boun...@amsat.org [mailto:amsat-bb-boun...@amsat.org] On Behalf Of Mark L. Hammond Sent: Wednesday, 17 August 2011 20:36 To: Phil Karn; Amsat - BBs Subject: [amsat-bb] Re: Turn off AGC when receiving BPSK-1000 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 ----- No virus found in this message. Checked by AVG - www.avg.com Version: 10.0.1392 / Virus Database: 1520/3841 - Release Date: 08/17/11 _______________________________________________ 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
