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-bounces@AMSAT.Org [mailto:amsat-bb-bounces@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.
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